.. _ghci:
Using GHCi
==========
.. index::
single: GHCi
single: interpreter
single: interactive
single: Hugs
single: Foreign Function Interface; GHCi support
single: FFI; GHCi support
GHCi [1]_ is GHC's interactive environment, in which Haskell expressions
can be interactively evaluated and programs can be interpreted. If
you're familiar with `Hugs `__, then
you'll be right at home with GHCi. However, GHCi also has support for
interactively loading compiled code, as well as supporting all [2]_ the
language extensions that GHC provides. GHCi also includes an interactive
debugger (see :ref:`ghci-debugger`).
.. [1]
The ‘i’ stands for “Interactive”
.. [2]
except ``foreign export``, at the moment
.. _ghci-introduction:
Introduction to GHCi
--------------------
Let's start with an example GHCi session. You can fire up GHCi with the
command ``ghci``:
::
$ ghci
GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help
Prelude>
There may be a short pause while GHCi loads the prelude and standard
libraries, after which the prompt is shown. As the banner says, you can
type ``:?`` to see the list of commands available, and a half line
description of each of them. We'll explain most of these commands as we
go along, and there is complete documentation for all the commands in
:ref:`ghci-commands`.
Haskell expressions can be typed at the prompt:
.. index::
single: prompt; GHCi
::
Prelude> 1+2
3
Prelude> let x = 42 in x / 9
4.666666666666667
Prelude>
GHCi interprets the whole line as an expression to evaluate. The
expression may not span several lines - as soon as you press enter, GHCi
will attempt to evaluate it.
In Haskell, a ``let`` expression is followed by ``in``. However, in
GHCi, since the expression can also be interpreted in the ``IO`` monad,
a ``let`` binding with no accompanying ``in`` statement can be signalled
by an empty line, as in the above example.
.. _loading-source-files:
Loading source files
--------------------
Suppose we have the following Haskell source code, which we place in a
file ``Main.hs``:
::
main = print (fac 20)
fac 0 = 1
fac n = n * fac (n-1)
You can save ``Main.hs`` anywhere you like, but if you save it somewhere
other than the current directory [3]_ then we will need to change to the
right directory in GHCi:
::
Prelude> :cd dir
where ⟨dir⟩ is the directory (or folder) in which you saved ``Main.hs``.
To load a Haskell source file into GHCi, use the ``:load`` command:
.. index::
single: :load
::
Prelude> :load Main
Compiling Main ( Main.hs, interpreted )
Ok, modules loaded: Main.
*Main>
GHCi has loaded the ``Main`` module, and the prompt has changed to
“\ ``*Main>``\ ” to indicate that the current context for expressions
typed at the prompt is the ``Main`` module we just loaded (we'll explain
what the ``*`` means later in :ref:`ghci-scope`). So we can now type
expressions involving the functions from ``Main.hs``:
::
*Main> fac 17
355687428096000
Loading a multi-module program is just as straightforward; just give the
name of the “topmost” module to the ``:load`` command (hint: ``:load``
can be abbreviated to ``:l``). The topmost module will normally be
``Main``, but it doesn't have to be. GHCi will discover which modules
are required, directly or indirectly, by the topmost module, and load
them all in dependency order.
.. [3]
If you started up GHCi from the command line then GHCi's current
directory is the same as the current directory of the shell from
which it was started. If you started GHCi from the “Start” menu in
Windows, then the current directory is probably something like
``C:\Documents and Settings\user name``.
.. _ghci-modules-filenames:
Modules vs. filenames
~~~~~~~~~~~~~~~~~~~~~
.. index::
single: modules; and filenames
single: filenames; of modules
Question: How does GHC find the filename which contains module ⟨M⟩?
Answer: it looks for the file ``M.hs``, or ``M.lhs``. This means that
for most modules, the module name must match the filename. If it
doesn't, GHCi won't be able to find it.
There is one exception to this general rule: when you load a program
with ``:load``, or specify it when you invoke ``ghci``, you can give a
filename rather than a module name. This filename is loaded if it
exists, and it may contain any module you like. This is particularly
convenient if you have several ``Main`` modules in the same directory
and you can't call them all ``Main.hs``.
The search path for finding source files is specified with the ``-i``
option on the GHCi command line, like so:
::
ghci -idir1:...:dirn
or it can be set using the ``:set`` command from within GHCi (see
:ref:`ghci-cmd-line-options`) [4]_
One consequence of the way that GHCi follows dependencies to find
modules to load is that every module must have a source file. The only
exception to the rule is modules that come from a package, including the
``Prelude`` and standard libraries such as ``IO`` and ``Complex``. If
you attempt to load a module for which GHCi can't find a source file,
even if there are object and interface files for the module, you'll get
an error message.
.. [4]
Note that in GHCi, and ``--make`` mode, the ``-i`` option is used to
specify the search path for *source* files, whereas in standard
batch-compilation mode the ``-i`` option is used to specify the
search path for interface files, see :ref:`search-path`.
Making changes and recompilation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. index::
single: :reload
If you make some changes to the source code and want GHCi to recompile
the program, give the ``:reload`` command. The program will be
recompiled as necessary, with GHCi doing its best to avoid actually
recompiling modules if their external dependencies haven't changed. This
is the same mechanism we use to avoid re-compiling modules in the batch
compilation setting (see :ref:`recomp`).
.. _ghci-compiled:
Loading compiled code
---------------------
.. index::
single: compiled code; in GHCi
When you load a Haskell source module into GHCi, it is normally
converted to byte-code and run using the interpreter. However,
interpreted code can also run alongside compiled code in GHCi; indeed,
normally when GHCi starts, it loads up a compiled copy of the ``base``
package, which contains the ``Prelude``.
Why should we want to run compiled code? Well, compiled code is roughly
10x faster than interpreted code, but takes about 2x longer to produce
(perhaps longer if optimisation is on). So it pays to compile the parts
of a program that aren't changing very often, and use the interpreter
for the code being actively developed.
When loading up source modules with ``:load``, GHCi normally looks for
any corresponding compiled object files, and will use one in preference
to interpreting the source if possible. For example, suppose we have a 4-module
program consisting of modules ``A``, ``B``, ``C``, and ``D``. Modules ``B`` and
``C`` both import ``D`` only, and ``A`` imports both ``B`` and ``C``:
::
A
/ \
B C
\ /
D
We can compile ``D``, then load the whole program, like this:
::
Prelude> :! ghc -c -dynamic D.hs
Prelude> :load A
Compiling B ( B.hs, interpreted )
Compiling C ( C.hs, interpreted )
Compiling A ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.
*Main>
In the messages from the compiler, we see that there is no line for
``D``. This is because it isn't necessary to compile ``D``, because the
source and everything it depends on is unchanged since the last
compilation.
Note the ``-dynamic`` flag to GHC: GHCi uses dynamically-linked object
code (if you are on a platform that supports it), and so in order to use
compiled code with GHCi it must be compiled for dynamic linking.
At any time you can use the command ``:show modules`` to get a list of
the modules currently loaded into GHCi:
::
*Main> :show modules
D ( D.hs, D.o )
C ( C.hs, interpreted )
B ( B.hs, interpreted )
A ( A.hs, interpreted )
*Main>
If we now modify the source of ``D`` (or pretend to: using the Unix command
``touch`` on the source file is handy for this), the compiler will no
longer be able to use the object file, because it might be out of date:
::
*Main> :! touch D.hs
*Main> :reload
Compiling D ( D.hs, interpreted )
Ok, modules loaded: A, B, C, D.
*Main>
Note that module ``D`` was compiled, but in this instance because its source
hadn't really changed, its interface remained the same, and the
recompilation checker determined that ``A``, ``B`` and ``C`` didn't need to be
recompiled.
So let's try compiling one of the other modules:
::
*Main> :! ghc -c C.hs
*Main> :load A
Compiling D ( D.hs, interpreted )
Compiling B ( B.hs, interpreted )
Compiling C ( C.hs, interpreted )
Compiling A ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.
We didn't get the compiled version of ``C``! What happened? Well, in GHCi a
compiled module may only depend on other compiled modules, and in this
case ``C`` depends on ``D``, which doesn't have an object file, so GHCi also
rejected ``C``\'s object file. Ok, so let's also compile ``D``:
::
*Main> :! ghc -c D.hs
*Main> :reload
Ok, modules loaded: A, B, C, D.
Nothing happened! Here's another lesson: newly compiled modules aren't
picked up by ``:reload``, only ``:load``:
::
*Main> :load A
Compiling B ( B.hs, interpreted )
Compiling A ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.
The automatic loading of object files can sometimes lead to confusion,
because non-exported top-level definitions of a module are only
available for use in expressions at the prompt when the module is
interpreted (see :ref:`ghci-scope`). For this reason, you might
sometimes want to force GHCi to load a module using the interpreter.
This can be done by prefixing a ``*`` to the module name or filename
when using ``:load``, for example
::
Prelude> :load *A
Compiling A ( A.hs, interpreted )
*A>
When the ``*`` is used, GHCi ignores any pre-compiled object code and
interprets the module. If you have already loaded a number of modules as
object code and decide that you wanted to interpret one of them, instead
of re-loading the whole set you can use ``:add *M`` to specify that you
want ``M`` to be interpreted (note that this might cause other modules
to be interpreted too, because compiled modules cannot depend on
interpreted ones).
To always compile everything to object code and never use the
interpreter, use the ``-fobject-code`` option (see :ref:`ghci-obj`).
.. hint::
Since GHCi will only use a compiled object file if it can be sure
that the compiled version is up-to-date, a good technique when working
on a large program is to occasionally run ``ghc --make`` to compile the
whole project (say before you go for lunch :-), then continue working in
the interpreter. As you modify code, the changed modules will be
interpreted, but the rest of the project will remain compiled.
.. _interactive-evaluation:
Interactive evaluation at the prompt
------------------------------------
When you type an expression at the prompt, GHCi immediately evaluates
and prints the result:
::
Prelude> reverse "hello"
"olleh"
Prelude> 5+5
10
.. _actions-at-prompt:
I/O actions at the prompt
~~~~~~~~~~~~~~~~~~~~~~~~~
GHCi does more than simple expression evaluation at the prompt. If you
enter an expression of type ``IO a`` for some ``a``, then GHCi
*executes* it as an IO-computation.
::
Prelude> "hello"
"hello"
Prelude> putStrLn "hello"
hello
This works even if the type of the expression is more general, provided
it can be *instantiated* to ``IO a``. For example
::
Prelude> return True
True
Furthermore, GHCi will print the result of the I/O action if (and only
if):
- The result type is an instance of ``Show``.
- The result type is not ``()``.
For example, remembering that ``putStrLn :: String -> IO ()``:
::
Prelude> putStrLn "hello"
hello
Prelude> do { putStrLn "hello"; return "yes" }
hello
"yes"
.. _ghci-stmts:
Using ``do-``\ notation at the prompt
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. index::
single: do-notation; in GHCi
single: statements; in GHCi
GHCi actually accepts statements rather than just expressions at the
prompt. This means you can bind values and functions to names, and use
them in future expressions or statements.
The syntax of a statement accepted at the GHCi prompt is exactly the
same as the syntax of a statement in a Haskell ``do`` expression.
However, there's no monad overloading here: statements typed at the
prompt must be in the ``IO`` monad.
::
Prelude> x <- return 42
Prelude> print x
42
Prelude>
The statement ``x <- return 42`` means “execute ``return 42`` in the
``IO`` monad, and bind the result to ``x``\ ”. We can then use ``x`` in
future statements, for example to print it as we did above.
If ``-fprint-bind-result`` is set then GHCi will print the result of a
statement if and only if:
- The statement is not a binding, or it is a monadic binding
(``p <- e``) that binds exactly one variable.
- The variable's type is not polymorphic, is not ``()``, and is an
instance of ``Show``.
.. index::
single: -fprint-bind-result
Of course, you can also bind normal non-IO expressions using the
``let``\-statement:
::
Prelude> let x = 42
Prelude> x
42
Prelude>
Another important difference between the two types of binding is that
the monadic bind (``p <- e``) is *strict* (it evaluates ``e``), whereas
with the ``let`` form, the expression isn't evaluated immediately:
::
Prelude> let x = error "help!"
Prelude> print x
*** Exception: help!
Prelude>
Note that ``let`` bindings do not automatically print the value bound,
unlike monadic bindings.
You can also define functions at the prompt:
::
Prelude> add a b = a + b
Prelude> add 1 2
3
Prelude>
However, this quickly gets tedious when defining functions with multiple
clauses, or groups of mutually recursive functions, because the complete
definition has to be given on a single line, using explicit semicolons
instead of layout:
::
Prelude> f op n [] = n ; f op n (h:t) = h `op` f op n t
Prelude> f (+) 0 [1..3]
6
Prelude>
To alleviate this issue, GHCi commands can be split over multiple lines,
by wrapping them in ``:{`` and ``:}`` (each on a single line of its
own):
::
Prelude> :{
Prelude| g op n [] = n
Prelude| g op n (h:t) = h `op` g op n t
Prelude| :}
Prelude> g (*) 1 [1..3]
6
Such multiline commands can be used with any GHCi command, and note that
the layout rule is in effect. The main purpose of multiline commands is
not to replace module loading but to make definitions in .ghci-files
(see :ref:`ghci-dot-files`) more readable and maintainable.
Any exceptions raised during the evaluation or execution of the
statement are caught and printed by the GHCi command line interface (for
more information on exceptions, see the module ``Control.Exception`` in
the libraries documentation).
Every new binding shadows any existing bindings of the same name,
including entities that are in scope in the current module context.
.. warning::
Temporary bindings introduced at the prompt only last until the
next ``:load`` or ``:reload`` command, at which time they will be simply
lost. However, they do survive a change of context with ``:module``: the
temporary bindings just move to the new location.
.. hint::
To get a list of the bindings currently in scope, use the
``:show bindings`` command:
::
Prelude> :show bindings
x :: Int
Prelude>
.. hint::
If you turn on the ``+t`` option, GHCi will show the type of each
variable bound by a statement. For example:
::
Prelude> :set +t
Prelude> let (x:xs) = [1..]
x :: Integer
xs :: [Integer]
.. index::
single: +t option; in GHCi
.. _ghci-multiline:
Multiline input
~~~~~~~~~~~~~~~
Apart from the ``:{ ... :}`` syntax for multi-line input mentioned
above, GHCi also has a multiline mode, enabled by ``:set +m``,
``:set +m`` in which GHCi detects automatically when the current
statement is unfinished and allows further lines to be added. A
multi-line input is terminated with an empty line. For example:
::
Prelude> :set +m
Prelude> let x = 42
Prelude|
Further bindings can be added to this ``let`` statement, so GHCi
indicates that the next line continues the previous one by changing the
prompt. Note that layout is in effect, so to add more bindings to this
``let`` we have to line them up:
::
Prelude> :set +m
Prelude> let x = 42
Prelude| y = 3
Prelude|
Prelude>
Explicit braces and semicolons can be used instead of layout:
::
Prelude> do {
Prelude| putStrLn "hello"
Prelude| ;putStrLn "world"
Prelude| }
hello
world
Prelude>
Note that after the closing brace, GHCi knows that the current statement
is finished, so no empty line is required.
Multiline mode is useful when entering monadic ``do`` statements:
::
Control.Monad.State> flip evalStateT 0 $ do
Control.Monad.State| i <- get
Control.Monad.State| lift $ do
Control.Monad.State| putStrLn "Hello World!"
Control.Monad.State| print i
Control.Monad.State|
"Hello World!"
0
Control.Monad.State>
During a multiline interaction, the user can interrupt and return to the
top-level prompt.
::
Prelude> do
Prelude| putStrLn "Hello, World!"
Prelude| ^C
Prelude>
.. _ghci-decls:
Type, class and other declarations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
At the GHCi prompt you can also enter any top-level Haskell declaration,
including ``data``, ``type``, ``newtype``, ``class``, ``instance``,
``deriving``, and ``foreign`` declarations. For example:
::
Prelude> data T = A | B | C deriving (Eq, Ord, Show, Enum)
Prelude> [A ..]
[A,B,C]
Prelude> :i T
data T = A | B | C -- Defined at :2:6
instance Enum T -- Defined at :2:45
instance Eq T -- Defined at :2:30
instance Ord T -- Defined at :2:34
instance Show T -- Defined at :2:39
As with ordinary variable bindings, later definitions shadow earlier
ones, so you can re-enter a declaration to fix a problem with it or
extend it. But there's a gotcha: when a new type declaration shadows an
older one, there might be other declarations that refer to the old type.
The thing to remember is that the old type still exists, and these other
declarations still refer to the old type. However, while the old and the
new type have the same name, GHCi will treat them as distinct. For
example:
::
Prelude> data T = A | B
Prelude> let f A = True; f B = False
Prelude> data T = A | B | C
Prelude> f A
:2:3:
Couldn't match expected type `main::Interactive.T'
with actual type `T'
In the first argument of `f', namely `A'
In the expression: f A
In an equation for `it': it = f A
Prelude>
The old, shadowed, version of ``T`` is displayed as
``main::Interactive.T`` by GHCi in an attempt to distinguish it from the
new ``T``, which is displayed as simply ``T``.
Class and type-family instance declarations are simply added to the list
of available instances, with one exception. Since you might want to
re-define one, a class or type-family instance *replaces* any earlier
instance with an identical head or left hand side (respectively). (See
:ref:`type-families`.)
.. _ghci-scope:
What's really in scope at the prompt?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When you type an expression at the prompt, what identifiers and types
are in scope? GHCi provides a flexible way to control exactly how the
context for an expression is constructed:
- The ``:load``, ``:add``, and ``:reload`` commands
(:ref:`ghci-load-scope`).
- The ``import`` declaration (:ref:`ghci-import-decl`).
- The ``:module`` command (:ref:`ghci-module-cmd`).
The command ``:show imports`` will show a summary of which modules
contribute to the top-level scope.
.. hint::
GHCi will tab-complete names that are in scope; for example, if
you run GHCi and type ``J`` then GHCi will expand it to
“\ ``Just``\ ”.
.. _ghci-load-scope:
The effect of ``:load`` on what is in scope
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The ``:load``, ``:add``, and ``:reload`` commands
(:ref:`loading-source-files` and :ref:`ghci-compiled`) affect the
top-level scope. Let's start with the simple cases; when you start GHCi
the prompt looks like this:
::
Prelude>
which indicates that everything from the module ``Prelude`` is currently
in scope; the visible identifiers are exactly those that would be
visible in a Haskell source file with no ``import`` declarations.
If we now load a file into GHCi, the prompt will change:
::
Prelude> :load Main.hs
Compiling Main ( Main.hs, interpreted )
*Main>
The new prompt is ``*Main``, which indicates that we are typing
expressions in the context of the top-level of the ``Main`` module.
Everything that is in scope at the top-level in the module ``Main`` we
just loaded is also in scope at the prompt (probably including
``Prelude``, as long as ``Main`` doesn't explicitly hide it).
The syntax in the prompt ``*module`` indicates that it is the full
top-level scope of ⟨module⟩ that is contributing to the scope for
expressions typed at the prompt. Without the ``*``, just the exports of
the module are visible.
.. note::
For technical reasons, GHCi can only support the ``*``-form for
modules that are interpreted. Compiled modules and package modules can
only contribute their exports to the current scope. To ensure that GHCi
loads the interpreted version of a module, add the ``*`` when loading
the module, e.g. ``:load *M``.
In general, after a ``:load`` command, an automatic import is added to
the scope for the most recently loaded "target" module, in a ``*``-form
if possible. For example, if you say ``:load foo.hs bar.hs`` and
``bar.hs`` contains module ``Bar``, then the scope will be set to
``*Bar`` if ``Bar`` is interpreted, or if ``Bar`` is compiled it will be
set to ``Prelude Bar`` (GHCi automatically adds ``Prelude`` if it isn't
present and there aren't any ``*``-form modules). These
automatically-added imports can be seen with ``:show imports``:
::
Prelude> :load hello.hs
[1 of 1] Compiling Main ( hello.hs, interpreted )
Ok, modules loaded: Main.
*Main> :show imports
:module +*Main -- added automatically
*Main>
and the automatically-added import is replaced the next time you use
``:load``, ``:add``, or ``:reload``. It can also be removed by
``:module`` as with normal imports.
.. _ghci-import-decl:
Controlling what is in scope with ``import``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
We are not limited to a single module: GHCi can combine scopes from
multiple modules, in any mixture of ``*`` and non-\ ``*`` forms. GHCi
combines the scopes from all of these modules to form the scope that is
in effect at the prompt.
To add modules to the scope, use ordinary Haskell ``import`` syntax:
::
Prelude> import System.IO
Prelude System.IO> hPutStrLn stdout "hello\n"
hello
Prelude System.IO>
The full Haskell import syntax is supported, including ``hiding`` and
``as`` clauses. The prompt shows the modules that are currently
imported, but it omits details about ``hiding``, ``as``, and so on. To
see the full story, use ``:show imports``:
::
Prelude> import System.IO
Prelude System.IO> import Data.Map as Map
Prelude System.IO Map> :show imports
import Prelude -- implicit
import System.IO
import Data.Map as Map
Prelude System.IO Map>
Note that the ``Prelude`` import is marked as implicit. It can be
overridden with an explicit ``Prelude`` import, just like in a Haskell
module.
With multiple modules in scope, especially multiple ``*``-form modules,
it is likely that name clashes will occur. Haskell specifies that name
clashes are only reported when an ambiguous identifier is used, and GHCi
behaves in the same way for expressions typed at the prompt.
.. _ghci-module-cmd:
Controlling what is in scope with the ``:module`` command
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Another way to manipulate the scope is to use the ``:module`` command,
whose syntax is this:
::
:module +|- *mod1 ... *modn
Using the ``+`` form of the ``module`` commands adds modules to the
current scope, and ``-`` removes them. Without either ``+`` or ``-``,
the current scope is replaced by the set of modules specified. Note that
if you use this form and leave out ``Prelude``, an implicit ``Prelude``
import will be added automatically.
The ``:module`` command provides a way to do two things that cannot be
done with ordinary ``import`` declarations:
- ``:module`` supports the ``*`` modifier on modules, which opens the
full top-level scope of a module, rather than just its exports.
- Imports can be *removed* from the context, using the syntax
``:module -M``. The ``import`` syntax is cumulative (as in a Haskell
module), so this is the only way to subtract from the scope.
.. _ghci-import-qualified:
Qualified names
^^^^^^^^^^^^^^^
To make life slightly easier, the GHCi prompt also behaves as if there
is an implicit ``import qualified`` declaration for every module in
every package, and every module currently loaded into GHCi. This
behaviour can be disabled with the ``-fno-implicit-import-qualified`` flag.
.. index::
single: -fno-implicit-import-qualified
``:module`` and ``:load``
^^^^^^^^^^^^^^^^^^^^^^^^^
It might seem that ``:module``/``import`` and
``:load``/``:add``/``:reload`` do similar things: you can use both to
bring a module into scope. However, there is a very important
difference. GHCi is concerned with two sets of modules:
- The set of modules that are currently *loaded*. This set is modified
by ``:load``, ``:add`` and ``:reload``, and can be shown with
``:show modules``.
- The set of modules that are currently *in scope* at the prompt. This
set is modified by ``import`` and ``:module``, and it is also
modified automatically after ``:load``, ``:add``, and ``:reload``, as
described above. The set of modules in scope can be shown with
``:show imports``.
You can add a module to the scope (via ``:module`` or ``import``) only
if either (a) it is loaded, or (b) it is a module from a package that
GHCi knows about. Using ``:module`` or ``import`` to try bring into
scope a non-loaded module may result in the message
“\ ``module M is not loaded``\ ”.
The ``:main`` and ``:run`` commands
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When a program is compiled and executed, it can use the ``getArgs``
function to access the command-line arguments. However, we cannot simply
pass the arguments to the ``main`` function while we are testing in
ghci, as the ``main`` function doesn't take its directly.
Instead, we can use the ``:main`` command. This runs whatever ``main``
is in scope, with any arguments being treated the same as command-line
arguments, e.g.:
::
Prelude> main = System.Environment.getArgs >>= print
Prelude> :main foo bar
["foo","bar"]
We can also quote arguments which contains characters like spaces, and
they are treated like Haskell strings, or we can just use Haskell list
syntax:
::
Prelude> :main foo "bar baz"
["foo","bar baz"]
Prelude> :main ["foo", "bar baz"]
["foo","bar baz"]
Finally, other functions can be called, either with the ``-main-is``
flag or the ``:run`` command:
::
Prelude> foo = putStrLn "foo" >> System.Environment.getArgs >>= print
Prelude> bar = putStrLn "bar" >> System.Environment.getArgs >>= print
Prelude> :set -main-is foo
Prelude> :main foo "bar baz"
foo
["foo","bar baz"]
Prelude> :run bar ["foo", "bar baz"]
bar
["foo","bar baz"]
The ``it`` variable
~~~~~~~~~~~~~~~~~~~
.. index::
single: it variable
Whenever an expression (or a non-binding statement, to be precise) is
typed at the prompt, GHCi implicitly binds its value to the variable
``it``. For example:
::
Prelude> 1+2
3
Prelude> it * 2
6
What actually happens is that GHCi typechecks the expression, and if it
doesn't have an ``IO`` type, then it transforms it as follows: an
expression ``e`` turns into
::
let it = e;
print it
which is then run as an IO-action.
Hence, the original expression must have a type which is an instance of
the ``Show`` class, or GHCi will complain:
::
Prelude> id
:1:0:
No instance for (Show (a -> a))
arising from use of `print' at :1:0-1
Possible fix: add an instance declaration for (Show (a -> a))
In the expression: print it
In a 'do' expression: print it
The error message contains some clues as to the transformation happening
internally.
If the expression was instead of type ``IO a`` for some ``a``, then
``it`` will be bound to the result of the ``IO`` computation, which is
of type ``a``. eg.:
::
Prelude> Time.getClockTime
Wed Mar 14 12:23:13 GMT 2001
Prelude> print it
Wed Mar 14 12:23:13 GMT 2001
The corresponding translation for an IO-typed ``e`` is
::
it <- e
Note that ``it`` is shadowed by the new value each time you evaluate a
new expression, and the old value of ``it`` is lost.
.. _extended-default-rules:
Type defaulting in GHCi
~~~~~~~~~~~~~~~~~~~~~~~
.. index::
single: Type defaulting; in GHCi
single: Show class
Consider this GHCi session:
::
ghci> reverse []
What should GHCi do? Strictly speaking, the program is ambiguous.
``show (reverse [])`` (which is what GHCi computes here) has type
``Show a => String`` and how that displays depends on the type ``a``.
For example:
::
ghci> reverse ([] :: String)
""
ghci> reverse ([] :: [Int])
[]
However, it is tiresome for the user to have to specify the type, so
GHCi extends Haskell's type-defaulting rules (Section 4.3.4 of the
Haskell 2010 Report) as follows. The standard rules take each group of
constraints ``(C1 a, C2 a, ..., Cn a)`` for each type variable ``a``,
and defaults the type variable if
1. The type variable ``a`` appears in no other constraints
2. All the classes ``Ci`` are standard.
3. At least one of the classes ``Ci`` is numeric.
At the GHCi prompt, or with GHC if the ``-XExtendedDefaultRules`` flag
is given, the following additional differences apply:
- Rule 2 above is relaxed thus: *All* of the classes ``Ci`` are
single-parameter type classes.
- Rule 3 above is relaxed this: At least one of the classes ``Ci`` is
numeric, or is ``Show``, ``Eq``, ``Ord``, ``Foldable`` or ``Traversable``.
- The unit type ``()`` and the list type ``[]`` are added to the start of
the standard list of types which are tried when doing type defaulting.
The last point means that, for example, this program:
::
main :: IO ()
main = print def
instance Num ()
def :: (Num a, Enum a) => a
def = toEnum 0
prints ``()`` rather than ``0`` as the type is defaulted to ``()``
rather than ``Integer``.
The motivation for the change is that it means ``IO a`` actions default
to ``IO ()``, which in turn means that ghci won't try to print a result
when running them. This is particularly important for ``printf``, which
has an instance that returns ``IO a``. However, it is only able to
return ``undefined`` (the reason for the instance having this type is so
that printf doesn't require extensions to the class system), so if the
type defaults to ``Integer`` then ghci gives an error when running a
printf.
See also :ref:`actions-at-prompt` for how the monad of a computational
expression defaults to ``IO`` if possible.
.. _ghci-interactive-print:
Using a custom interactive printing function
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. index::
single: Custom printing function; in GHCi
[**New in version 7.6.1**] By default, GHCi prints the result of
expressions typed at the prompt using the function ``System.IO.print``.
Its type signature is ``Show a => a -> IO ()``, and it works by
converting the value to ``String`` using ``show``.
This is not ideal in certain cases, like when the output is long, or
contains strings with non-ascii characters.
The ``-interactive-print`` flag allows to specify any function of type
``C a => a -> IO ()``, for some constraint ``C``, as the function for
printing evaluated expressions. The function can reside in any loaded
module or any registered package.
As an example, suppose we have following special printing module:
::
module SpecPrinter where
import System.IO
sprint a = putStrLn $ show a ++ "!"
The ``sprint`` function adds an exclamation mark at the end of any
printed value. Running GHCi with the command:
::
ghci -interactive-print=SpecPrinter.sprinter SpecPrinter
will start an interactive session where values with be printed using
``sprint``:
::
*SpecPrinter> [1,2,3]
[1,2,3]!
*SpecPrinter> 42
42!
A custom pretty printing function can be used, for example, to format
tree-like and nested structures in a more readable way.
The ``-interactive-print`` flag can also be used when running GHC in
``-e mode``:
::
% ghc -e "[1,2,3]" -interactive-print=SpecPrinter.sprint SpecPrinter
[1,2,3]!
.. _ghci-debugger:
The GHCi Debugger
-----------------
.. index::
single: debugger; in GHCi
GHCi contains a simple imperative-style debugger in which you can stop a
running computation in order to examine the values of variables. The
debugger is integrated into GHCi, and is turned on by default: no flags
are required to enable the debugging facilities. There is one major
restriction: breakpoints and single-stepping are only available in
interpreted modules; compiled code is invisible to the debugger [5]_.
The debugger provides the following:
- The ability to set a breakpoint on a function definition or
expression in the program. When the function is called, or the
expression evaluated, GHCi suspends execution and returns to the
prompt, where you can inspect the values of local variables before
continuing with the execution.
- Execution can be single-stepped: the evaluator will suspend execution
approximately after every reduction, allowing local variables to be
inspected. This is equivalent to setting a breakpoint at every point
in the program.
- Execution can take place in tracing mode, in which the evaluator
remembers each evaluation step as it happens, but doesn't suspend
execution until an actual breakpoint is reached. When this happens,
the history of evaluation steps can be inspected.
- Exceptions (e.g. pattern matching failure and ``error``) can be
treated as breakpoints, to help locate the source of an exception in
the program.
There is currently no support for obtaining a “stack trace”, but the
tracing and history features provide a useful second-best, which will
often be enough to establish the context of an error. For instance, it
is possible to break automatically when an exception is thrown, even if
it is thrown from within compiled code (see
:ref:`ghci-debugger-exceptions`).
.. _breakpoints:
Breakpoints and inspecting variables
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Let's use quicksort as a running example. Here's the code:
::
qsort [] = []
qsort (a:as) = qsort left ++ [a] ++ qsort right
where (left,right) = (filter (<=a) as, filter (>a) as)
main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
First, load the module into GHCi:
::
Prelude> :l qsort.hs
[1 of 1] Compiling Main ( qsort.hs, interpreted )
Ok, modules loaded: Main.
*Main>
Now, let's set a breakpoint on the right-hand-side of the second
equation of qsort:
::
*Main> :break 2
Breakpoint 0 activated at qsort.hs:2:15-46
*Main>
The command ``:break 2`` sets a breakpoint on line 2 of the most
recently-loaded module, in this case ``qsort.hs``. Specifically, it
picks the leftmost complete subexpression on that line on which to set
the breakpoint, which in this case is the expression
``(qsort left ++ [a] ++ qsort right)``.
Now, we run the program:
::
*Main> main
Stopped at qsort.hs:2:15-46
_result :: [a]
a :: a
left :: [a]
right :: [a]
[qsort.hs:2:15-46] *Main>
Execution has stopped at the breakpoint. The prompt has changed to
indicate that we are currently stopped at a breakpoint, and the
location: ``[qsort.hs:2:15-46]``. To further clarify the location, we
can use the ``:list`` command:
::
[qsort.hs:2:15-46] *Main> :list
1 qsort [] = []
2 qsort (a:as) = qsort left ++ [a] ++ qsort right
3 where (left,right) = (filter (<=a) as, filter (>a) as)
The ``:list`` command lists the source code around the current
breakpoint. If your output device supports it, then GHCi will highlight
the active subexpression in bold.
GHCi has provided bindings for the free variables [6]_ of the expression
on which the breakpoint was placed (``a``, ``left``, ``right``), and
additionally a binding for the result of the expression (``_result``).
These variables are just like other variables that you might define in
GHCi; you can use them in expressions that you type at the prompt, you
can ask for their types with ``:type``, and so on. There is one
important difference though: these variables may only have partial
types. For example, if we try to display the value of ``left``:
::
[qsort.hs:2:15-46] *Main> left
:1:0:
Ambiguous type variable `a' in the constraint:
`Show a' arising from a use of `print' at :1:0-3
Cannot resolve unknown runtime types: a
Use :print or :force to determine these types
This is because ``qsort`` is a polymorphic function, and because GHCi
does not carry type information at runtime, it cannot determine the
runtime types of free variables that involve type variables. Hence, when
you ask to display ``left`` at the prompt, GHCi can't figure out which
instance of ``Show`` to use, so it emits the type error above.
Fortunately, the debugger includes a generic printing command,
``:print``, which can inspect the actual runtime value of a variable and
attempt to reconstruct its type. If we try it on ``left``:
::
[qsort.hs:2:15-46] *Main> :set -fprint-evld-with-show
[qsort.hs:2:15-46] *Main> :print left
left = (_t1::[a])
This isn't particularly enlightening. What happened is that ``left`` is
bound to an unevaluated computation (a suspension, or thunk), and
``:print`` does not force any evaluation. The idea is that ``:print``
can be used to inspect values at a breakpoint without any unfortunate
side effects. It won't force any evaluation, which could cause the
program to give a different answer than it would normally, and hence it
won't cause any exceptions to be raised, infinite loops, or further
breakpoints to be triggered (see :ref:`nested-breakpoints`). Rather than
forcing thunks, ``:print`` binds each thunk to a fresh variable
beginning with an underscore, in this case ``_t1``.
The flag ``-fprint-evld-with-show`` instructs ``:print`` to reuse
available ``Show`` instances when possible. This happens only when the
contents of the variable being inspected are completely evaluated.
If we aren't concerned about preserving the evaluatedness of a variable,
we can use ``:force`` instead of ``:print``. The ``:force`` command
behaves exactly like ``:print``, except that it forces the evaluation of
any thunks it encounters:
::
[qsort.hs:2:15-46] *Main> :force left
left = [4,0,3,1]
Now, since ``:force`` has inspected the runtime value of ``left``, it
has reconstructed its type. We can see the results of this type
reconstruction:
::
[qsort.hs:2:15-46] *Main> :show bindings
_result :: [Integer]
a :: Integer
left :: [Integer]
right :: [Integer]
_t1 :: [Integer]
Not only do we now know the type of ``left``, but all the other partial
types have also been resolved. So we can ask for the value of ``a``, for
example:
::
[qsort.hs:2:15-46] *Main> a
8
You might find it useful to use Haskell's ``seq`` function to evaluate
individual thunks rather than evaluating the whole expression with
``:force``. For example:
::
[qsort.hs:2:15-46] *Main> :print right
right = (_t1::[Integer])
[qsort.hs:2:15-46] *Main> seq _t1 ()
()
[qsort.hs:2:15-46] *Main> :print right
right = 23 : (_t2::[Integer])
We evaluated only the ``_t1`` thunk, revealing the head of the list, and
the tail is another thunk now bound to ``_t2``. The ``seq`` function is
a little inconvenient to use here, so you might want to use ``:def`` to
make a nicer interface (left as an exercise for the reader!).
Finally, we can continue the current execution:
::
[qsort.hs:2:15-46] *Main> :continue
Stopped at qsort.hs:2:15-46
_result :: [a]
a :: a
left :: [a]
right :: [a]
[qsort.hs:2:15-46] *Main>
The execution continued at the point it previously stopped, and has now
stopped at the breakpoint for a second time.
.. _setting-breakpoints:
Setting breakpoints
^^^^^^^^^^^^^^^^^^^
Breakpoints can be set in various ways. Perhaps the easiest way to set a
breakpoint is to name a top-level function:
::
:break identifier
Where ⟨identifier⟩ names any top-level function in an interpreted module
currently loaded into GHCi (qualified names may be used). The breakpoint
will be set on the body of the function, when it is fully applied but
before any pattern matching has taken place.
Breakpoints can also be set by line (and optionally column) number:
::
:break line
:break line column
:break module line
:break module line column
When a breakpoint is set on a particular line, GHCi sets the breakpoint
on the leftmost subexpression that begins and ends on that line. If two
complete subexpressions start at the same column, the longest one is
picked. If there is no complete subexpression on the line, then the
leftmost expression starting on the line is picked, and failing that the
rightmost expression that partially or completely covers the line.
When a breakpoint is set on a particular line and column, GHCi picks the
smallest subexpression that encloses that location on which to set the
breakpoint. Note: GHC considers the TAB character to have a width of 1,
wherever it occurs; in other words it counts characters, rather than
columns. This matches what some editors do, and doesn't match others.
The best advice is to avoid tab characters in your source code
altogether (see ``-fwarn-tabs`` in :ref:`options-sanity`).
If the module is omitted, then the most recently-loaded module is used.
Not all subexpressions are potential breakpoint locations. Single
variables are typically not considered to be breakpoint locations
(unless the variable is the right-hand-side of a function definition,
lambda, or case alternative). The rule of thumb is that all redexes are
breakpoint locations, together with the bodies of functions, lambdas,
case alternatives and binding statements. There is normally no
breakpoint on a let expression, but there will always be a breakpoint on
its body, because we are usually interested in inspecting the values of
the variables bound by the let.
Listing and deleting breakpoints
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The list of breakpoints currently enabled can be displayed using
``:show breaks``:
::
*Main> :show breaks
[0] Main qsort.hs:1:11-12
[1] Main qsort.hs:2:15-46
To delete a breakpoint, use the ``:delete`` command with the number
given in the output from ``:show breaks``:
::
*Main> :delete 0
*Main> :show breaks
[1] Main qsort.hs:2:15-46
To delete all breakpoints at once, use ``:delete *``.
.. _single-stepping:
Single-stepping
~~~~~~~~~~~~~~~
Single-stepping is a great way to visualise the execution of your
program, and it is also a useful tool for identifying the source of a
bug. GHCi offers two variants of stepping. Use ``:step`` to enable all
the breakpoints in the program, and execute until the next breakpoint is
reached. Use ``:steplocal`` to limit the set of enabled breakpoints to
those in the current top level function. Similarly, use ``:stepmodule``
to single step only on breakpoints contained in the current module. For
example:
::
*Main> :step main
Stopped at qsort.hs:5:7-47
_result :: IO ()
The command ``:step expr`` begins the evaluation of ⟨expr⟩ in
single-stepping mode. If ⟨expr⟩ is omitted, then it single-steps from
the current breakpoint. ``:steplocal`` and ``:stepmodule`` work
similarly.
The ``:list`` command is particularly useful when single-stepping, to
see where you currently are:
::
[qsort.hs:5:7-47] *Main> :list
4
5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
6
[qsort.hs:5:7-47] *Main>
In fact, GHCi provides a way to run a command when a breakpoint is hit,
so we can make it automatically do ``:list``:
::
[qsort.hs:5:7-47] *Main> :set stop :list
[qsort.hs:5:7-47] *Main> :step
Stopped at qsort.hs:5:14-46
_result :: [Integer]
4
5 main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
6
[qsort.hs:5:14-46] *Main>
.. _nested-breakpoints:
Nested breakpoints
~~~~~~~~~~~~~~~~~~
When GHCi is stopped at a breakpoint, and an expression entered at the
prompt triggers a second breakpoint, the new breakpoint becomes the
"current" one, and the old one is saved on a stack. An arbitrary number
of breakpoint contexts can be built up in this way. For example:
::
[qsort.hs:2:15-46] *Main> :st qsort [1,3]
Stopped at qsort.hs:(1,0)-(3,55)
_result :: [a]
... [qsort.hs:(1,0)-(3,55)] *Main>
While stopped at the breakpoint on line 2 that we set earlier, we
started a new evaluation with ``:step qsort [1,3]``. This new evaluation
stopped after one step (at the definition of ``qsort``). The prompt has
changed, now prefixed with ``...``, to indicate that there are saved
breakpoints beyond the current one. To see the stack of contexts, use
``:show context``:
::
... [qsort.hs:(1,0)-(3,55)] *Main> :show context
--> main
Stopped at qsort.hs:2:15-46
--> qsort [1,3]
Stopped at qsort.hs:(1,0)-(3,55)
... [qsort.hs:(1,0)-(3,55)] *Main>
To abandon the current evaluation, use ``:abandon``:
::
... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
[qsort.hs:2:15-46] *Main> :abandon
*Main>
.. _ghci-debugger-result:
The ``_result`` variable
~~~~~~~~~~~~~~~~~~~~~~~~
When stopped at a breakpoint or single-step, GHCi binds the variable
``_result`` to the value of the currently active expression. The value
of ``_result`` is presumably not available yet, because we stopped its
evaluation, but it can be forced: if the type is known and showable,
then just entering ``_result`` at the prompt will show it. However,
there's one caveat to doing this: evaluating ``_result`` will be likely
to trigger further breakpoints, starting with the breakpoint we are
currently stopped at (if we stopped at a real breakpoint, rather than
due to ``:step``). So it will probably be necessary to issue a
``:continue`` immediately when evaluating ``_result``. Alternatively,
you can use ``:force`` which ignores breakpoints.
.. _tracing:
Tracing and history
~~~~~~~~~~~~~~~~~~~
A question that we often want to ask when debugging a program is “how
did I get here?”. Traditional imperative debuggers usually provide some
kind of stack-tracing feature that lets you see the stack of active
function calls (sometimes called the “lexical call stack”), describing a
path through the code to the current location. Unfortunately this is
hard to provide in Haskell, because execution proceeds on a
demand-driven basis, rather than a depth-first basis as in strict
languages. The “stack“ in GHC's execution engine bears little
resemblance to the lexical call stack. Ideally GHCi would maintain a
separate lexical call stack in addition to the dynamic call stack, and
in fact this is exactly what our profiling system does
(:ref:`profiling`), and what some other Haskell debuggers do. For the
time being, however, GHCi doesn't maintain a lexical call stack (there
are some technical challenges to be overcome). Instead, we provide a way
to backtrack from a breakpoint to previous evaluation steps: essentially
this is like single-stepping backwards, and should in many cases provide
enough information to answer the "how did I get here?" question.
To use tracing, evaluate an expression with the ``:trace`` command. For
example, if we set a breakpoint on the base case of ``qsort``:
::
*Main> :list qsort
1 qsort [] = []
2 qsort (a:as) = qsort left ++ [a] ++ qsort right
3 where (left,right) = (filter (<=a) as, filter (>a) as)
4
*Main> :b 1
Breakpoint 1 activated at qsort.hs:1:11-12
*Main>
and then run a small ``qsort`` with tracing:
::
*Main> :trace qsort [3,2,1]
Stopped at qsort.hs:1:11-12
_result :: [a]
[qsort.hs:1:11-12] *Main>
We can now inspect the history of evaluation steps:
::
[qsort.hs:1:11-12] *Main> :hist
-1 : qsort.hs:3:24-38
-2 : qsort.hs:3:23-55
-3 : qsort.hs:(1,0)-(3,55)
-4 : qsort.hs:2:15-24
-5 : qsort.hs:2:15-46
-6 : qsort.hs:3:24-38
-7 : qsort.hs:3:23-55
-8 : qsort.hs:(1,0)-(3,55)
-9 : qsort.hs:2:15-24
-10 : qsort.hs:2:15-46
-11 : qsort.hs:3:24-38
-12 : qsort.hs:3:23-55
-13 : qsort.hs:(1,0)-(3,55)
-14 : qsort.hs:2:15-24
-15 : qsort.hs:2:15-46
-16 : qsort.hs:(1,0)-(3,55)
To examine one of the steps in the history, use ``:back``:
::
[qsort.hs:1:11-12] *Main> :back
Logged breakpoint at qsort.hs:3:24-38
_result :: [a]
as :: [a]
a :: a
[-1: qsort.hs:3:24-38] *Main>
Note that the local variables at each step in the history have been
preserved, and can be examined as usual. Also note that the prompt has
changed to indicate that we're currently examining the first step in the
history: ``-1``. The command ``:forward`` can be used to traverse
forward in the history.
The ``:trace`` command can be used with or without an expression. When
used without an expression, tracing begins from the current breakpoint,
just like ``:step``.
The history is only available when using ``:trace``; the reason for this
is we found that logging each breakpoint in the history cuts performance
by a factor of 2 or more. By default, GHCi remembers the last 50 steps
in the history, but this can be changed with the ``-fghci-hist-size=n`` option).
.. index::
single: -fghci-hist-size
.. _ghci-debugger-exceptions:
Debugging exceptions
~~~~~~~~~~~~~~~~~~~~
Another common question that comes up when debugging is "where did this
exception come from?". Exceptions such as those raised by ``error`` or
``head []`` have no context information attached to them. Finding which
particular call to ``head`` in your program resulted in the error can be
a painstaking process, usually involving ``Debug.Trace.trace``, or
compiling with profiling and using ``Debug.Trace.traceStack`` or
``+RTS -xc`` (see :ref:`prof-time-options`).
The GHCi debugger offers a way to hopefully shed some light on these
errors quickly and without modifying or recompiling the source code. One
way would be to set a breakpoint on the location in the source code that
throws the exception, and then use ``:trace`` and ``:history`` to
establish the context. However, ``head`` is in a library and we can't
set a breakpoint on it directly. For this reason, GHCi provides the
flags ``-fbreak-on-exception`` which causes the evaluator to stop when
an exception is thrown, and ``-fbreak-on-error``, which works similarly
but stops only on uncaught exceptions. When stopping at an exception,
GHCi will act just as it does when a breakpoint is hit, with the
deviation that it will not show you any source code location. Due to
this, these commands are only really useful in conjunction with
``:trace``, in order to log the steps leading up to the exception. For
example:
::
*Main> :set -fbreak-on-exception
*Main> :trace qsort ("abc" ++ undefined)
“Stopped at
_exception :: e
[] *Main> :hist
-1 : qsort.hs:3:24-38
-2 : qsort.hs:3:23-55
-3 : qsort.hs:(1,0)-(3,55)
-4 : qsort.hs:2:15-24
-5 : qsort.hs:2:15-46
-6 : qsort.hs:(1,0)-(3,55)
[] *Main> :back
Logged breakpoint at qsort.hs:3:24-38
_result :: [a]
as :: [a]
a :: a
[-1: qsort.hs:3:24-38] *Main> :force as
*** Exception: Prelude.undefined
[-1: qsort.hs:3:24-38] *Main> :print as
as = 'b' : 'c' : (_t1::[Char])
The exception itself is bound to a new variable, ``_exception``.
Breaking on exceptions is particularly useful for finding out what your
program was doing when it was in an infinite loop. Just hit Control-C,
and examine the history to find out what was going on.
Example: inspecting functions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It is possible to use the debugger to examine function values. When we
are at a breakpoint and a function is in scope, the debugger cannot show
you the source code for it; however, it is possible to get some
information by applying it to some arguments and observing the result.
The process is slightly complicated when the binding is polymorphic. We
show the process by means of an example. To keep things simple, we will
use the well known ``map`` function:
::
import Prelude hiding (map)
map :: (a->b) -> [a] -> [b]
map f [] = []
map f (x:xs) = f x : map f xs
We set a breakpoint on ``map``, and call it.
::
*Main> :break 5
Breakpoint 0 activated at map.hs:5:15-28
*Main> map Just [1..5]
Stopped at map.hs:(4,0)-(5,12)
_result :: [b]
x :: a
f :: a -> b
xs :: [a]
GHCi tells us that, among other bindings, ``f`` is in scope. However,
its type is not fully known yet, and thus it is not possible to apply it
to any arguments. Nevertheless, observe that the type of its first
argument is the same as the type of ``x``, and its result type is shared
with ``_result``.
As we demonstrated earlier (:ref:`breakpoints`), the debugger has some
intelligence built-in to update the type of ``f`` whenever the types of
``x`` or ``_result`` are discovered. So what we do in this scenario is
force ``x`` a bit, in order to recover both its type and the argument
part of ``f``.
::
*Main> seq x ()
*Main> :print x
x = 1
We can check now that as expected, the type of ``x`` has been
reconstructed, and with it the type of ``f`` has been too:
::
*Main> :t x
x :: Integer
*Main> :t f
f :: Integer -> b
From here, we can apply f to any argument of type Integer and observe
the results.
::
*Main> let b = f 10
*Main> :t b
b :: b
*Main> b
:1:0:
Ambiguous type variable `b' in the constraint:
`Show b' arising from a use of `print' at :1:0
*Main> :p b
b = (_t2::a)
*Main> seq b ()
()
*Main> :t b
b :: a
*Main> :p b
b = Just 10
*Main> :t b
b :: Maybe Integer
*Main> :t f
f :: Integer -> Maybe Integer
*Main> f 20
Just 20
*Main> map f [1..5]
[Just 1, Just 2, Just 3, Just 4, Just 5]
In the first application of ``f``, we had to do some more type
reconstruction in order to recover the result type of ``f``. But after
that, we are free to use ``f`` normally.
Limitations
~~~~~~~~~~~
- When stopped at a breakpoint, if you try to evaluate a variable that
is already under evaluation, the second evaluation will hang. The
reason is that GHC knows the variable is under evaluation, so the new
evaluation just waits for the result before continuing, but of course
this isn't going to happen because the first evaluation is stopped at
a breakpoint. Control-C can interrupt the hung evaluation and return
to the prompt.
The most common way this can happen is when you're evaluating a CAF
(e.g. main), stop at a breakpoint, and ask for the value of the CAF
at the prompt again.
- Implicit parameters (see :ref:`implicit-parameters`) are only
available at the scope of a breakpoint if there is an explicit type
signature.
.. _ghci-invocation:
Invoking GHCi
-------------
.. index::
single: invoking; GHCi
single: --interactive
GHCi is invoked with the command ``ghci`` or ``ghc --interactive``. One
or more modules or filenames can also be specified on the command line;
this instructs GHCi to load the specified modules or filenames (and all
the modules they depend on), just as if you had said ``:load modules``
at the GHCi prompt (see :ref:`ghci-commands`). For example, to start
GHCi and load the program whose topmost module is in the file
``Main.hs``, we could say:
::
$ ghci Main.hs
Most of the command-line options accepted by GHC (see :ref:`using-ghc`)
also make sense in interactive mode. The ones that don't make sense are
mostly obvious.
Packages
~~~~~~~~
.. index::
single: packages; with GHCi
Most packages (see :ref:`using-packages`) are available without needing
to specify any extra flags at all: they will be automatically loaded the
first time they are needed.
For hidden packages, however, you need to request the package be loaded
by using the ``-package`` flag:
::
$ ghci -package readline
GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help
Loading package base ... linking ... done.
Loading package readline-1.0 ... linking ... done.
Prelude>
The following command works to load new packages into a running GHCi:
::
Prelude> :set -package name
But note that doing this will cause all currently loaded modules to be
unloaded, and you'll be dumped back into the ``Prelude``.
Extra libraries
~~~~~~~~~~~~~~~
.. index::
single: libraries; with GHCi
Extra libraries may be specified on the command line using the normal
``-llib`` option. (The term *library* here refers to libraries of
foreign object code; for using libraries of Haskell source code, see
:ref:`ghci-modules-filenames`.) For example, to load the “m” library:
::
$ ghci -lm
On systems with ``.so``-style shared libraries, the actual library
loaded will the ``liblib.so``. GHCi searches the following places for
libraries, in this order:
- Paths specified using the ``-Lpath`` command-line option,
- the standard library search path for your system, which on some
systems may be overridden by setting the ``LD_LIBRARY_PATH``
environment variable.
On systems with ``.dll``-style shared libraries, the actual library
loaded will be ``lib.dll``. Again, GHCi will signal an error if it can't
find the library.
GHCi can also load plain object files (``.o`` or ``.obj`` depending on
your platform) from the command-line. Just add the name the object file
to the command line.
Ordering of ``-l`` options matters: a library should be mentioned
*before* the libraries it depends on (see :ref:`options-linker`).
.. _ghci-commands:
GHCi commands
-------------
GHCi commands all begin with "``:``" and consist of a single command
name followed by zero or more parameters. The command name may be
abbreviated, with ambiguities being resolved in favour of the more
commonly used commands.
``:abandon``
.. index::
single: :abandon
Abandons the current evaluation (only available when stopped at a
breakpoint).
``:add[*] ⟨module⟩``
.. index::
single: :add
Add ⟨module⟩(s) to the current target set, and perform a reload.
Normally pre-compiled code for the module will be loaded if
available, or otherwise the module will be compiled to byte-code.
Using the ``*`` prefix forces the module to be loaded as byte-code.
``:back ⟨n⟩``
.. index::
single: :back
Travel back ⟨n⟩ steps in the history. ⟨n⟩ is one if omitted. See
:ref:`tracing` for more about GHCi's debugging facilities. See also:
``:trace``, ``:history``, ``:forward``.
``:break [⟨identifier⟩ | [⟨module⟩] ⟨line⟩ [⟨column⟩]]``
.. index::
single: :break
Set a breakpoint on the specified function or line and column. See
:ref:`setting-breakpoints`.
``:browse[!] [[*] ⟨module⟩]``
.. index::
single: :browse
Displays the identifiers exported by the module ⟨module⟩, which must
be either loaded into GHCi or be a member of a package. If ⟨module⟩
is omitted, the most recently-loaded module is used.
Like all other GHCi commands, the output is always displayed in the
current GHCi scope (:ref:`ghci-scope`).
There are two variants of the browse command:
- If the ``*`` symbol is placed before the module name, then *all*
the identifiers in scope in ⟨module⟩ (rather that just its
exports) are shown.
The ``*``-form is only available for modules which are
interpreted; for compiled modules (including modules from
packages) only the non-\ ``*`` form of ``:browse`` is available.
- Data constructors and class methods are usually displayed in the
context of their data type or class declaration. However, if the
``!`` symbol is appended to the command, thus ``:browse!``, they
are listed individually. The ``!``-form also annotates the
listing with comments giving possible imports for each group of
entries. Here is an example:
::
Prelude> :browse! Data.Maybe
-- not currently imported
Data.Maybe.catMaybes :: [Maybe a] -> [a]
Data.Maybe.fromJust :: Maybe a -> a
Data.Maybe.fromMaybe :: a -> Maybe a -> a
Data.Maybe.isJust :: Maybe a -> Bool
Data.Maybe.isNothing :: Maybe a -> Bool
Data.Maybe.listToMaybe :: [a] -> Maybe a
Data.Maybe.mapMaybe :: (a -> Maybe b) -> [a] -> [b]
Data.Maybe.maybeToList :: Maybe a -> [a]
-- imported via Prelude
Just :: a -> Maybe a
data Maybe a = Nothing | Just a
Nothing :: Maybe a
maybe :: b -> (a -> b) -> Maybe a -> b
This output shows that, in the context of the current session (ie
in the scope of ``Prelude``), the first group of items from
``Data.Maybe`` are not in scope (althought they are available in
fully qualified form in the GHCi session - see
:ref:`ghci-scope`), whereas the second group of items are in
scope (via ``Prelude``) and are therefore available either
unqualified, or with a ``Prelude.`` qualifier.
``:cd ⟨dir⟩``
.. index::
single: :cd
Changes the current working directory to ⟨dir⟩. A "``~``" symbol
at the beginning of ⟨dir⟩ will be replaced by the contents of the
environment variable ``HOME``. See also the ``:show paths`` command
for showing the current working directory.
Note: changing directories causes all currently loaded modules to be
unloaded. This is because the search path is usually expressed using
relative directories, and changing the search path in the middle of
a session is not supported.
``:cmd ⟨expr⟩``
.. index::
single: :cmd
Executes ⟨expr⟩ as a computation of type ``IO String``, and then
executes the resulting string as a list of GHCi commands. Multiple
commands are separated by newlines. The ``:cmd`` command is useful
with ``:def`` and ``:set stop``.
``:complete ⟨type⟩ [⟨n⟩-][⟨m⟩] ⟨string-literal⟩``
.. index::
single: :complete
This command allows to request command completions from GHCi even
when interacting over a pipe instead of a proper terminal and is
designed for integrating GHCi's completion with text editors and
IDEs.
When called, ``:complete`` prints the ⟨n⟩\ :sup:`th` to
⟨m⟩\ :sup:`th` completion candidates for the partial input
⟨string-literal⟩ for the completion domain denoted by ⟨type⟩.
Currently, only the ``repl`` domain is supported which denotes the
kind of completion that would be provided interactively by GHCi at
the input prompt.
If omitted, ⟨n⟩ and ⟨m⟩ default to the first or last available
completion candidate respectively. If there are less candidates than
requested via the range argument, ⟨n⟩ and ⟨m⟩ are implicitly capped
to the number of available completition candidates.
The output of ``:complete`` begins with a header line containing
three space-delimited fields:
- An integer denoting the number ``l`` of printed completions,
- an integer denoting the total number of completions available,
and finally
- a string literal denoting a common prefix to be added to the
returned completion candidates.
The header line is followed by ⟨l⟩ lines each containing one
completion candidate encoded as (quoted) string literal. Here are
some example invocations showing the various cases:
::
Prelude> :complete repl 0 ""
0 470 ""
Prelude> :complete repl 5 "import For"
5 21 "import "
"Foreign"
"Foreign.C"
"Foreign.C.Error"
"Foreign.C.String"
"Foreign.C.Types"
Prelude> :complete repl 5-10 "import For"
6 21 "import "
"Foreign.C.Types"
"Foreign.Concurrent"
"Foreign.ForeignPtr"
"Foreign.ForeignPtr.Safe"
"Foreign.ForeignPtr.Unsafe"
"Foreign.Marshal"
Prelude> :complete repl 20- "import For"
2 21 "import "
"Foreign.StablePtr"
"Foreign.Storable"
Prelude> :complete repl "map"
3 3 ""
"map"
"mapM"
"mapM_"
Prelude> :complete repl 5-10 "map"
0 3 ""
``:continue``
.. index::
single: :continue
Continue the current evaluation, when stopped at a breakpoint.
``:ctags [⟨filename⟩]``, ``:etags [⟨filename⟩]``
.. index::
single: :ctags
single: :etags
Generates a “tags” file for Vi-style editors (``:ctags``) or
Emacs-style editors (``:etags``). If no filename is specified, the
default ``tags`` or ``TAGS`` is used, respectively. Tags for all the
functions, constructors and types in the currently loaded modules
are created. All modules must be interpreted for these commands to
work.
``:def! ⟨name⟩ ⟨expr⟩``
.. index::
single: :def
``:def`` is used to define new commands, or macros, in GHCi. The
command ``:def`` ⟨name⟩ ⟨expr⟩ defines a new GHCi command ``:name``,
implemented by the Haskell expression ⟨expr⟩, which must have type
``String -> IO String``. When ``:name args`` is typed at the prompt,
GHCi will run the expression ``(name args)``, take the resulting
``String``, and feed it back into GHCi as a new sequence of
commands. Separate commands in the result must be separated by
"``\n``".
That's all a little confusing, so here's a few examples. To start
with, here's a new GHCi command which doesn't take any arguments or
produce any results, it just outputs the current date & time:
::
Prelude> let date _ = Time.getClockTime >>= print >> return ""
Prelude> :def date date
Prelude> :date
Fri Mar 23 15:16:40 GMT 2001
Here's an example of a command that takes an argument. It's a
re-implementation of ``:cd``:
::
Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
Prelude> :def mycd mycd
Prelude> :mycd ..
Or I could define a simple way to invoke "``ghc --make Main``"
in the current directory:
::
Prelude> :def make (\_ -> return ":! ghc --make Main")
We can define a command that reads GHCi input from a file. This
might be useful for creating a set of bindings that we want to
repeatedly load into the GHCi session:
::
Prelude> :def . readFile
Prelude> :. cmds.ghci
Notice that we named the command ``:.``, by analogy with the
"``.``" Unix shell command that does the same thing.
Typing ``:def`` on its own lists the currently-defined macros.
Attempting to redefine an existing command name results in an error
unless the ``:def!`` form is used, in which case the old command
with that name is silently overwritten.
``:delete * | ⟨num⟩ ...``
.. index::
single: :delete
Delete one or more breakpoints by number (use ``:show breaks`` to
see the number of each breakpoint). The ``*`` form deletes all the
breakpoints.
``:edit ⟨file⟩``
.. index::
single: :edit
Opens an editor to edit the file ⟨file⟩, or the most recently loaded
module if ⟨file⟩ is omitted. If there were errors during the last
loading, the cursor will be positioned at the line of the first
error. The editor to invoke is taken from the ``EDITOR`` environment
variable, or a default editor on your system if ``EDITOR`` is not
set. You can change the editor using ``:set editor``.
``:etags``
See ``:ctags``.
``:force ⟨identifier⟩ ...``
.. index::
single: :force
Prints the value of ⟨identifier⟩ in the same way as ``:print``.
Unlike ``:print``, ``:force`` evaluates each thunk that it
encounters while traversing the value. This may cause exceptions or
infinite loops, or further breakpoints (which are ignored, but
displayed).
``:forward ⟨n⟩``
.. index::
single: :forward
Move forward ⟨n⟩ steps in the history. ⟨n⟩ is one if omitted. See
:ref:`tracing` for more about GHCi's debugging facilities. See also:
``:trace``, ``:history``, ``:back``.
``:help``, ``:?``
.. index::
single: :help
single: :?
Displays a list of the available commands.
``:``
.. index::
single: :
Repeat the previous command.
``:history [num]``
.. index::
single: :history
Display the history of evaluation steps. With a number, displays
that many steps (default: 20). For use with ``:trace``; see
:ref:`tracing`. To set the number of history entries stored by GHCi,
use ``-fghci-hist-size=n``.
``:info[!] ⟨name⟩``
.. index::
single: :info
Displays information about the given name(s). For example, if ⟨name⟩
is a class, then the class methods and their types will be printed;
if ⟨name⟩ is a type constructor, then its definition will be
printed; if ⟨name⟩ is a function, then its type will be printed. If
⟨name⟩ has been loaded from a source file, then GHCi will also
display the location of its definition in the source.
For types and classes, GHCi also summarises instances that mention
them. To avoid showing irrelevant information, an instance is shown
only if (a) its head mentions ⟨name⟩, and (b) all the other things
mentioned in the instance are in scope (either qualified or
otherwise) as a result of a ``:load`` or ``:module`` commands.
The command ``:info!`` works in a similar fashion but it removes
restriction (b), showing all instances that are in scope and mention
⟨name⟩ in their head.
``:issafe [⟨module⟩]``
.. index::
single: :issafe
Displays Safe Haskell information about the given module (or the
current module if omitted). This includes the trust type of the
module and its containing package.
``:kind[!] ⟨type⟩``
.. index::
single: :kind
Infers and prints the kind of ⟨type⟩. The latter can be an arbitrary
type expression, including a partial application of a type
constructor, such as ``Either Int``. In fact, ``:kind`` even allows
you to write a partial application of a type synonym (usually
disallowed), so that this works:
::
ghci> type T a b = (a,b,a)
ghci> :k T Int Bool
T Int Bool :: *
ghci> :k T
T :: * -> * -> *
ghci> :k T Int
T Int :: * -> *
If you specify the optional "``!``", GHC will in addition normalise
the type by expanding out type synonyms and evaluating type-function
applications, and display the normalised result.
``:list ⟨identifier⟩``
.. index::
single: :list
Lists the source code around the definition of ⟨identifier⟩ or the
current breakpoint if not given. This requires that the identifier
be defined in an interpreted module. If your output device supports
it, then GHCi will highlight the active subexpression in bold.
``:list [⟨module⟩] ⟨line⟩``
.. index::
single: :list
Lists the source code around the given line number of ⟨module⟩. This
requires that the module be interpreted. If your output device
supports it, then GHCi will highlight the active subexpression in
bold.
``:load[!] [*]⟨module⟩``
.. index::
single: :load
Recursively loads the specified ⟨module⟩s, and all the modules they
depend on. Here, each ⟨module⟩ must be a module name or filename,
but may not be the name of a module in a package.
All previously loaded modules, except package modules, are
forgotten. The new set of modules is known as the target set. Note
that ``:load`` can be used without any arguments to unload all the
currently loaded modules and bindings.
Normally pre-compiled code for a module will be loaded if available,
or otherwise the module will be compiled to byte-code. Using the
``*`` prefix forces a module to be loaded as byte-code.
Adding the optional "``!``" turns type errors into warnings while
loading. This allows to use the portions of the module that are
correct, even if there are type errors in some definitions.
Effectively, the "-fdefer-type-errors" flag is set before loading
and unset after loading if the flag has not already been set before.
See :ref:`defer-type-errors` for further motivation and details.
After a ``:load`` command, the current context is set to:
- ⟨module⟩, if it was loaded successfully, or
- the most recently successfully loaded module, if any other
modules were loaded as a result of the current ``:load``, or
- ``Prelude`` otherwise.
``:main ⟨arg1⟩ ... ⟨argn⟩``
.. index::
single: :main
When a program is compiled and executed, it can use the ``getArgs``
function to access the command-line arguments. However, we cannot
simply pass the arguments to the ``main`` function while we are
testing in ghci, as the ``main`` function doesn't take its arguments
directly.
Instead, we can use the ``:main`` command. This runs whatever
``main`` is in scope, with any arguments being treated the same as
command-line arguments, e.g.:
::
Prelude> main = System.Environment.getArgs >>= print
Prelude> :main foo bar
["foo","bar"]
We can also quote arguments which contains characters like spaces,
and they are treated like Haskell strings, or we can just use
Haskell list syntax:
::
Prelude> :main foo "bar baz"
["foo","bar baz"]
Prelude> :main ["foo", "bar baz"]
["foo","bar baz"]
Finally, other functions can be called, either with the ``-main-is``
flag or the ``:run`` command:
::
Prelude> foo = putStrLn "foo" >> System.Environment.getArgs >>= print
Prelude> bar = putStrLn "bar" >> System.Environment.getArgs >>= print
Prelude> :set -main-is foo
Prelude> :main foo "bar baz"
foo
["foo","bar baz"]
Prelude> :run bar ["foo", "bar baz"]
bar
["foo","bar baz"]
``:module +|- [*]⟨mod1⟩ ...``, ``import mod``
.. index::
single: :module
Sets or modifies the current context for statements typed at the
prompt. The form ``import mod`` is equivalent to ``:module +mod``.
See :ref:`ghci-scope` for more details.
``:print ⟨names⟩``
.. index::
single: :print
Prints a value without forcing its evaluation. ``:print`` may be
used on values whose types are unknown or partially known, which
might be the case for local variables with polymorphic types at a
breakpoint. While inspecting the runtime value, ``:print`` attempts
to reconstruct the type of the value, and will elaborate the type in
GHCi's environment if possible. If any unevaluated components
(thunks) are encountered, then ``:print`` binds a fresh variable
with a name beginning with ``_t`` to each thunk. See
:ref:`breakpoints` for more information. See also the ``:sprint``
command, which works like ``:print`` but does not bind new
variables.
``:quit``
.. index::
single: :quit
Quits GHCi. You can also quit by typing control-D at the prompt.
``:reload[!]``
.. index::
single: :reload
Attempts to reload the current target set (see ``:load``) if any of
the modules in the set, or any dependent module, has changed. Note
that this may entail loading new modules, or dropping modules which
are no longer indirectly required by the target.
Adding the optional "``!``" turns type errors into warnings while
loading. This allows to use the portions of the module that are
correct, even if there are type errors in some definitions.
Effectively, the "-fdefer-type-errors" flag is set before loading
and unset after loading if the flag has not already been set before.
See :ref:`defer-type-errors` for further motivation and details.
``:run``
.. index::
single: :run
See ``:main``.
``:script [⟨n⟩] ⟨filename⟩``
.. index::
single: :script
Executes the lines of a file as a series of GHCi commands. This
command is compatible with multiline statements as set by
``:set +m``
``:set [⟨option⟩ ...]``
.. index::
single: :set
Sets various options. See :ref:`ghci-set` for a list of available
options and :ref:`interactive-mode-options` for a list of
GHCi-specific flags. The ``:set`` command by itself shows which
options are currently set. It also lists the current dynamic flag
settings, with GHCi-specific flags listed separately.
``:set args ⟨arg⟩``
.. index::
single: :set args
single: getArgs, behavior in GHCi
Sets the list of arguments which are returned when the program calls
``System.getArgs``.
``:set editor ⟨cmd⟩``
Sets the command used by ``:edit`` to ⟨cmd⟩.
``:set prog ⟨prog⟩``
.. index::
single: :set prog
single: getProgName, behavior in GHCi
Sets the string to be returned when the program calls
``System.getProgName``.
``:set prompt ⟨prompt⟩``
.. index::
single: GHCi prompt; setting
Sets the string to be used as the prompt in GHCi. Inside ⟨prompt⟩,
the sequence ``%s`` is replaced by the names of the modules
currently in scope, ``%l`` is replaced by the line number (as
referenced in compiler messages) of the current prompt, and ``%%``
is replaced by ``%``. If ⟨prompt⟩ starts with ``"`` then it is parsed as
a Haskell String; otherwise it is treated as a literal string.
``:set prompt2 ⟨prompt⟩``
Sets the string to be used as the continuation prompt (used when
using the ``:{`` command) in GHCi.
``:set stop ⟨num⟩ ⟨cmd⟩``
Set a command to be executed when a breakpoint is hit, or a new item
in the history is selected. The most common use of ``:set stop`` is
to display the source code at the current location, e.g.
``:set stop :list``.
If a number is given before the command, then the commands are run
when the specified breakpoint (only) is hit. This can be quite
useful: for example, ``:set stop 1 :continue`` effectively disables
breakpoint 1, by running ``:continue`` whenever it is hit (although
GHCi will still emit a message to say the breakpoint was hit).
What's more, with cunning use of ``:def`` and ``:cmd`` you can use
``:set stop`` to implement conditional breakpoints:
::
*Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
*Main> :set stop 0 :cond (x < 3)
Ignoring breakpoints for a specified number of iterations is also
possible using similar techniques.
``:seti [⟨option⟩ ...]``
.. index::
single: :seti
Like ``:set``, but options set with ``:seti`` affect only
expressions and commands typed at the prompt, and not modules loaded
with ``:load`` (in contrast, options set with ``:set`` apply
everywhere). See :ref:`ghci-interactive-options`.
Without any arguments, displays the current set of options that are
applied to expressions and commands typed at the prompt.
``:show bindings``
.. index::
single: :show bindings
Show the bindings made at the prompt and their types.
``:show breaks``
.. index::
single: :show breaks
List the active breakpoints.
``:show context``
.. index::
single: :show context
List the active evaluations that are stopped at breakpoints.
``:show imports``
.. index::
single: :show imports
Show the imports that are currently in force, as created by
``import`` and ``:module`` commands.
``:show modules``
.. index::
single: :show modules
Show the list of modules currently loaded.
``:show packages``
.. index::
single: :show packages
Show the currently active package flags, as well as the list of
packages currently loaded.
``:show paths``
.. index::
single: :show paths
Show the current working directory (as set via ``:cd`` command), as
well as the list of directories searched for source files (as set by
the ``-i`` option).
``:show language``
.. index::
single: :show language
Show the currently active language flags for source files.
``:showi language``
.. index::
single: :showi language
Show the currently active language flags for expressions typed at
the prompt (see also ``:seti``).
``:show [args|prog|prompt|editor|stop]``
.. index::
single: :show
Displays the specified setting (see ``:set``).
``:sprint``
.. index::
single: :sprint
Prints a value without forcing its evaluation. ``:sprint`` is
similar to ``:print``, with the difference that unevaluated subterms
are not bound to new variables, they are simply denoted by ‘\_’.
``:step [⟨expr⟩]``
.. index::
single: :step
Enable all breakpoints and begin evaluating an expression in
single-stepping mode. In this mode evaluation will be stopped after
every reduction, allowing local variables to be inspected. If ⟨expr⟩
is not given, evaluation will resume at the last breakpoint. See
:ref:`single-stepping`.
``:steplocal``
.. index::
single: :steplocal
Enable only breakpoints in the current top-level binding and resume
evaluation at the last breakpoint.
``:stepmodule``
.. index::
single: :stepmodule
Enable only breakpoints in the current module and resume evaluation
at the last breakpoint.
``:trace expr``
.. index::
single: :trace
Evaluates the given expression (or from the last breakpoint if no
expression is given), and additionally logs the evaluation steps for
later inspection using ``:history``. See :ref:`tracing`.
``:type ⟨expression⟩``
.. index::
single: :type
Infers and prints the type of ⟨expression⟩, including explicit
forall quantifiers for polymorphic types. The monomorphism
restriction is *not* applied to the expression during type
inference.
``:undef ⟨name⟩``
.. index::
single: :undef
Undefines the user-defined command ⟨name⟩ (see ``:def`` above).
``:unset ⟨option⟩``
.. index::
single: :unset
Unsets certain options. See :ref:`ghci-set` for a list of available
options.
``:! ⟨command⟩``
.. index::
single: :!
single: shell commands; in GHCi
Executes the shell command ⟨command⟩.
.. _ghci-set:
The ``:set`` and ``:seti`` commands
-----------------------------------
.. index::
single: :set
single: :seti
The ``:set`` command sets two types of options: GHCi options, which
begin with "``+``", and "command-line" options, which begin with
"``-``".
.. note::
At the moment, the ``:set`` command doesn't support any kind of
quoting in its arguments: quotes will not be removed and cannot be used
to group words together. For example, ``:set -DFOO='BAR BAZ'`` will not
do what you expect.
GHCi options
~~~~~~~~~~~~
.. index::
single: options; GHCi
GHCi options may be set using ``:set`` and unset using ``:unset``.
The available GHCi options are:
``+m``
.. index::
single: +m
Enable parsing of multiline commands. A multiline command is
prompted for when the current input line contains open layout
contexts (see :ref:`ghci-multiline`).
``+r``
.. index::
single: +r
single: CAFs; in GHCi
single: Constant Applicative Form
Normally, any evaluation of top-level expressions (otherwise known
as CAFs or Constant Applicative Forms) in loaded modules is retained
between evaluations. Turning on ``+r`` causes all evaluation of
top-level expressions to be discarded after each evaluation (they
are still retained *during* a single evaluation).
This option may help if the evaluated top-level expressions are
consuming large amounts of space, or if you need repeatable
performance measurements.
``+s``
.. index::
single: +s
Display some stats after evaluating each expression, including the
elapsed time and number of bytes allocated. NOTE: the allocation
figure is only accurate to the size of the storage manager's
allocation area, because it is calculated at every GC. Hence, you
might see values of zero if no GC has occurred.
``+t``
.. index::
single: +t
Display the type of each variable bound after a statement is entered
at the prompt. If the statement is a single expression, then the
only variable binding will be for the variable ‘\ ``it``\ ’.
.. _ghci-cmd-line-options:
Setting GHC command-line options in GHCi
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Normal GHC command-line options may also be set using ``:set``. For
example, to turn on ``-fwarn-missing-signatures``, you would say:
::
Prelude> :set -fwarn-missing-signatures
Any GHC command-line option that is designated as dynamic (see the table
in :ref:`flag-reference`), may be set using ``:set``. To unset an
option, you can set the reverse option:
.. index::
single: dynamic; options
::
Prelude> :set -fno-warn-incomplete-patterns -XNoMultiParamTypeClasses
:ref:`flag-reference` lists the reverse for each option where
applicable.
Certain static options (``-package``, ``-I``, ``-i``, and ``-l`` in
particular) will also work, but some may not take effect until the next
reload.
.. index::
single: static; options
.. _ghci-interactive-options:
Setting options for interactive evaluation only
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
GHCi actually maintains *two* sets of options:
- The *loading options* apply when loading modules
- The *interactive options* apply when evaluating expressions and
commands typed at the GHCi prompt.
The ``:set`` command modifies both, but there is also a ``:seti``
command (for "set interactive") that affects only the interactive
options set.
It is often useful to change the interactive options, without having
that option apply to loaded modules too. For example
::
:seti -XMonoLocalBinds
It would be undesirable if ``-XMonoLocalBinds`` were to apply to loaded
modules too: that might cause a compilation error, but more commonly it
will cause extra recompilation, because GHC will think that it needs to
recompile the module because the flags have changed.
If you are setting language options in your ``.ghci`` file, it is good
practice to use ``:seti`` rather than ``:set``, unless you really do
want them to apply to all modules you load in GHCi.
The two sets of options can be inspected using the ``:set`` and
``:seti`` commands respectively, with no arguments. For example, in a
clean GHCi session we might see something like this:
::
Prelude> :seti
base language is: Haskell2010
with the following modifiers:
-XNoMonomorphismRestriction
-XNoDatatypeContexts
-XNondecreasingIndentation
-XExtendedDefaultRules
GHCi-specific dynamic flag settings:
other dynamic, non-language, flag settings:
-fimplicit-import-qualified
warning settings:
The two sets of options are initialised as follows. First, both sets of
options are initialised as described in :ref:`ghci-dot-files`. Then the
interactive options are modified as follows:
- The option ``-XExtendedDefaultRules`` is enabled, in order to apply
special defaulting rules to expressions typed at the prompt (see
:ref:`extended-default-rules`).
- The Monomorphism Restriction is disabled (see :ref:`monomorphism`).
.. _ghci-dot-files:
The ``.ghci`` and ``.haskeline`` files
--------------------------------------
.. _dot-ghci-files:
The ``.ghci`` files
~~~~~~~~~~~~~~~~~~~
.. index::
single: .ghci; file
single: startup; files, GHCi
When it starts, unless the ``-ignore-dot-ghci`` flag is given, GHCi
reads and executes commands from the following files, in this order, if
they exist:
1. ``./.ghci``
2. ``appdata/ghc/ghci.conf``, where ⟨appdata⟩ depends on your system,
but is usually something like
``C:/Documents and Settings/user/Application Data``
3. On Unix: ``$HOME/.ghc/ghci.conf``
4. ``$HOME/.ghci``
The ``ghci.conf`` file is most useful for turning on favourite options
(eg. ``:set +s``), and defining useful macros. Note: when setting
language options in this file it is usually desirable to use ``:seti``
rather than ``:set`` (see :ref:`ghci-interactive-options`).
Placing a ``.ghci`` file in a directory with a Haskell project is a
useful way to set certain project-wide options so you don't have to type
them every time you start GHCi: eg. if your project uses multi-parameter
type classes, scoped type variables, and CPP, and has source files in
three subdirectories A, B and C, you might put the following lines in
``.ghci``:
::
:set -XMultiParamTypeClasses -XScopedTypeVariables -cpp
:set -iA:B:C
(Note that strictly speaking the ``-i`` flag is a static one, but in
fact it works to set it using ``:set`` like this. The changes won't take
effect until the next ``:load``, though.)
Once you have a library of GHCi macros, you may want to source them from
separate files, or you may want to source your ``.ghci`` file into your
running GHCi session while debugging it
::
:def source readFile
With this macro defined in your ``.ghci`` file, you can use
``:source file`` to read GHCi commands from ``file``. You can find (and
contribute!-) other suggestions for ``.ghci`` files on this Haskell wiki
page: `GHC/GHCi `__
Additionally, any files specified with ``-ghci-script`` flags will be
read after the standard files, allowing the use of custom .ghci files.
Two command-line options control whether the startup files files are
read:
``-ignore-dot-ghci``
.. index::
single: -ignore-dot-ghci
Don't read either ``./.ghci`` or the other startup files when
starting up.
``-ghci-script``
.. index::
single: -ghci-script
Read a specific file after the usual startup files. Maybe be
specified repeatedly for multiple inputs.
When defining GHCi macros, there is some important behavior you should
be aware of when names may conflict with built-in commands, especially
regarding tab completion.
For example, consider if you had a macro named ``:time`` and in the
shell, typed ``:t 3`` - what should happen? The current algorithm we use
for completing commands is:
1. First, look up an exact match on the name from the defined macros.
2. Look for the exact match on the name in the built-in command list.
3. Do a prefix lookup on the list of built-in commands - if a built-in
command matches, but a macro is defined with the same name as the
built-in defined, pick the macro.
4. Do a prefix lookup on the list of built-in commands.
5. Do a prefix lookup on the list of defined macros.
Here are some examples:
1. You have a macro ``:time`` and enter ``:t 3``
You get ``:type 3``
2. You have a macro ``:type`` and enter ``:t 3``
You get ``:type 3`` with your defined macro, not the builtin.
3. You have a macro ``:time`` and a macro ``:type``, and enter ``:t 3``
You get ``:type 3`` with your defined macro.
.. _dot-haskeline-file:
The ``.haskeline`` file
~~~~~~~~~~~~~~~~~~~~~~~
.. index::
single: .haskeline; file
single: startup; files, GHCi
GHCi uses `Haskeline `__ under
the hood. You can configure it to, among other
things, prune duplicates from GHCi history. See:
`Haskeline user preferences `__.
.. _ghci-obj:
Compiling to object code inside GHCi
------------------------------------
By default, GHCi compiles Haskell source code into byte-code that is
interpreted by the runtime system. GHCi can also compile Haskell code to
object code: to turn on this feature, use the ``-fobject-code`` flag
either on the command line or with ``:set`` (the option ``-fbyte-code``
restores byte-code compilation again). Compiling to object code takes
longer, but typically the code will execute 10-20 times faster than
byte-code.
Compiling to object code inside GHCi is particularly useful if you are
developing a compiled application, because the ``:reload`` command
typically runs much faster than restarting GHC with ``--make`` from the
command-line, because all the interface files are already cached in
memory.
There are disadvantages to compiling to object-code: you can't set
breakpoints in object-code modules, for example. Only the exports of an
object-code module will be visible in GHCi, rather than all top-level
bindings as in interpreted modules.
.. _ghci-faq:
FAQ and Things To Watch Out For
-------------------------------
The interpreter can't load modules with foreign export declarations!
Unfortunately not. We haven't implemented it yet. Please compile any
offending modules by hand before loading them into GHCi.
``-O`` doesn't work with GHCi!
.. index::
single: -O
For technical reasons, the bytecode compiler doesn't interact well
with one of the optimisation passes, so we have disabled
optimisation when using the interpreter. This isn't a great loss:
you'll get a much bigger win by compiling the bits of your code that
need to go fast, rather than interpreting them with optimisation
turned on.
Unboxed tuples don't work with GHCi
That's right. You can always compile a module that uses unboxed
tuples and load it into GHCi, however. (Incidentally the previous
point, namely that ``-O`` is incompatible with GHCi, is because the
bytecode compiler can't deal with unboxed tuples).
Concurrent threads don't carry on running when GHCi is waiting for input.
This should work, as long as your GHCi was built with the
``-threaded`` switch, which is the default. Consult whoever supplied
your GHCi installation.
After using ``getContents``, I can't use ``stdin``, until I do ``:load`` or ``:reload``
This is the defined behaviour of ``getContents``: it puts the stdin
Handle in a state known as semi-closed, wherein any further I/O
operations on it are forbidden. Because I/O state is retained
between computations, the semi-closed state persists until the next
``:load`` or ``:reload`` command.
You can make ``stdin`` reset itself after every evaluation by giving
GHCi the command ``:set +r``. This works because ``stdin`` is just a
top-level expression that can be reverted to its unevaluated state
in the same way as any other top-level expression (CAF).
I can't use Control-C to interrupt computations in GHCi on Windows.
See :ref:`ghci-windows`.
The default buffering mode is different in GHCi to GHC.
In GHC, the stdout handle is line-buffered by default. However, in
GHCi we turn off the buffering on stdout, because this is normally
what you want in an interpreter: output appears as it is generated.
If you want line-buffered behaviour, as in GHC, you can start your
program thus:
::
main = do { hSetBuffering stdout LineBuffering; ... }
.. [5]
Note that packages only contain compiled code, so debugging a package
requires finding its source and loading that directly.
.. [6]
We originally provided bindings for all variables in scope, rather
than just the free variables of the expression, but found that this
affected performance considerably, hence the current restriction to
just the free variables.