3. Release notes for version 7.12.1¶

The significant changes to the various parts of the compiler are listed in the following sections. There have also been numerous bug fixes and performance improvements over the 7.10 branch.

3.1. Highlights¶

The highlights, since the 7.10 branch, are:

TODO FIXME

3.2. Full details¶

3.2.1. Language¶

TODO FIXME.

The parser now supports Haddock comments on GADT data constructors. For example,

data Expr a where
  -- | Just a normal sum
  Sum :: Int -> Int -> Expr Int

Implicit parameters of the new ghc-prim type GHC.Types.CallStack are treated specially, and automatically solved for the current source location. For example
```
f = print (?stk :: CallStack)
```
will print the singleton stack containing the occurrence of ?stk. If there is another CallStack implicit in-scope, the new location will be appended to the existing stack, e.g.
```
f :: (?stk :: CallStack) => IO ()
f = print (?stk :: CallStack)
```
will print the occurrence of ?stk and the call-site of f. The name of the implicit parameter does not matter.

See the release notes for ghc-prim for a description of the CallStack type.
To conform to the common case, the default role assigned to parameters of datatypes declared in hs-boot files is representational. However, if the constructor(s) for the datatype are given, it makes sense to do normal role inference. This is now implemented, effectively making the default role for non-abstract datatypes in hs-boot files to be phantom, like it is in regular Haskell code.
Wildcards can be used in the type arguments of type/data family instance declarations to indicate that the name of a type variable doesn’t matter. They will be replaced with new unique type variables. See Data instance declarations for more details.
GHC now allows to declare type families as injective. Injectivity information can then be used by the typechecker. See Injective type families for details.
Due to a security issue, Safe Haskell now forbids annotations in programs marked as -XSafe.
Generic instances can be derived for data types whose constructors have arguments with certain unlifted types. See Generic programming for more details.
The -XDeriveAnyClass extension now fills in associated type family default instances when deriving a class that contains them.
Users can now define record pattern synonyms. This allows pattern synonyms to behave more like normal data constructors. For example,
```
pattern P :: a -> b -> (a, b)
pattern P{x,y} = (x,y)
```
will allow P to be used like a record data constructor and also defines selector functions x :: (a, b) -> a and y :: (a, b) -> b.
Pattern synonyms can now be bundled with type constructors. For a pattern synonym P and a type constructor T, P can be bundled with T so that when T is imported P is also imported. With this change a library author can provide either real data constructors or pattern synonyms in an opaque manner. See Pattern synonyms for details.
```
-- Foo.hs
module Foo ( T(P) ) where

data T = T

pattern P = T

-- Baz.hs
module Baz where

-- P is imported
import Foo (T(..))
```
Whenever a data instance is exported, the corresponding data family is exported, too. This allows one to write
```
-- Foo.hs
module Foo where

data family T a

-- Bar.hs
module Bar where

import Foo

data instance T Int = MkT

-- Baz.hs
module Baz where

import Bar (T(MkT))
```
In previous versions of GHC, this required a workaround via an explicit export list in Bar.

3.2.2. Compiler¶

Added the option -dth-dec-file. This dumps out a .th.hs file of all Template Haskell declarations in a corresponding .hs file. The idea is that application developers can check this into their repository so that they can grep for identifiers used elsewhere that were defined in Template Haskell. This is similar to using -ddump-to-file with -ddump-splices but it always generates a file instead of being coupled to -ddump-to-file and only outputs code that does not exist in the .hs file and a comment for the splice location in the original file.
Added the option -fprint-expanded-types. When enabled, GHC also prints type-synonym-expanded types in type errors.
Added the option -fcpr-anal. When enabled, the demand analyser performs CPR analysis. It is implied by -O. Consequently, -fcpr-off is now removed, run with -fno-cpr-anal to get the old -fcpr-off behaviour.
Added the option -fworker-wrapper. When enabled, the worker-wrapper transformation is performed after a strictness analysis pass. It is implied by -O and by -fstrictness. It is disabled by -fno-strictness. Enabling -fworker-wrapper while strictness analysis is disabled (by -fno-strictness) has no effect.
Added the options -fwarn-missed-specialisations and -fwarn-all-missed-specialisations. When enabled, the simplifier will produce a warning when a overloaded imported function cannot be specialised (typically due to a missing INLINEABLE pragma). This is intended to alert users to cases where they apply INLINEABLE but may not get the speed-up they expect.
Added the option -fwarn-noncanonical-monad-instances which helps detect noncanonical Applicative/Monad instance definitions. See flag description in Warnings and sanity-checking for more details.
When printing an out-of-scope error message, GHC will give helpful advice if the error might be caused by too restrictive imports.
Added the -Wcompat warning group, along with its opposite -Wno-compat. Turns on warnings that will be enabled by default in the future, but remain off in normal compilations for the time being. This allows library authors eager to make their code future compatible to adapt to new features before they even generate warnings.
Added the -fwarn-missing-monadfail-instance flag. When enabled, this will issue a warning if a failable pattern is used in a context that does not have a MonadFail constraint. This flag represents phase 1 of the MonadFail Proposal (MFP).
Added the -fwarn-semigroup flag. When enabled, this will issue a warning if a type is an instance of Monoid but not Semigroup, and when a custom definition (<>) is made. Fixing these warnings makes sure the definition of Semigroup as a superclass of Monoid does not break any code.
Added the -fwarn-missing-pat-syn-sigs flag. When enabled, this will issue a warning when a pattern synonym definition doesn’t have a type signature. It is turned off by default but enabled by -Wall.

3.2.3. GHCi¶

Main with an explicit module header but without main is now an error (#7765).
The :back and :forward commands now take an optional count allowing the user to move forward or backward in history several steps at a time.
Added commands :load! and :reload!, effectively setting “-fdefer-type-errors” before loading a module and unsetting it after loading if it has not been set before (#8353).
ghci -e now behaves like ghc -e (#9360).
Added support for top-level function declarations (#7253).

3.2.4. Template Haskell¶

The new -XTemplateHaskellQuotes flag allows to use the quotes (not quasi-quotes) subset of TemplateHaskell. This is particularly useful for use with a stage 1 compiler (i.e. GHC without interpreter support). Also, -XTemplateHaskellQuotes is considered safe under Safe Haskell.
Partial type signatures can now be used in splices, see Where can they occur?.
Template Haskell now fully supports typed holes and quoting unbound variables. This means it is now possible to use pattern splices nested inside quotation brackets.
Template Haskell now supports the use of UInfixT in types to resolve infix operator fixities, in the same vein as UInfixP and UInfixE in patterns and expressions. ParensT and InfixT have also been introduced, serving the same functions as their pattern and expression counterparts.
Primitive chars (e.g., [| 'a'# |]) and primitive strings (e.g., [| "abc"# |]) can now be quoted with Template Haskell. The Lit data type also has a new constructor, CharPrimL, for primitive char literals.
addTopDecls now accepts annotation pragmas.
Internally, the implementation of quasi-quotes has been unified with that of normal Template Haskell splices. Under the previous implementation, top-level declaration quasi-quotes did not cause a break in the declaration groups, unlike splices of the form $(...). This behavior has been preserved under the new implementation, and is now recognized and documented in Syntax.
The Lift class is now derivable via the -XDeriveLift extension. See Deriving Lift instances for more information.

3.2.5. Runtime system¶

Support for performance monitoring with PAPI has been dropped.

3.2.6. Build system¶

TODO FIXME.

3.2.7. Package system¶

TODO FIXME.

3.3. Libraries¶

3.3.1. array¶

Version number XXXXX (was 0.5.0.0)

3.3.2. base¶

Version number 4.9.0.0 (was 4.7.0.0)
A new module GHC.SrcLoc was added, exporting a new type SrcLoc. A SrcLoc contains package, module, and file names, as well as start and end positions.
A new type CallStack was added for use with the new implicit callstack parameters. A CallStack is a [(String, SrcLoc)], sorted by most-recent call.
A new function, interruptible, was added to GHC.IO allowing an IO action to be run such that it can be interrupted by an asynchronous exception, even if exceptions are masked (except if masked with interruptibleMask).

This was introduced to fix the behavior of allowInterrupt, which would previously incorrectly allow exceptions in uninterruptible regions (see Trac #9516).
Per-thread allocation counters (setAllocationCounter and getAllocationCounter) and limits (enableAllocationLimit, disableAllocationLimit are now available from System.Mem. Previously this functionality was only available from GHC.Conc.
forever, filterM, mapAndUnzipM, zipWithM, zipWithM_, replicateM, and replicateM were generalized from Monad to Applicative. If this causes performance regressions, try to make the implementation of (*>) match that of (>>).
Add GHC.TypeLits.TypeError and ErrorMessage to allow users to define custom compile-time error messages.

3.3.3. binary¶

Version number XXXXX (was 0.7.1.0)

3.3.4. bytestring¶

Version number XXXXX (was 0.10.4.0)

3.3.5. Cabal¶

Version number XXXXX (was 1.18.1.3)

3.3.6. containers¶

Version number XXXXX (was 0.5.4.0)

3.3.7. deepseq¶

Version number XXXXX (was 1.3.0.2)

3.3.8. directory¶

Version number XXXXX (was 1.2.0.2)

3.3.9. filepath¶

Version number XXXXX (was 1.3.0.2)

3.3.10. ghc¶

TODO FIXME.
The HsBang type has been removed in favour of HsSrcBang and HsImplBang. Data constructors now always carry around their strictness annotations as the user wrote them, whether from an imported module or not.
Moved startsVarSym, startsVarId, startsConSym, startsConId, startsVarSymASCII, and isVarSymChar from Lexeme to the GHC.Lemexe module of the ghc-boot library.
Add isImport, isDecl, and isStmt functions.

3.3.11. ghc-boot¶

This is an internal package. Use with caution.
This package was renamed from bin-package-db to reflect its new purpose of containing intra-GHC functionality that needs to be shared across multiple GHC boot libraries.
Added GHC.Lexeme, which contains functions for determining if a character can be the first letter of a variable or data constructor in Haskell, as defined by GHC. (These functions were moved from Lexeme in ghc.)

3.3.12. ghc-prim¶

Version number XXXXX (was 0.3.1.0)

3.3.13. haskell98¶

Version number XXXXX (was 2.0.0.3)

3.3.14. haskell2010¶

Version number XXXXX (was 1.1.1.1)

3.3.15. hoopl¶

Version number XXXXX (was 3.10.0.0)

3.3.16. hpc¶

Version number XXXXX (was 0.6.0.1)

3.3.17. integer-gmp¶

Version number XXXXX (was 0.5.1.0)

3.3.18. old-locale¶

Version number XXXXX (was 1.0.0.6)

3.3.19. old-time¶

Version number XXXXX (was 1.1.0.2)

3.3.20. process¶

Version number XXXXX (was 1.2.0.0)

3.3.21. template-haskell¶

Version number XXXXX (was 2.9.0.0)
The Lift type class for lifting values into Template Haskell splices now has a default signature lift :: Data a => a -> Q Exp, which means that you do not have to provide an explicit implementation of lift for types which have a Data instance. To manually use this default implementation, you can use the liftData function which is now exported from Language.Haskell.TH.Syntax.
Info‘s constructors no longer have Fixity fields. A qReifyFixity function was added to the Quasi type class (as well as the reifyFixity function, specialized for Q) to allow lookup of fixity information for any given Name.

3.3.22. time¶

Version number XXXXX (was 1.4.1)

3.3.23. unix¶

Version number XXXXX (was 2.7.0.0)

3.3.24. Win32¶

Version number XXXXX (was 2.3.0.1)

3.4. Known bugs¶

TODO FIXME

Glasgow Haskell Compiler Users Guide