.. _options-sanity: Warnings and sanity-checking ---------------------------- .. index:: single: sanity-checking options single: warnings GHC has a number of options that select which types of non-fatal error messages, otherwise known as warnings, can be generated during compilation. By default, you get a standard set of warnings which are generally likely to indicate bugs in your program. These are: ``-fwarn-overlapping-patterns``, ``-fwarn-warnings-deprecations``, ``-fwarn-deprecated-flags``, ``-fwarn-unrecognised-pragmas``, ``-fwarn-missed-specialisations``, ``-fwarn-duplicate-constraints``, ``-fwarn-duplicate-exports``, ``-fwarn-overflowed-literals``, ``-fwarn-empty-enumerations``, ``-fwarn-missing-fields``, ``-fwarn-missing-methods``, ``-fwarn-wrong-do-bind``, ``-fwarn-unsupported-calling-conventions``, ``-fwarn-dodgy-foreign-imports``, ``-fwarn-inline-rule-shadowing``, ``-fwarn-unsupported-llvm-version``, ``-fwarn-context-quantification``, and ``-fwarn-tabs``. The following flags are simple ways to select standard “packages” of warnings: ``-W`` .. index:: single: -W option Provides the standard warnings plus ``-fwarn-unused-binds``, ``-fwarn-unused-matches``, ``-fwarn-unused-imports``, ``-fwarn-incomplete-patterns``, ``-fwarn-dodgy-exports``, and ``-fwarn-dodgy-imports``. ``-Wall`` .. index:: single: -Wall Turns on all warning options that indicate potentially suspicious code. The warnings that are *not* enabled by ``-Wall`` are ``-fwarn-incomplete-uni-patterns``, ``-fwarn-incomplete-record-updates``, ``-fwarn-monomorphism-restriction``, ``-fwarn-implicit-prelude``, ``-fwarn-missing-local-sigs``, ``-fwarn-missing-exported-sigs``, ``-fwarn-missing-import-lists`` and ``-fwarn-identities``. ``-Wcompat`` .. index:: single: -Wcompat 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. This currently enables ``-fwarn-missing-monadfail-instance``, ``-fwarn-semigroup``, and ``-fwarn-noncanonical-monoid-instances``. ``-Wno-compat`` .. index:: single: -Wno-compat Disables all warnings enabled by ``-Wcompat``. ``-w`` .. index:: single: -w Turns off all warnings, including the standard ones and those that ``-Wall`` doesn't enable. ``-Werror`` .. index:: single: -Werror Makes any warning into a fatal error. Useful so that you don't miss warnings when doing batch compilation. ``-Wwarn`` .. index:: single: -Wwarn Warnings are treated only as warnings, not as errors. This is the default, but can be useful to negate a ``-Werror`` flag. The full set of warning options is described below. To turn off any warning, simply give the corresponding ``-fno-warn-...`` option on the command line. ``-fwarn-typed-holes`` .. index:: single: -fwarn-typed-holes single: warnings Determines whether the compiler reports typed holes warnings. Has no effect unless typed holes errors are deferred until runtime. See :ref:`typed-holes` and :ref:`defer-type-errors` This warning is on by default. ``-fwarn-type-errors`` .. index:: single: -fwarn-type-errors single: warnings Causes a warning to be reported when a type error is deferred until runtime. See :ref:`defer-type-errors` This warning is on by default. ``-fdefer-type-errors`` .. index:: single: -fdefer-type-errors single: warnings Defer as many type errors as possible until runtime. At compile time you get a warning (instead of an error). At runtime, if you use a value that depends on a type error, you get a runtime error; but you can run any type-correct parts of your code just fine. See :ref:`defer-type-errors` ``-fdefer-typed-holes`` .. index:: single: -fdefer-typed-holes single: warnings Defer typed holes errors until runtime. This will turn the errors produced by :ref:`typed holes ` into warnings. Using a value that depends on a typed hole produces a runtime error, the same as ``-fdefer-type-errors`` (which implies this option). See :ref:`typed-holes` and :ref:`defer-type-errors`. Implied by ``-fdefer-type-errors``. See also ``-fwarn-typed-holes``. ``-fwarn-partial-type-signatures`` .. index:: single: -fwarn-partial-type-signatures single: warnings Determines whether the compiler reports holes in partial type signatures as warnings. Has no effect unless ``-XPartialTypeSignatures`` is enabled, which controls whether errors should be generated for holes in types or not. See :ref:`partial-type-signatures`. This warning is on by default. ``-fhelpful-errors`` .. index:: single: -fhelpful-errors single: warnings When a name or package is not found in scope, make suggestions for the name or package you might have meant instead. This option is on by default. ``-fwarn-unrecognised-pragmas`` .. index:: single: -fwarn-unrecognised-pragmas single: warnings single: pragmas Causes a warning to be emitted when a pragma that GHC doesn't recognise is used. As well as pragmas that GHC itself uses, GHC also recognises pragmas known to be used by other tools, e.g. ``OPTIONS_HUGS`` and ``DERIVE``. This option is on by default. ``-fwarn-missed-specialisations``, ``-fwarn-all-missed-specialisations`` .. index:: single: -fwarn-missed-specialisations single: -fwarn-all-missed-specialisations single: warnings single: pragmas Emits a warning if GHC cannot specialise an overloaded function, usually because the function needs an ``INLINEABLE`` pragma. The "all" form reports all such situations whereas the "non-all" form only reports when the situation arises during specialisation of an imported function. The "non-all" form is intended to catch cases where an imported function that is marked as ``INLINEABLE`` (presumably to enable specialisation) cannot be specialised as it calls other functions that are themselves not specialised. These options are both off by default. ``-fwarn-warnings-deprecations`` .. index:: single: -fwarn-warnings-deprecations single: warnings single: deprecations Causes a warning to be emitted when a module, function or type with a ``WARNING`` or ``DEPRECATED pragma`` is used. See :ref:`warning-deprecated-pragma` for more details on the pragmas. This option is on by default. ``-fwarn-amp`` .. index:: single: -fwarn-amp single: AMP single: Applicative-Monad Proposal This option is deprecated. Caused a warning to be emitted when a definition was in conflict with the AMP (Applicative-Monad proosal). ``-fwarn-noncanonical-monad-instances`` .. index:: single: -fwarn-noncanonical-monad-instances Warn if noncanonical ``Applicative`` or ``Monad`` instances declarations are detected. When this warning is enabled, the following conditions are verified: In ``Monad`` instances declarations warn if any of the following conditions does not hold: * If ``return`` is defined it must be canonical (i.e. ``return = pure``). * If ``(>>)`` is defined it must be canonical (i.e. ``(>>) = (*>)``). Moreover, in 'Applicative' instance declarations: * Warn if ``pure`` is defined backwards (i.e. ``pure = return``). * Warn if ``(*>)`` is defined backwards (i.e. ``(*>) = (>>)``). This option is off by default. ``-fwarn-noncanonical-monoid-instances`` .. index:: single: -fwarn-noncanonical-monoid-instances Warn if noncanonical ``Semigroup`` or ``Monoid`` instances declarations are detected. When this warning is enabled, the following conditions are verified: In ``Monoid`` instances declarations warn if any of the following conditions does not hold: * If ``mappend`` is defined it must be canonical (i.e. ``mappend = (Data.Semigroup.<>)``). Moreover, in 'Semigroup' instance declarations: * Warn if ``(<>)`` is defined backwards (i.e. ``(<>) = mappend``). This warning is off by default. However, it is part of the ``-Wcompat`` option group. ``-fwarn-missing-monadfail-instance`` .. index:: single: -fwarn-missing-monadfail-instance single: MFP single: MonadFail Proposal Warn when a failable pattern is used in a do-block that does not have a ``MonadFail`` instance. Being part of the ``-Wcompat`` option group, this warning is off by default, but will be switched on in a future GHC release, as part of the `MonadFail Proposal (MFP) `__. ``-fwarn-semigroup`` .. index:: single: -fwarn-semigroup single: semigroup Warn when definitions are in conflict with the future inclusion of ``Semigroup`` into the standard typeclasses. 1. Instances of ``Monoid`` should also be instances of ``Semigroup`` 2. The ``Semigroup`` operator ``(<>)`` will be in ``Prelude``, which clashes with custom local definitions of such an operator Being part of the ``-Wcompat`` option group, this warning is off by default, but will be switched on in a future GHC release. ``-fwarn-deprecated-flags`` .. index:: single: -fwarn-deprecated-flags single: deprecated-flags Causes a warning to be emitted when a deprecated command-line flag is used. This option is on by default. ``-fwarn-unsupported-calling-conventions`` .. index:: single: -fwarn-unsupported-calling-conventions Causes a warning to be emitted for foreign declarations that use unsupported calling conventions. In particular, if the ``stdcall`` calling convention is used on an architecture other than i386 then it will be treated as ``ccall``. ``-fwarn-dodgy-foreign-imports`` .. index:: single: -fwarn-dodgy-foreign-imports Causes a warning to be emitted for foreign imports of the following form: :: foreign import "f" f :: FunPtr t on the grounds that it probably should be :: foreign import "&f" f :: FunPtr t The first form declares that \`f\` is a (pure) C function that takes no arguments and returns a pointer to a C function with type \`t\`, whereas the second form declares that \`f\` itself is a C function with type \`t\`. The first declaration is usually a mistake, and one that is hard to debug because it results in a crash, hence this warning. ``-fwarn-dodgy-exports`` .. index:: single: -fwarn-dodgy-exports Causes a warning to be emitted when a datatype ``T`` is exported with all constructors, i.e. ``T(..)``, but is it just a type synonym. Also causes a warning to be emitted when a module is re-exported, but that module exports nothing. ``-fwarn-dodgy-imports`` .. index:: single: -fwarn-dodgy-imports Causes a warning to be emitted in the following cases: - When a datatype ``T`` is imported with all constructors, i.e. ``T(..)``, but has been exported abstractly, i.e. ``T``. - When an ``import`` statement hides an entity that is not exported. ``-fwarn-overflowed-literals`` .. index:: single: -fwarn-overflowed-literals Causes a warning to be emitted if a literal will overflow, e.g. ``300 :: Word8``. ``-fwarn-empty-enumerations`` .. index:: single: -fwarn-empty-enumerations Causes a warning to be emitted if an enumeration is empty, e.g. ``[5 .. 3]``. ``-fwarn-lazy-unlifted-bindings`` .. index:: single: -fwarn-lazy-unlifted-bindings This flag is a no-op, and will be removed in GHC 7.10. ``-fwarn-duplicate-constraints`` .. index:: single: -fwarn-duplicate-constraints single: duplicate constraints, warning Have the compiler warn about duplicate constraints in a type signature. For example :: f :: (Eq a, Show a, Eq a) => a -> a The warning will indicate the duplicated ``Eq a`` constraint. This option is now deprecated in favour of ``-fwarn-redundant-constraints``. ``-fwarn-redundant-constraints`` .. index:: single: -fwarn-redundant-constraints single: redundant constraints, warning Have the compiler warn about redundant constraints in a type signature. In particular: - A redundant constraint within the type signature itself: :: f :: (Eq a, Ord a) => a -> a The warning will indicate the redundant ``Eq a`` constraint: it is subsumed by the ``Ord a`` constraint. - A constraint in the type signature is not used in the code it covers: :: f :: Eq a => a -> a -> Bool f x y = True The warning will indicate the redundant ``Eq a`` constraint: : it is not used by the definition of ``f``.) Similar warnings are given for a redundant constraint in an instance declaration. This option is on by default. As usual you can suppress it on a per-module basis with ``-fno-warn-redundant-constraints``. Occasionally you may specifically want a function to have a more constrained signature than necessary, perhaps to leave yourself wiggle-room for changing the implementation without changing the API. In that case, you can suppress the warning on a per-function basis, using a call in a dead binding. For example: :: f :: Eq a => a -> a -> Bool f x y = True where _ = x == x -- Suppress the redundant-constraint warning for (Eq a) Here the call to ``(==)`` makes GHC think that the ``(Eq a)`` constraint is needed, so no warning is issued. ``-fwarn-duplicate-exports`` .. index:: single: -fwarn-duplicate-exports single: duplicate exports, warning single: export lists, duplicates Have the compiler warn about duplicate entries in export lists. This is useful information if you maintain large export lists, and want to avoid the continued export of a definition after you've deleted (one) mention of it in the export list. This option is on by default. ``-fwarn-hi-shadowing`` .. index:: single: -fwarn-hi-shadowing single: shadowing; interface files Causes the compiler to emit a warning when a module or interface file in the current directory is shadowing one with the same module name in a library or other directory. ``-fwarn-identities`` .. index:: single: -fwarn-identities Causes the compiler to emit a warning when a Prelude numeric conversion converts a type T to the same type T; such calls are probably no-ops and can be omitted. The functions checked for are: ``toInteger``, ``toRational``, ``fromIntegral``, and ``realToFrac``. ``-fwarn-implicit-prelude`` .. index:: single: -fwarn-implicit-prelude single: implicit prelude, warning Have the compiler warn if the Prelude is implicitly imported. This happens unless either the Prelude module is explicitly imported with an ``import ... Prelude ...`` line, or this implicit import is disabled (either by ``-XNoImplicitPrelude`` or a ``LANGUAGE NoImplicitPrelude`` pragma). Note that no warning is given for syntax that implicitly refers to the Prelude, even if ``-XNoImplicitPrelude`` would change whether it refers to the Prelude. For example, no warning is given when ``368`` means ``Prelude.fromInteger (368::Prelude.Integer)`` (where ``Prelude`` refers to the actual Prelude module, regardless of the imports of the module being compiled). This warning is off by default. ``-fwarn-incomplete-patterns``, ``-fwarn-incomplete-uni-patterns`` .. index:: single: -fwarn-incomplete-patterns single: -fwarn-incomplete-uni-patterns single: incomplete patterns, warning single: patterns, incomplete The option ``-fwarn-incomplete-patterns`` warns about places where a pattern-match might fail at runtime. The function ``g`` below will fail when applied to non-empty lists, so the compiler will emit a warning about this when ``-fwarn-incomplete-patterns`` is enabled. :: g [] = 2 This option isn't enabled by default because it can be a bit noisy, and it doesn't always indicate a bug in the program. However, it's generally considered good practice to cover all the cases in your functions, and it is switched on by ``-W``. The flag ``-fwarn-incomplete-uni-patterns`` is similar, except that it applies only to lambda-expressions and pattern bindings, constructs that only allow a single pattern: :: h = \[] -> 2 Just k = f y ``-fwarn-incomplete-record-updates`` .. index:: single: -fwarn-incomplete-record-updates single: incomplete record updates, warning single: record updates, incomplete The function ``f`` below will fail when applied to ``Bar``, so the compiler will emit a warning about this when ``-fwarn-incomplete-record-updates`` is enabled. :: data Foo = Foo { x :: Int } | Bar f :: Foo -> Foo f foo = foo { x = 6 } This option isn't enabled by default because it can be very noisy, and it often doesn't indicate a bug in the program. ``-fwarn-missing-fields`` .. index:: single: -fwarn-missing-fields single: missing fields, warning single: fields, missing This option is on by default, and warns you whenever the construction of a labelled field constructor isn't complete, missing initialisers for one or more fields. While not an error (the missing fields are initialised with bottoms), it is often an indication of a programmer error. ``-fwarn-missing-import-lists`` .. index:: single: -fwarn-import-lists single: missing import lists, warning single: import lists, missing This flag warns if you use an unqualified ``import`` declaration that does not explicitly list the entities brought into scope. For example :: module M where import X( f ) import Y import qualified Z p x = f x x The ``-fwarn-import-lists`` flag will warn about the import of ``Y`` but not ``X`` If module ``Y`` is later changed to export (say) ``f``, then the reference to ``f`` in ``M`` will become ambiguous. No warning is produced for the import of ``Z`` because extending ``Z``\'s exports would be unlikely to produce ambiguity in ``M``. ``-fwarn-missing-methods`` .. index:: single: -fwarn-missing-methods single: missing methods, warning single: methods, missing This option is on by default, and warns you whenever an instance declaration is missing one or more methods, and the corresponding class declaration has no default declaration for them. The warning is suppressed if the method name begins with an underscore. Here's an example where this is useful: :: class C a where _simpleFn :: a -> String complexFn :: a -> a -> String complexFn x y = ... _simpleFn ... The idea is that: (a) users of the class will only call ``complexFn``; never ``_simpleFn``; and (b) instance declarations can define either ``complexFn`` or ``_simpleFn``. The ``MINIMAL`` pragma can be used to change which combination of methods will be required for instances of a particular class. See :ref:`minimal-pragma`. ``-fwarn-missing-signatures`` .. index:: single: -fwarn-missing-signatures single: type signatures, missing If you would like GHC to check that every top-level function/value has a type signature, use the ``-fwarn-missing-signatures`` option. As part of the warning GHC also reports the inferred type. The option is off by default. ``-fwarn-missing-exported-sigs`` .. index:: single: -fwarn-missing-exported-sigs single: type signatures, missing If you would like GHC to check that every exported top-level function/value has a type signature, but not check unexported values, use the ``-fwarn-missing-exported-sigs`` option. This option takes precedence over ``-fwarn-missing-signatures``. As part of the warning GHC also reports the inferred type. The option is off by default. ``-fwarn-missing-local-sigs`` .. index:: single: -fwarn-missing-local-sigs single: type signatures, missing If you use the ``-fwarn-missing-local-sigs`` flag GHC will warn you about any polymorphic local bindings. As part of the warning GHC also reports the inferred type. The option is off by default. ``-fwarn-missing-pat-syn-sigs`` .. index :: single: -fwarn-missing-pat-syn-sigs single: type signatures, missing, pattern synonyms If you would like GHC to check that every pattern synonym has a type signature, use the ``-fwarn-missing-pat-syn-sigs`` option. If this option is used in conjunction with ``-fwarn-missing-exported-sigs`` then only exported pattern synonyms must have a type signature. GHC also reports the inferred type. This option is off by default. ``-fwarn-name-shadowing`` .. index:: single: -fwarn-name-shadowing single: shadowing, warning This option causes a warning to be emitted whenever an inner-scope value has the same name as an outer-scope value, i.e. the inner value shadows the outer one. This can catch typographical errors that turn into hard-to-find bugs, e.g., in the inadvertent capture of what would be a recursive call in ``f = ... let f = id in ... f ...``. The warning is suppressed for names beginning with an underscore. For example :: f x = do { _ignore <- this; _ignore <- that; return (the other) } ``-fwarn-orphans`` .. index:: single: -fwarn-orphans single: orphan instances, warning single: orphan rules, warning These flags cause a warning to be emitted whenever the module contains an "orphan" instance declaration or rewrite rule. An instance declaration is an orphan if it appears in a module in which neither the class nor the type being instanced are declared in the same module. A rule is an orphan if it is a rule for a function declared in another module. A module containing any orphans is called an orphan module. The trouble with orphans is that GHC must pro-actively read the interface files for all orphan modules, just in case their instances or rules play a role, whether or not the module's interface would otherwise be of any use. See :ref:`orphan-modules` for details. The flag ``-fwarn-orphans`` warns about user-written orphan rules or instances. ``-fwarn-overlapping-patterns`` .. index:: single: -fwarn-overlapping-patterns single: overlapping patterns, warning single: patterns, overlapping By default, the compiler will warn you if a set of patterns are overlapping, e.g., :: f :: String -> Int f [] = 0 f (_:xs) = 1 f "2" = 2 where the last pattern match in ``f`` won't ever be reached, as the second pattern overlaps it. More often than not, redundant patterns is a programmer mistake/error, so this option is enabled by default. ``-fwarn-tabs`` .. index:: single: -fwarn-tabs single: tabs, warning Have the compiler warn if there are tabs in your source file. ``-fwarn-type-defaults`` .. index:: single: -fwarn-type-defaults single: defaulting mechanism, warning Have the compiler warn/inform you where in your source the Haskell defaulting mechanism for numeric types kicks in. This is useful information when converting code from a context that assumed one default into one with another, e.g., the ‘default default’ for Haskell 1.4 caused the otherwise unconstrained value ``1`` to be given the type ``Int``, whereas Haskell 98 and later defaults it to ``Integer``. This may lead to differences in performance and behaviour, hence the usefulness of being non-silent about this. This warning is off by default. ``-fwarn-monomorphism-restriction`` .. index:: single: -fwarn-monomorphism-restriction single: monomorphism restriction, warning Have the compiler warn/inform you where in your source the Haskell Monomorphism Restriction is applied. If applied silently the MR can give rise to unexpected behaviour, so it can be helpful to have an explicit warning that it is being applied. This warning is off by default. ``-fwarn-unticked-promoted-constructors`` .. index:: single: -fwarn-unticked-promoted-constructors single: promoted constructor, warning Warn if a promoted data constructor is used without a tick preceding its name. For example: :: data Nat = Succ Nat | Zero data Vec n s where Nil :: Vec Zero a Cons :: a -> Vec n a -> Vec (Succ n) a Will raise two warnings because ``Zero`` and ``Succ`` are not written as ``'Zero`` and ``'Succ``. This warning is is enabled by default in ``-Wall`` mode. ``-fwarn-unused-binds`` .. index:: single: -fwarn-unused-binds single: unused binds, warning single: binds, unused Report any function definitions (and local bindings) which are unused. An alias for - ``-fwarn-unused-top-binds`` - ``-fwarn-unused-local-binds`` - ``-fwarn-unused-pattern-binds`` ``-fwarn-unused-top-binds`` .. index:: single: -fwarn-unused-top-binds single: unused binds, warning single: binds, unused Report any function definitions which are unused. More precisely, warn if a binding brings into scope a variable that is not used, except if the variable's name starts with an underscore. The "starts-with-underscore" condition provides a way to selectively disable the warning. A variable is regarded as "used" if - It is exported, or - It appears in the right hand side of a binding that binds at least one used variable that is used For example :: module A (f) where f = let (p,q) = rhs1 in t p -- No warning: q is unused, but is locally bound t = rhs3 -- No warning: f is used, and hence so is t g = h x -- Warning: g unused h = rhs2 -- Warning: h is only used in the -- right-hand side of another unused binding _w = True -- No warning: _w starts with an underscore ``-fwarn-unused-local-binds`` .. index:: single: -fwarn-unused-local-binds single: unused binds, warning single: binds, unused Report any local definitions which are unused. For example :: module A (f) where f = let (p,q) = rhs1 in t p -- Warning: q is unused g = h x -- No warning: g is unused, but is a top-level binding ``-fwarn-unused-pattern-binds`` .. index:: single: -fwarn-unused-pattern-binds single: unused binds, warning single: binds, unused Warn if a pattern binding binds no variables at all, unless it is a lone, possibly-banged, wild-card pattern. For example: :: Just _ = rhs3 -- Warning: unused pattern binding (_, _) = rhs4 -- Warning: unused pattern binding _ = rhs3 -- No warning: lone wild-card pattern !_ = rhs4 -- No warning: banged wild-card pattern; behaves like seq The motivation for allowing lone wild-card patterns is they are not very different from ``_v = rhs3``, which elicits no warning; and they can be useful to add a type constraint, e.g. ``_ = x::Int``. A lone banged wild-card pattern is useful as an alternative (to ``seq``) way to force evaluation. ``-fwarn-unused-imports`` .. index:: single: -fwarn-unused-imports single: unused imports, warning single: imports, unused Report any modules that are explicitly imported but never used. However, the form ``import M()`` is never reported as an unused import, because it is a useful idiom for importing instance declarations, which are anonymous in Haskell. ``-fwarn-unused-matches`` .. index:: single: -fwarn-unused-matches single: unused matches, warning single: matches, unused Report all unused variables which arise from pattern matches, including patterns consisting of a single variable. For instance ``f x y = []`` would report ``x`` and ``y`` as unused. The warning is suppressed if the variable name begins with an underscore, thus: :: f _x = True ``-fwarn-unused-do-bind`` .. index:: single: -fwarn-unused-do-bind single: unused do binding, warning single: do binding, unused Report expressions occurring in ``do`` and ``mdo`` blocks that appear to silently throw information away. For instance ``do { mapM popInt xs ; return 10 }`` would report the first statement in the ``do`` block as suspicious, as it has the type ``StackM [Int]`` and not ``StackM ()``, but that ``[Int]`` value is not bound to anything. The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away: :: do { _ <- mapM popInt xs ; return 10 } Of course, in this particular situation you can do even better: :: do { mapM_ popInt xs ; return 10 } ``-fwarn-context-quantification`` .. index:: single: -fwarn-context-quantification single: implicit context quantification, warning single: context, implicit quantification Report if a variable is quantified only due to its presence in a context (see :ref:`universal-quantification`). For example, :: type T a = Monad m => a -> f a It is recommended to write this polymorphic type as :: type T a = forall m. Monad m => a -> f a instead. ``-fwarn-wrong-do-bind`` .. index:: single: -fwarn-wrong-do-bind single: apparently erroneous do binding, warning single: do binding, apparently erroneous Report expressions occurring in ``do`` and ``mdo`` blocks that appear to lack a binding. For instance ``do { return (popInt 10) ; return 10 }`` would report the first statement in the ``do`` block as suspicious, as it has the type ``StackM (StackM Int)`` (which consists of two nested applications of the same monad constructor), but which is not then "unpacked" by binding the result. The warning is suppressed by explicitly mentioning in the source code that your program is throwing something away: :: do { _ <- return (popInt 10) ; return 10 } For almost all sensible programs this will indicate a bug, and you probably intended to write: :: do { popInt 10 ; return 10 } ``-fwarn-inline-rule-shadowing`` .. index:: single: -fwarn-inline-rule-shadowing Warn if a rewrite RULE might fail to fire because the function might be inlined before the rule has a chance to fire. See :ref:`rules-inline`. If you're feeling really paranoid, the ``-dcore-lint`` option is a good choice. It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's sanity, not yours.)