.. _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. Some options control individual warnings and others control collections of warnings. Use ``-W⟨wflag⟩`` to turn on an individual warning or a collection, or use ``-Wno-⟨wflag⟩`` to turn it off. Use ``-Werror`` to make all warnings into fatal errors, or ``-Werror=⟨wflag⟩`` to make a specific warning into an error. Reverse this with ``-Wwarn`` to make all warnings non-fatal, or ``-Wwarn=⟨wflag⟩`` to make a specific warning non-fatal. .. note:: In GHC < 8 the syntax for ``-W⟨wflag⟩`` was ``-fwarn-⟨wflag⟩`` (e.g. ``-fwarn-incomplete-patterns``). This spelling is deprecated, but still accepted for backwards compatibility. Likewise, ``-Wno-⟨wflag⟩`` used to be ``fno-warn-⟨wflag⟩`` (e.g. ``-fno-warn-incomplete-patterns``). Warning groups ============== The following flags are simple ways to select standard "packages" of warnings. They can be reversed using ``-Wno-⟨group⟩``, which has the same effect as ``-Wno-...`` for every individual warning in the group. .. ghc-flag:: -Wdefault :shortdesc: enable default flags :type: dynamic :reverse: -Wno-default :category: :since: 8.0 By default, you get a standard set of warnings which are generally likely to indicate bugs in your program. These are: .. hlist:: :columns: 3 * :ghc-flag:`-Woverlapping-patterns` * :ghc-flag:`-Wwarnings-deprecations` * :ghc-flag:`-Wdeprecations` * :ghc-flag:`-Wdeprecated-flags` * :ghc-flag:`-Wunrecognised-pragmas` * :ghc-flag:`-Wduplicate-exports` * :ghc-flag:`-Wderiving-defaults` * :ghc-flag:`-Woverflowed-literals` * :ghc-flag:`-Wempty-enumerations` * :ghc-flag:`-Wmissing-fields` * :ghc-flag:`-Wmissing-methods` * :ghc-flag:`-Wwrong-do-bind` * :ghc-flag:`-Wsimplifiable-class-constraints` * :ghc-flag:`-Wtyped-holes` * :ghc-flag:`-Wdeferred-type-errors` * :ghc-flag:`-Wpartial-type-signatures` * :ghc-flag:`-Wunsupported-calling-conventions` * :ghc-flag:`-Wdodgy-foreign-imports` * :ghc-flag:`-Winline-rule-shadowing` * :ghc-flag:`-Wunsupported-llvm-version` * :ghc-flag:`-Wmissed-extra-shared-lib` * :ghc-flag:`-Wtabs` * :ghc-flag:`-Wunrecognised-warning-flags` * :ghc-flag:`-Winaccessible-code` * :ghc-flag:`-Wstar-binder` * :ghc-flag:`-Wstar-is-type` * :ghc-flag:`-Woperator-whitespace-ext-conflict` * :ghc-flag:`-Wambiguous-fields` * :ghc-flag:`-Wunicode-bidirectional-format-characters` * :ghc-flag:`-Wforall-identifier` * :ghc-flag:`-Wgadt-mono-local-binds` * :ghc-flag:`-Wtype-equality-requires-operators` * :ghc-flag:`-Winconsistent-flags` .. ghc-flag:: -W :shortdesc: enable normal warnings :type: dynamic :reverse: -Wno-extra :category: Provides the standard warnings plus .. hlist:: :columns: 3 * :ghc-flag:`-Wunused-binds` * :ghc-flag:`-Wunused-matches` * :ghc-flag:`-Wunused-foralls` * :ghc-flag:`-Wunused-imports` * :ghc-flag:`-Wincomplete-patterns` * :ghc-flag:`-Wdodgy-exports` * :ghc-flag:`-Wdodgy-imports` * :ghc-flag:`-Wunbanged-strict-patterns` .. ghc-flag:: -Wextra :shortdesc: alias for :ghc-flag:`-W` :type: dynamic :reverse: -Wno-extra Alias for :ghc-flag:`-W` .. ghc-flag:: -Wall :shortdesc: enable almost all warnings (details in :ref:`options-sanity`) :type: dynamic :reverse: -Wno-all :category: Turns on all warning options that indicate potentially suspicious code. They include all warnings in :ghc-flag:`-Wextra`, plus: .. hlist:: :columns: 3 * :ghc-flag:`-Whi-shadowing` * :ghc-flag:`-Wincomplete-record-updates` * :ghc-flag:`-Wincomplete-uni-patterns` * :ghc-flag:`-Wmissing-pattern-synonym-signatures` * :ghc-flag:`-Wmissing-signatures` * :ghc-flag:`-Wname-shadowing` * :ghc-flag:`-Worphans` * :ghc-flag:`-Wredundant-record-wildcards` * :ghc-flag:`-Wstar-is-type` * :ghc-flag:`-Wtrustworthy-safe` * :ghc-flag:`-Wtype-defaults` * :ghc-flag:`-Wunused-do-bind` * :ghc-flag:`-Wunused-record-wildcards` * :ghc-flag:`-Wincomplete-export-warnings` .. ghc-flag:: -Weverything :shortdesc: enable all warnings supported by GHC :type: dynamic :reverse: -w :category: :since: 8.0 Turns on every single warning supported by the compiler. .. ghc-flag:: -Wcompat :shortdesc: enable future compatibility warnings (details in :ref:`options-sanity`) :type: dynamic :reverse: -Wno-compat :category: :since: 8.0 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 .. hlist:: :columns: 3 * :ghc-flag:`-Wsemigroup` * :ghc-flag:`-Wnoncanonical-monoid-instances` * :ghc-flag:`-Wnoncanonical-monad-instances` * :ghc-flag:`-Wcompat-unqualified-imports` * :ghc-flag:`-Wtype-equality-out-of-scope` * :ghc-flag:`-Wimplicit-rhs-quantification` .. ghc-flag:: -w :shortdesc: disable all warnings :type: dynamic :category: Turns off all warnings, including the standard ones and those that :ghc-flag:`-Wall` doesn't enable. .. ghc-flag:: -Wnot :shortdesc: *(deprecated)* Alias for :ghc-flag:`-w` :type: dynamic Deprecated alias for :ghc-flag:`-w` When a warning is emitted, the specific warning flag which controls it is shown, but the group can optionally be shown as well: .. ghc-flag:: -fshow-warning-groups :shortdesc: show which group an emitted warning belongs to. :type: dynamic :reverse: -fno-show-warning-groups :category: :default: off When showing which flag controls a warning, also show the respective warning group flag(s) that warning is contained in. Treating warnings as fatal errors ================================= These options control which warnings are considered fatal and cause compilation to abort. .. ghc-flag:: -Werror :shortdesc: make warnings fatal :type: dynamic :reverse: -Wwarn :category: :since: 6.8 (``-Wwarn``) Makes any warning into a fatal error. Useful so that you don't miss warnings when doing batch compilation. To reverse ``-Werror`` and stop treating any warnings as errors use ``-Wwarn``, or use ``-Wwarn=⟨wflag⟩`` to stop treating specific warnings as errors. .. ghc-flag:: -Werror=⟨wflag⟩ :shortdesc: make a specific warning fatal :type: dynamic :reverse: -Wwarn=⟨wflag⟩ :category: :noindex: :implies: ``-W⟨wflag⟩`` Makes a specific warning into a fatal error. The warning will be enabled if it hasn't been enabled yet. Can be reversed with ``-Wwarn=⟨wflag⟩``. ``-Werror=⟨group⟩`` has the same effect as ``-Werror=...`` for each warning flag in the group (for example, ``-Werror=compat`` will turn every warning in the :ghc-flag:`-Wcompat` group into a fatal error). .. ghc-flag:: -Wwarn :shortdesc: make warnings non-fatal :type: dynamic :reverse: -Werror :category: Warnings are treated only as warnings, not as errors. This is the default, but can be useful to negate a :ghc-flag:`-Werror` flag. .. ghc-flag:: -Wwarn=⟨wflag⟩ :shortdesc: make a specific warning non-fatal :type: dynamic :reverse: -Werror=⟨wflag⟩ :category: :noindex: Causes a specific warning to be treated as normal warning, not fatal error. Note that it doesn't fully negate the effects of ``-Werror=⟨wflag⟩`` - the warning will still be enabled. ``-Wwarn=⟨group⟩`` has the same effect as ``-Wwarn=...`` for each warning flag in the group (for example, ``-Wwarn=compat`` will mark every warning in the :ghc-flag:`-Wcompat` group as non-fatal). .. ghc-flag:: -Wno-error=⟨wflag⟩ :shortdesc: make a specific warning non-fatal :type: dynamic :reverse: -Werror=⟨wflag⟩ :category: :noindex: Alternative spelling for ``-Wwarn=⟨wflag⟩``. Individual warning options ========================== The full set of warning options is described below. To turn off any warning, simply give the corresponding ``-Wno-...`` option on the command line. For backwards compatibility with GHC versions prior to 8.0, all these warnings can still be controlled with ``-f(no-)warn-*`` instead of ``-W(no-)*``. .. ghc-flag:: -Wunrecognised-warning-flags :shortdesc: throw a warning when an unrecognised ``-W...`` flag is encountered on the command line. :type: dynamic :reverse: -Wno-unrecognised-warning-flags :category: :since: 8.0 :default: on Enables warnings when the compiler encounters a ``-W...`` flag that is not recognised. .. ghc-flag:: -Wcompat-unqualified-imports :shortdesc: Report unqualified imports of core libraries which are expected to cause compatibility problems in future releases. :type: dynamic :reverse: -Wno-compat-unqualified-imports :category: :since: 8.10 Warns on unqualified imports of core library modules which are subject to change in future GHC releases. Currently the following modules are covered by this warning: - ``Data.List`` due to the future addition of ``Data.List.singleton`` and specialisation of exports to the ``[]`` type. See the `mailing list `_ for details. This warning can be addressed by either adding an explicit import list or using a ``qualified`` import. .. ghc-flag:: -Wprepositive-qualified-module :shortdesc: Report imports with a leading/prepositive "qualified" :type: dynamic :reverse: -Wno-prepositive-qualified-module :category: :since: 8.10 Normally, imports are qualified prepositively: ``import qualified M``. By using :extension:`ImportQualifiedPost`, the qualified keyword can be used after the module name. Like so: ``import M qualified``. This will warn when the first, prepositive syntax is used. .. ghc-flag:: -Wtyped-holes :shortdesc: Report warnings when :ref:`typed hole ` errors are :ref:`deferred until runtime `. See :ghc-flag:`-fdefer-typed-holes`. :type: dynamic :reverse: -Wno-typed-holes :category: :since: 7.8 :default: on 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`. .. ghc-flag:: -Wdeferred-type-errors :shortdesc: Report warnings when :ref:`deferred type errors ` are enabled. This option is enabled by default. See :ghc-flag:`-fdefer-type-errors`. :type: dynamic :reverse: -Wno-deferred-type-errors :category: :since: 8.0 :default: on Causes a warning to be reported when a type error is deferred until runtime. See :ref:`defer-type-errors`. .. ghc-flag:: -Wdeferred-out-of-scope-variables :shortdesc: Report warnings when variable out-of-scope errors are :ref:`deferred until runtime `. See :ghc-flag:`-fdefer-out-of-scope-variables`. :type: dynamic :reverse: -Wno-deferred-out-of-scope-variables :category: :since: 8.0 Warn when a deferred out-of-scope variable is encountered. See :ref:`defer-type-errors`. .. ghc-flag:: -Wpartial-type-signatures :shortdesc: warn about holes in partial type signatures when :extension:`PartialTypeSignatures` is enabled. Not applicable when :extension:`PartialTypeSignatures` is not enabled, in which case errors are generated for such holes. :type: dynamic :reverse: -Wno-partial-type-signatures :category: :since: 7.10 :default: on Determines whether the compiler reports holes in partial type signatures as warnings. Has no effect unless :extension:`PartialTypeSignatures` is enabled, which controls whether errors should be generated for holes in types or not. See :ref:`partial-type-signatures`. .. ghc-flag:: -fhelpful-errors :shortdesc: Make suggestions for mis-spelled names. :type: dynamic :reverse: -fno-helpful-errors :category: :since: 7.4 :default: on When a name or package is not found in scope, make suggestions for the name or package you might have meant instead. .. ghc-flag:: -Wunrecognised-pragmas :shortdesc: warn about uses of pragmas that GHC doesn't recognise :type: dynamic :reverse: -Wno-unrecognised-pragmas :category: :since: 6.10 :default: on 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``. .. ghc-flag:: -Wmisplaced-pragmas :shortdesc: warn about uses of file header pragmas in the module body :type: dynamic :reverse: -Wno-misplaced-pragmas :category: :since: 9.4 :default: on Warn when a pragma that should only appear in the header of a module, such as a `LANGUAGE` or `OPTIONS_GHC` pragma, appears in the body of the module instead. .. ghc-flag:: -Wmissed-specialisations :shortdesc: warn when specialisation of an imported, overloaded function fails. :type: dynamic :reverse: -Wno-missed-specialisations :category: :since: 8.0 :default: off Emits a warning if GHC cannot specialise an overloaded function, usually because the function needs an ``INLINABLE`` pragma. Reports when the situation arises during specialisation of an imported function. This form is intended to catch cases where an imported function that is marked as ``INLINABLE`` (presumably to enable specialisation) cannot be specialised as it calls other functions that are themselves not specialised. Note that this warning will not throw errors if used with :ghc-flag:`-Werror`. .. ghc-flag:: -Wmissed-specializations :shortdesc: alias for :ghc-flag:`-Wmissed-specialisations` :type: dynamic :reverse: -Wno-missed-specializations Alias for :ghc-flag:`-Wmissed-specialisations` .. ghc-flag:: -Wall-missed-specialisations :shortdesc: warn when specialisation of any overloaded function fails. :type: dynamic :reverse: -Wno-all-missed-specialisations :category: :since: 8.0 :default: off Emits a warning if GHC cannot specialise an overloaded function, usually because the function needs an ``INLINABLE`` pragma. Reports all such situations. Note that this warning will not throw errors if used with :ghc-flag:`-Werror`. .. ghc-flag:: -Wall-missed-specializations :shortdesc: alias for :ghc-flag:`-Wall-missed-specialisations` :type: dynamic :reverse: -Wno-all-missed-specializations Alias for :ghc-flag:`-Wall-missed-specialisations` .. ghc-flag:: -Wextended-warnings :shortdesc: warn about uses of functions & types that have WARNING or DEPRECATED pragmas, across all categories :type: dynamic :reverse: -Wno-extended-warnings :category: :since: 9.8.1 :default: on .. index:: pair: deprecations; warnings Causes a warning to be emitted when a module, function or type with a ``WARNING`` or ``DEPRECATED`` pragma is used, regardless of the category which may be associated with the pragma. See :ref:`warning-deprecated-pragma` for more details on the pragmas. This implies :ghc-flag:`-Wdeprecations` and all ``-Wx-⟨category⟩`` flags. .. ghc-flag:: -Wx-⟨category⟩ :shortdesc: warn about uses of functions & types that have WARNING pragmas with the given category :type: dynamic :reverse: -Wno-x-⟨category⟩ :category: :since: 9.8.1 :default: on .. index:: pair: deprecations; warnings Causes a warning to be emitted when a module, function or type with a ``WARNING in "x-⟨category⟩"`` pragma is used. See :ref:`warning-deprecated-pragma` for more details on the pragmas. .. ghc-flag:: -Wdeprecations :shortdesc: warn about uses of functions & types that have DEPRECATED pragmas, or WARNING pragmas with the ``deprecated`` category. :type: dynamic :reverse: -Wno-deprecations :category: :default: on .. index:: pair: deprecations; warnings Causes a warning to be emitted when a module, function or type with ``DEPRECATED pragma``, or a ``WARNING`` pragma with the ``deprecated`` category, is used. See :ref:`warning-deprecated-pragma` for more details on the pragmas. .. ghc-flag:: -Wwarnings-deprecations :shortdesc: warn about uses of functions & types that have DEPRECATED pragmas, or WARNING pragmas with the ``deprecated`` category. Alias for :ghc-flag:`-Wdeprecations`. :type: dynamic :reverse: -Wno-warnings-deprecations :category: :since: 6.10 :default: on .. index:: pair: deprecations; warnings Causes a warning to be emitted when a module, function or type with ``DEPRECATED pragma``, or a ``WARNING`` pragma with the ``deprecated`` category, is used. See :ref:`warning-deprecated-pragma` for more details on the pragmas. An alias for :ghc-flag:`-Wdeprecations`. .. ghc-flag:: -Wnoncanonical-monad-instances :shortdesc: warn when ``Applicative`` or ``Monad`` instances have noncanonical definitions of ``return``, ``pure``, ``(>>)``, or ``(*>)``. See flag description in :ref:`options-sanity` for more details. :type: dynamic :reverse: -Wno-noncanonical-monad-instances :category: :since: 8.0 :default: on 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 warning is part of the :ghc-flag:`-Wcompat` option group. .. ghc-flag:: -Wnoncanonical-monadfail-instances :shortdesc: *(deprecated)* warn when ``Monad`` or ``MonadFail`` instances have noncanonical definitions of ``fail``. :type: dynamic :reverse: -Wno-noncanonical-monadfail-instances :category: :since: 8.0 This warning is deprecated. It no longer has any effect since GHC 8.8. It was used during the transition period of the MonadFail proposal, to detect when an instance of the ``Monad`` class was not defined via ``MonadFail``, or when a ``MonadFail`` instance was defined backwards, using the method in ``Monad``. .. ghc-flag:: -Wnoncanonical-monoid-instances :shortdesc: warn when ``Semigroup`` or ``Monoid`` instances have noncanonical definitions of ``(<>)`` or ``mappend``. See flag description in :ref:`options-sanity` for more details. :type: dynamic :reverse: -Wno-noncanonical-monoid-instances :category: :since: 8.0 :default: on 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 part of the :ghc-flag:`-Wcompat` option group. .. ghc-flag:: -Wmissing-monadfail-instances :shortdesc: *(deprecated)* Warn when a failable pattern is used in a do-block that does not have a ``MonadFail`` instance. :type: dynamic :reverse: -Wno-missing-monadfail-instances :category: :since: 8.0 This warning is deprecated. It no longer has any effect since GHC 8.8. It was used during the transition period of the MonadFail proposal, to warn when a failable pattern is used in a do-block that does not have a ``MonadFail`` instance. .. ghc-flag:: -Wsemigroup :shortdesc: warn when a ``Monoid`` is not ``Semigroup``, and on non- ``Semigroup`` definitions of ``(<>)``? :type: dynamic :reverse: -Wno-semigroup :category: :since: 8.0 .. index:: single: semigroup; warning 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 :ghc-flag:`-Wcompat` option group, this warning is off by default, but will be switched on in a future GHC release. .. ghc-flag:: -Wdeprecated-flags :shortdesc: warn about uses of commandline flags that are deprecated :type: dynamic :reverse: -Wno-deprecated-flags :category: :since: 6.10 :default: on .. index:: single: deprecated flags Causes a warning to be emitted when a deprecated command-line flag is used. .. ghc-flag:: -Wunsupported-calling-conventions :shortdesc: warn about use of an unsupported calling convention :type: dynamic :reverse: -Wno-unsupported-calling-conventions :category: :since: 7.6 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``. .. ghc-flag:: -Wdodgy-foreign-imports :shortdesc: warn about dodgy foreign imports :type: dynamic :reverse: -Wno-dodgy-foreign-imports :category: :since: 6.10 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. .. ghc-flag:: -Wdodgy-exports :shortdesc: warn about dodgy exports :type: dynamic :reverse: -Wno-dodgy-exports :category: :since: 6.12 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. .. ghc-flag:: -Wdodgy-imports :shortdesc: warn about dodgy imports :type: dynamic :reverse: -Wno-dodgy-imports :category: :since: 6.8 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. .. ghc-flag:: -Woverflowed-literals :shortdesc: warn about literals that will overflow their type :type: dynamic :reverse: -Wno-overflowed-literals :category: :since: 7.8 Causes a warning to be emitted if a literal will overflow, e.g. ``300 :: Word8``. .. ghc-flag:: -Wempty-enumerations :shortdesc: warn about enumerations that are empty :type: dynamic :reverse: -Wno-empty-enumerations :category: :since: 7.8 Causes a warning to be emitted if an enumeration is empty, e.g. ``[5 .. 3]``. .. ghc-flag:: -Wderiving-defaults :shortdesc: warn about default deriving when using both :extension:`DeriveAnyClass` and :extension:`GeneralizedNewtypeDeriving` :type: dynamic :reverse: -Wno-deriving-defaults :category: :since: 8.10 Causes a warning when both :extension:`DeriveAnyClass` and :extension:`GeneralizedNewtypeDeriving` are enabled and no explicit deriving strategy is in use. For example, this would result a warning: :: class C a newtype T a = MkT a deriving C .. ghc-flag:: -Wduplicate-constraints :shortdesc: warn when a constraint appears duplicated in a type signature :type: dynamic :reverse: -Wno-duplicate-constraints :category: :since: 7.8 .. index:: 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 :ghc-flag:`-Wredundant-constraints`. .. ghc-flag:: -Wredundant-constraints :shortdesc: Have the compiler warn about redundant constraints in type signatures. :type: dynamic :reverse: -Wno-redundant-constraints :category: :since: 8.0 .. index:: 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. When turning on, you can suppress it on a per-module basis with :ghc-flag:`-Wno-redundant-constraints <-Wredundant-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. .. ghc-flag:: -Wduplicate-exports :shortdesc: warn when an entity is exported multiple times :type: dynamic :reverse: -Wno-duplicate-exports :category: :since: at least 5.04 :default: on .. index:: 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. .. ghc-flag:: -Whi-shadowing :shortdesc: *(deprecated)* warn when a ``.hi`` file in the current directory shadows a library :type: dynamic :reverse: -Wno-hi-shadowing :category: :since: at least 5.04, deprecated .. index:: 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. This flag was not implemented correctly and is now deprecated. It will be removed in a later version of GHC. .. ghc-flag:: -Widentities :shortdesc: warn about uses of Prelude numeric conversions that are probably the identity (and hence could be omitted) :type: dynamic :reverse: -Wno-identities :category: :since: 7.2 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``. .. ghc-flag:: -Wimplicit-kind-vars :shortdesc: *(deprecated)* warn when kind variables are implicitly quantified over. :type: dynamic :reverse: -Wno-implicit-kind-vars :category: :since: 8.6 This warning is deprecated. It no longer has any effect since GHC 8.10. It was used to detect if a kind variable is not explicitly quantified over. For instance, the following would produce a warning: :: f :: forall (a :: k). Proxy a This is now an error and can be fixed by explicitly quantifying over ``k``: :: f :: forall k (a :: k). Proxy a or :: f :: forall {k} (a :: k). Proxy a .. ghc-flag:: -Wimplicit-lift :shortdesc: warn about implicit ``lift`` in Template Haskell quotes :type: dynamic :reverse: -Wno-implicit-lift :category: warnings :since: 9.2 Template Haskell quotes referring to local variables bound outside of the quote are implicitly converted to use ``lift``. For example, ``f x = [| reverse x |]`` becomes ``f x = [| reverse $(lift x) |])``. This flag issues a warning for every such implicit addition of ``lift``. This can be useful when debugging more complex staged programs, where an implicit ``lift`` can accidentally conceal a variable used at a wrong stage. .. ghc-flag:: -Wimplicit-prelude :shortdesc: warn when the Prelude is implicitly imported :type: dynamic :reverse: -Wno-implicit-prelude :category: :since: 6.8 :default: off .. index:: 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 :extension:`NoImplicitPrelude` or a ``LANGUAGE NoImplicitPrelude`` pragma). Note that no warning is given for syntax that implicitly refers to the Prelude, even if :extension:`NoImplicitPrelude` 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). .. ghc-flag:: -Wincomplete-patterns :shortdesc: warn when a pattern match could fail :type: dynamic :reverse: -Wno-incomplete-patterns :category: :since: 5.04 .. index:: single: incomplete patterns, warning single: patterns, incomplete The option :ghc-flag:`-Wincomplete-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 :ghc-flag:`-Wincomplete-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 :ghc-flag:`-W`. .. ghc-flag:: -Wincomplete-uni-patterns :shortdesc: warn when a pattern match in a lambda expression, pattern binding or a lazy pattern could fail :type: dynamic :reverse: -Wno-incomplete-uni-patterns :category: :since: 7.2 The flag :ghc-flag:`-Wincomplete-uni-patterns` is similar to :ghc-flag:`-Wincomplete-patterns`, except that it applies only to lambda-expressions and pattern bindings, constructs that only allow a single pattern: :: h = \[] -> 2 Just k = f y Furthermore, this flag also applies to lazy patterns, since they are syntactic sugar for pattern bindings. For example, ``f ~(Just x) = (x,x)`` is equivalent to ``f y = let Just x = y in (x,x)``. .. ghc-flag:: -fmax-pmcheck-models=⟨n⟩ :shortdesc: soft limit on the number of parallel models the pattern match checker should check a pattern match clause against :type: dynamic :category: :since: 8.10 :default: 30 The pattern match checker works by assigning symbolic values to each pattern. We call each such assignment a 'model'. Now, each pattern match clause leads to potentially multiple splits of that model, encoding different ways for the pattern match to fail. For example, when matching ``x`` against ``Just 4``, we split each incoming matching model into two uncovered sub-models: One where ``x`` is ``Nothing`` and one where ``x`` is ``Just y`` but ``y`` is not ``4``. This can be exponential in the arity of the pattern and in the number of guards in some cases. The :ghc-flag:`-fmax-pmcheck-models=⟨n⟩` limit makes sure we scale polynomially in the number of patterns, by forgetting refined information gained from a partially successful match. For the above example, if we had a limit of 1, we would continue checking the next clause with the original, unrefined model. .. ghc-flag:: -Wincomplete-record-updates :shortdesc: warn when a record update could fail :type: dynamic :reverse: -Wno-incomplete-record-updates :category: :since: 6.4 .. index:: 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 :ghc-flag:`-Wincomplete-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. .. ghc-flag:: -Wmissing-deriving-strategies :shortdesc: warn when a deriving clause is missing a deriving strategy :type: dynamic :reverse: -Wno-missing-deriving-strategies :category: :since: 8.8.1 :default: off The datatype below derives the ``Eq`` typeclass, but doesn't specify a strategy. When :ghc-flag:`-Wmissing-deriving-strategies` is enabled, the compiler will emit a warning about this. :: data Foo a = Foo a deriving (Eq) The compiler will warn here that the deriving clause doesn't specify a strategy. If the warning is enabled, but :extension:`DerivingStrategies` is not enabled, the compiler will suggest turning on the :extension:`DerivingStrategies` extension. .. ghc-flag:: -Wmissing-fields :shortdesc: warn when fields of a record are uninitialised :type: dynamic :reverse: -Wno-missing-fields :category: :since: at least 5.04 .. index:: 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. .. ghc-flag:: -Wmissing-export-lists :shortdesc: warn when a module declaration does not explicitly list all exports :type: dynamic :reverse: -Wno-missing-export-lists :category: :since: 8.4 .. index:: single: missing export lists, warning single: export lists, missing This flag warns if you declare a module without declaring an explicit export list. For example :: module M where p x = x The :ghc-flag:`-Wmissing-export-lists` flag will warn that ``M`` does not declare an export list. Declaring an explicit export list for ``M`` enables GHC dead code analysis, prevents accidental export of names and can ease optimizations like inlining. .. ghc-flag:: -Wmissing-import-lists :shortdesc: warn when an import declaration does not explicitly list all the names brought into scope :type: dynamic :reverse: -Wno-missing-import-lists :category: :since: 7.0 .. index:: 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 :ghc-flag:`-Wmissing-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``. .. ghc-flag:: -Wmissing-methods :shortdesc: warn when class methods are undefined :type: dynamic :reverse: -Wno-missing-methods :category: :since: at least 5.04 :default: on .. index:: single: missing methods, warning single: methods, missing This option warns you whenever an instance declaration is missing one or more methods, and the corresponding class declaration has no default declaration for them. 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`. .. ghc-flag:: -Wmissing-signatures :shortdesc: warn about top-level functions without signatures :type: dynamic :reverse: -Wno-missing-signatures :category: :since: at least 5.04 :default: off .. index:: single: type signatures, missing If you would like GHC to check that every top-level function/value has a type signature, use the :ghc-flag:`-Wmissing-signatures` option. As part of the warning GHC also reports the inferred type. .. ghc-flag:: -Wmissing-exported-sigs :shortdesc: *(deprecated)* warn about top-level functions without signatures, only if they are exported. takes precedence over -Wmissing-signatures :type: dynamic :reverse: -Wno-missing-exported-sigs :category: :since: 7.10 .. index:: single: type signatures, missing This option is now deprecated in favour of :ghc-flag:`-Wmissing-exported-signatures`. .. ghc-flag:: -Wmissing-exported-signatures :shortdesc: warn about top-level functions without signatures, only if they are exported :type: dynamic :reverse: -Wno-missing-exported-signatures :category: :since: 8.0 :default: off .. index:: 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 :ghc-flag:`-Wmissing-exported-signatures` option. If this option is used in conjunction with :ghc-flag:`-Wmissing-signatures` then every top-level function/value must have a type signature. As part of the warning GHC also reports the inferred type. .. ghc-flag:: -Wmissing-local-sigs :shortdesc: *(deprecated)* warn about polymorphic local bindings without signatures :type: dynamic :reverse: -Wno-missing-local-sigs :category: :since: 7.0 .. index:: single: type signatures, missing This option is now deprecated in favour of :ghc-flag:`-Wmissing-local-signatures`. .. ghc-flag:: -Wmissing-local-signatures :shortdesc: warn about polymorphic local bindings without signatures :type: dynamic :reverse: -Wno-missing-local-signatures :category: :since: 8.0 .. index:: single: type signatures, missing If you use the :ghc-flag:`-Wmissing-local-signatures` 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. .. ghc-flag:: -Wmissing-pattern-synonym-signatures :shortdesc: warn when pattern synonyms do not have type signatures :type: dynamic :reverse: -Wno-missing-pattern-synonym-signatures :category: :since: 8.0 :default: off .. index:: single: type signatures, missing, pattern synonyms If you would like GHC to check that every pattern synonym has a type signature, use the :ghc-flag:`-Wmissing-pattern-synonym-signatures` option. If this option is used in conjunction with :ghc-flag:`-Wmissing-exported-signatures` then only exported pattern synonyms must have a type signature. GHC also reports the inferred type. .. ghc-flag:: -Wmissing-kind-signatures :shortdesc: warn when type declarations don't have kind signatures nor CUSKs :type: dynamic :reverse: -Wno-missing-kind-signatures :category: :since: 9.2 :default: off .. index:: single: kind signatures, missing If you would like GHC to check that every data, type family, type-class definition has a :ref:`standalone kind signature ` or a :ref:`CUSK `, use the :ghc-flag:`-Wmissing-kind-signatures` option. You can specify the kind via :extension:`StandaloneKindSignatures` or :extension:`CUSKs`. Note that :ghc-flag:`-Wmissing-kind-signatures` does not warn about associated type families, as GHC considers an associated type family declaration to have a CUSK if its enclosing class has a CUSK. (See :ref:`complete-kind-signatures` for more on this point.) Therefore, giving the parent class a standalone kind signature or CUSK is sufficient to fix the warning for the class's associated type families as well. .. ghc-flag:: -Wmissing-poly-kind-signatures :shortdesc: warn when inferred polykinded type or class declaration don't have kind signatures nor CUSKs :type: dynamic :reverse: -Wno-missing-poly-kind-signatures :category: :since: 9.8 :default: off .. index:: single: kind signatures, missing This is a restricted version of :ghc-flag:`-Wmissing-kind-signatures`. It warns when a declaration defines a type constructor that lacks a :ref:`standalone kind signature ` and whose inferred kind is polymorphic (which happens with `-PolyKinds`. For example :: data T a = MkT (a -> Int) -- T :: Type -> Type -- Not polymorphic, hence no warning data W f a = MkW (f a) -- W :: forall k. (k->Type) -> k -> Type -- Polymorphic, hence warning! It is useful to catch accidentally polykinded types, or to make that polymorphism explicit, without requiring a kind signature for every type. .. ghc-flag:: -Wmissing-exported-pattern-synonym-signatures :shortdesc: warn about pattern synonyms without signatures, only if they are exported :type: dynamic :reverse: -Wno-missing-exported-pattern-synonym-signatures :category: :default: off .. index:: single: type signatures, missing, pattern synonyms If you would like GHC to check that every exported pattern synonym has a type signature, but not check unexported pattern synonyms, use the :ghc-flag:`-Wmissing-exported-pattern-synonym-signatures` option. If this option is used in conjunction with :ghc-flag:`-Wmissing-pattern-synonym-signatures` then every pattern synonym must have a type signature. As part of the warning GHC also reports the inferred type. .. ghc-flag:: -Wname-shadowing :shortdesc: warn when names are shadowed :type: dynamic :reverse: -Wno-name-shadowing :category: :since: at least 5.04 .. index:: 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) } .. ghc-flag:: -Worphans :shortdesc: warn when the module contains :ref:`orphan instance declarations or rewrite rules ` :type: dynamic :reverse: -Wno-orphans :category: :since: 6.4 .. index:: 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 :ghc-flag:`-Worphans` warns about user-written orphan rules or instances. .. ghc-flag:: -Woverlapping-patterns :shortdesc: warn about overlapping patterns :type: dynamic :reverse: -Wno-overlapping-patterns :category: :since: at least 5.04 .. index:: 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. If the programmer is dead set on keeping a redundant clause, for example to prevent bitrot, they can make use of a guard scrutinising ``GHC.Exts.considerAccessible`` to prevent the checker from flagging the parent clause as redundant: :: g :: String -> Int g [] = 0 g (_:xs) = 1 g "2" | considerAccessible = 2 -- No warning! Note that ``considerAccessible`` should come as the last statement of the guard in order not to impact the results of the checker. E.g., if you write :: h :: Bool -> Int h x = case (x, x) of (True, True) -> 1 (False, False) -> 2 (True, False) | considerAccessible, False <- x -> 3 The pattern-match checker takes you by your word, will conclude that ``False <- x`` might fail and warn that the pattern-match is inexhaustive. Put ``considerAccessible`` last to avoid such confusions. Note that due to technical limitations, ``considerAccessible`` will not suppress :ghc-flag:`-Winaccessible-code` warnings. .. ghc-flag:: -Winaccessible-code :shortdesc: warn about inaccessible code :type: dynamic :reverse: -Wno-inaccessible-code :category: :since: 8.6 .. index:: single: inaccessible code, warning single: inaccessible By default, the compiler will warn you if types make a branch inaccessible. This generally requires GADTs or similar extensions. Take, for example, the following program :: {-# LANGUAGE GADTs #-} data Foo a where Foo1 :: Foo Char Foo2 :: Foo Int data TyEquality a b where Refl :: TyEquality a a checkTEQ :: Foo t -> Foo u -> Maybe (TyEquality t u) checkTEQ x y = error "unimportant" step2 :: Bool step2 = case checkTEQ Foo1 Foo2 of Just Refl -> True -- Inaccessible code Nothing -> False The ``Just Refl`` case in ``step2`` is inaccessible, because in order for ``checkTEQ`` to be able to produce a ``Just``, ``t ~ u`` must hold, but since we're passing ``Foo1`` and ``Foo2`` here, it follows that ``t ~ Char``, and ``u ~ Int``, and thus ``t ~ u`` cannot hold. .. ghc-flag:: -Wstar-is-type :shortdesc: warn when ``*`` is used to mean ``Data.Kind.Type`` :type: dynamic :reverse: -Wno-star-is-type :category: :since: 8.6 The use of ``*`` to denote the kind of inhabited types relies on the :extension:`StarIsType` extension, which in a future release will be turned off by default and then possibly removed. The reasons for this and the deprecation schedule are described in `GHC proposal #143 `__. This warning allows to detect such uses of ``*`` before the actual breaking change takes place. The recommended fix is to replace ``*`` with ``Type`` imported from ``Data.Kind``. .. ghc-flag:: -Wstar-binder :shortdesc: warn about binding the ``(*)`` type operator despite :extension:`StarIsType` :type: dynamic :reverse: -Wno-star-binder :since: 8.6 Under :extension:`StarIsType`, a ``*`` in types is not an operator nor even a name, it is special syntax that stands for ``Data.Kind.Type``. This means that an expression like ``Either * Char`` is parsed as ``Either (*) Char`` and not ``(*) Either Char``. In binding positions, we have similar parsing rules. Consider the following example :: {-# LANGUAGE TypeOperators, TypeFamilies, StarIsType #-} type family a + b type family a * b While ``a + b`` is parsed as ``(+) a b`` and becomes a binding position for the ``(+)`` type operator, ``a * b`` is parsed as ``a (*) b`` and is rejected. As a workaround, we allow to bind ``(*)`` in prefix form:: type family (*) a b This is a rather fragile arrangement, as generally a programmer expects ``(*) a b`` to be equivalent to ``a * b``. With :ghc-flag:`-Wstar-binder` we warn when this special treatment of ``(*)`` takes place. .. ghc-flag:: -Wsimplifiable-class-constraints :shortdesc: Warn about class constraints in a type signature that can be simplified using a top-level instance declaration. :type: dynamic :reverse: -Wno-simplifiable-class-constraints :category: :since: 8.2 :default: on .. index:: single: simplifiable class constraints, warning Warn about class constraints in a type signature that can be simplified using a top-level instance declaration. For example: :: f :: Eq [a] => a -> a Here the ``Eq [a]`` in the signature overlaps with the top-level instance for ``Eq [a]``. GHC goes to some efforts to use the former, but if it should use the latter, it would then have an insoluble ``Eq a`` constraint. Best avoided by instead writing: :: f :: Eq a => a -> a .. ghc-flag:: -Wtabs :shortdesc: warn if there are tabs in the source file :type: dynamic :reverse: -Wno-tabs :category: :since: 6.8 .. index:: single: tabs, warning Have the compiler warn if there are tabs in your source file. .. ghc-flag:: -Wtype-defaults :shortdesc: warn when defaulting happens :type: dynamic :reverse: -Wno-type-defaults :category: :since: at least 5.04 :default: off .. index:: 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. .. ghc-flag:: -Wmonomorphism-restriction :shortdesc: warn when the Monomorphism Restriction is applied :type: dynamic :reverse: -Wno-monomorphism-restriction :category: :since: 6.8 :default: off .. index:: 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. .. ghc-flag:: -Wunsupported-llvm-version :shortdesc: Warn when using :ghc-flag:`-fllvm` with an unsupported version of LLVM. :type: dynamic :reverse: -Wno-unsupported-llvm-version :category: :since: 7.8 Warn when using :ghc-flag:`-fllvm` with an unsupported version of LLVM. .. ghc-flag:: -Wmissed-extra-shared-lib :shortdesc: Warn when GHCi can't load a shared lib. :type: dynamic :reverse: -Wno-missed-extra-shared-lib :category: :since: 8.8 Warn when GHCi can't load a shared lib it deduced it should load when loading a package and analyzing the extra-libraries stanza of the target package description. .. ghc-flag:: -Wunticked-promoted-constructors :shortdesc: warn if promoted constructors are not ticked :type: dynamic :reverse: -Wno-unticked-promoted-constructors :category: :since: 7.10 .. index:: 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 also applies to list literals since 9.4. For example: :: type L = [Int, Char, Bool] will raise a warning, because ``[Int, Char, Bool]`` is a promoted list which lacks a tick. .. ghc-flag:: -Wunused-binds :shortdesc: warn about bindings that are unused. Alias for :ghc-flag:`-Wunused-top-binds`, :ghc-flag:`-Wunused-local-binds` and :ghc-flag:`-Wunused-pattern-binds` :type: dynamic :reverse: -Wno-unused-binds :category: :since: at least 5.04 .. index:: single: unused binds, warning single: binds, unused Report any function definitions (and local bindings) which are unused. An alias for - :ghc-flag:`-Wunused-top-binds` - :ghc-flag:`-Wunused-local-binds` - :ghc-flag:`-Wunused-pattern-binds` .. ghc-flag:: -Wunused-top-binds :shortdesc: warn about top-level bindings that are unused :type: dynamic :reverse: -Wno-unused-top-binds :category: :since: 8.0 .. index:: 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 .. ghc-flag:: -Wunused-local-binds :shortdesc: warn about local bindings that are unused :type: dynamic :reverse: -Wno-unused-local-binds :category: :since: 8.0 .. index:: 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 .. ghc-flag:: -Wunused-pattern-binds :shortdesc: warn about pattern match bindings that are unused :type: dynamic :reverse: -Wno-unused-pattern-binds :category: :since: 8.0 .. index:: single: unused binds, warning single: binds, unused Warn if a pattern binding binds no variables at all, unless it is a lone wild-card pattern, or a banged 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 pattern; behaves like seq In general a lazy pattern binding `p = e` is a no-op if `p` does not bind any variables. 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 banged pattern (see :ref:`bang-patterns`) is *not* a no-op, because it forces evaluation, and is useful as an alternative to ``seq``. .. ghc-flag:: -Wunused-imports :shortdesc: warn about unnecessary imports :type: dynamic :reverse: -Wno-unused-imports :category: :since: at least 5.04 .. index:: 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. .. ghc-flag:: -Wunused-matches :shortdesc: warn about variables in patterns that aren't used :type: dynamic :reverse: -Wno-unused-matches :category: :since: at least 5.04 .. index:: single: unused matches, warning single: matches, unused Report all unused variables which arise from term-level 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 Note that :ghc-flag:`-Wunused-matches` does not warn about variables which arise from type-level patterns, as found in type family and data family instances. This must be enabled separately through the :ghc-flag:`-Wunused-type-patterns` flag. .. ghc-flag:: -Wunused-do-bind :shortdesc: warn about do bindings that appear to throw away values of types other than ``()`` :type: dynamic :reverse: -Wno-unused-do-bind :category: :since: 6.12 .. index:: 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 } .. ghc-flag:: -Wunused-type-patterns :shortdesc: warn about unused type variables which arise from patterns in in type family and data family instances :type: dynamic :reverse: -Wno-unused-type-patterns :category: :since: 8.0 .. index:: single: unused type patterns, warning single: type patterns, unused Report all unused implicitly bound type variables which arise from patterns in type family and data family instances. For instance: :: type instance F x y = [] would report ``x`` and ``y`` as unused on the right hand side. The warning is suppressed if the type variable name begins with an underscore, like so: :: type instance F _x _y = [] When :extension:`ExplicitForAll` is enabled, explicitly quantified type variables may also be identified as unused. For instance: :: type instance forall x y. F x y = [] would still report ``x`` and ``y`` as unused on the right hand side Unlike :ghc-flag:`-Wunused-matches`, :ghc-flag:`-Wunused-type-patterns` is not implied by :ghc-flag:`-Wall`. The rationale for this decision is that unlike term-level pattern names, type names are often chosen expressly for documentation purposes, so using underscores in type names can make the documentation harder to read. .. ghc-flag:: -Wunused-foralls :shortdesc: warn about type variables in user-written ``forall``\\s that are unused :type: dynamic :reverse: -Wno-unused-foralls :category: :since: 8.0 .. index:: single: unused foralls, warning single: foralls, unused Report all unused type variables which arise from explicit, user-written ``forall`` statements. For instance: :: g :: forall a b c. (b -> b) would report ``a`` and ``c`` as unused. .. ghc-flag:: -Wunused-record-wildcards :shortdesc: Warn about record wildcard matches when none of the bound variables are used. :type: dynamic :reverse: -Wno-unused-record-wildcards :category: :since: 8.10 .. index:: single: unused, warning, record wildcards Report all record wildcards where none of the variables bound implicitly are used. For instance: :: data P = P { x :: Int, y :: Int } f1 :: P -> Int f1 P{..} = 1 + 3 would report that the ``P{..}`` match is unused. .. ghc-flag:: -Wredundant-bang-patterns :shortdesc: Warn about redundant bang patterns. :type: dynamic :reverse: -Wno-redundant-bang-patterns :category: :since: 9.2 .. index:: single: redundant, warning, bang patterns Report dead bang patterns, where dead bangs are bang patterns that under no circumstances can force a thunk that wasn't already forced. Dead bangs are a form of redundant bangs. The new check is performed in pattern-match coverage checker along with other checks (namely, redundant and inaccessible RHSs). Given :: f :: Bool -> Int f True = 1 f !x = 2 The bang pattern on ``!x`` is dead. By the time the ``x`` in the second equation is reached, ``x`` will already have been forced due to the first equation (``f True = 1``). Moreover, there is no way to reach the second equation without going through the first one. Note that ``-Wredundant-bang-patterns`` will not warn about dead bangs that appear on a redundant clause. That is because in that case, it is recommended to delete the clause wholly, including its leading pattern match. Dead bang patterns are redundant. But there are bang patterns which are redundant that aren't dead, for example: :: f !() = 0 the bang still forces the argument, before we attempt to match on ``()``. But it is redundant with the forcing done by the ``()`` match. Currently such redundant bangs are not considered dead, and ``-Wredundant-bang-patterns`` will not warn about them. .. ghc-flag:: -Wredundant-record-wildcards :shortdesc: Warn about record wildcard matches when the wildcard binds no patterns. :type: dynamic :reverse: -Wno-redundant-record-wildcards :category: :since: 8.10 .. index:: single: unused, warning, record wildcards Report all record wildcards where the wild card match binds no patterns. For instance: :: data P = P { x :: Int, y :: Int } f1 :: P -> Int f1 P{x,y,..} = x + y would report that the ``P{x, y, ..}`` match has a redundant use of ``..``. .. ghc-flag:: -Wredundant-strictness-flags :shortdesc: Warn about redundant strictness flags. :type: dynamic :reverse: -Wno-redundant-strictness-flags :category: :since: 9.4 Report strictness flags applied to unlifted types. An unlifted type is always strict, and applying a strictness flag has no effect. For example: :: data T = T !Int# .. ghc-flag:: -Wwrong-do-bind :shortdesc: warn about do bindings that appear to throw away monadic values that you should have bound instead :type: dynamic :reverse: -Wno-wrong-do-bind :category: :since: 6.12 .. index:: 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 } .. ghc-flag:: -Winline-rule-shadowing :shortdesc: 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`. :type: dynamic :reverse: -Wno-inline-rule-shadowing :category: :since: 7.8 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`. .. ghc-flag:: -Wcpp-undef :shortdesc: warn on uses of the `#if` directive on undefined identifiers :type: dynamic :category: :since: 8.2 This flag passes ``-Wundef`` to the C pre-processor (if its being used) which causes the pre-processor to warn on uses of the `#if` directive on undefined identifiers. .. ghc-flag:: -Wunbanged-strict-patterns :shortdesc: warn on pattern bind of unlifted variable that is neither bare nor banged :type: dynamic :reverse: -Wno-unbanged-strict-patterns :category: :since: 8.2 This flag warns whenever you write a pattern that binds a variable whose type is unlifted, and yet the pattern is not a bang pattern nor a bare variable. See :ref:`glasgow-unboxed` for information about unlifted types. .. ghc-flag:: -Wmissing-home-modules :shortdesc: warn when encountering a home module imported, but not listed on the command line. Useful for cabal to ensure GHC won't pick up modules, not listed neither in ``exposed-modules``, nor in ``other-modules``. :type: dynamic :reverse: -Wno-missing-home-modules :category: :since: 8.2 When a module provided by the package currently being compiled (i.e. the "home" package) is imported, but not explicitly listed in command line as a target. Useful for Cabal to ensure GHC won't pick up modules, not listed neither in ``exposed-modules``, nor in ``other-modules``. .. ghc-flag:: -Wpartial-fields :shortdesc: warn when defining a partial record field. :type: dynamic :reverse: -Wno-partial-fields :category: :since: 8.4 The option :ghc-flag:`-Wpartial-fields` warns about a record field `f` that is defined in some, but not all, the contructors of a data type, because `f`'s record selector function may fail. For exampe, the record selector function `f`, defined in the `Foo` constructor record below, will fail when applied to ``Bar``, so the compiler will emit a warning at its definition when :ghc-flag:`-Wpartial-fields` is enabled. The warning is suppressed if the field name begins with an underscore. :: data Foo = Foo { f :: Int } | Bar .. ghc-flag:: -Wunused-packages :shortdesc: warn when package is requested on command line, but not needed. :type: dynamic :reverse: -Wno-unused-packages :category: :since: 8.10 The option :ghc-flag:`-Wunused-packages` warns about packages, specified on command line via :ghc-flag:`-package ⟨pkg⟩` or :ghc-flag:`-package-id ⟨unit-id⟩`, but were not needed during compilation. If the warning fires it means the specified package wasn't needed for compilation. This warning interacts poorly with GHCi because most invocations will pass a large number of ``-package`` arguments on the initial load. Therefore if you modify the targets using ``:load`` or ``:cd`` then the warning will be silently disabled if it's enabled (see :ghc-ticket:`21110`). .. ghc-flag:: -Winvalid-haddock :shortdesc: warn when a Haddock comment occurs in an invalid position :type: dynamic :reverse: -Wno-invalid-haddock :category: :since: 9.0 When the ``-haddock`` option is enabled, GHC collects documentation comments and associates them with declarations, function arguments, data constructors, and other syntactic elements. Documentation comments in invalid positions are discarded:: myValue = -- | Invalid (discarded) comment in an expression 2 + 2 This warning informs you about discarded documentation comments. It has no effect when :ghc-flag:`-haddock` is disabled. .. ghc-flag:: -Woperator-whitespace-ext-conflict :shortdesc: warn on uses of infix operators that would be parsed differently were a particular GHC extension enabled :type: dynamic :reverse: -Wno-operator-whitespace-ext-conflict :category: :since: 9.2 When :extension:`TemplateHaskell` is enabled, ``f $x`` is parsed as ``f`` applied to an untyped splice. But when the extension is disabled, the expression is parsed as a use of the ``$`` infix operator. To make it easy to read ``f $x`` without checking the enabled extensions, one could rewrite it as ``f $ x``, which is what this warning suggests. Currently, it detects the following cases: * ``$x`` could mean an untyped splice under :extension:`TemplateHaskell` * ``$$x`` could mean a typed splice under :extension:`TemplateHaskell` * ``%m`` could mean a multiplicity annotation under :extension:`LinearTypes` It only covers extensions that currently exist. If you want to enforce a stricter policy and always require whitespace around all infix operators, use :ghc-flag:`-Woperator-whitespace`. .. ghc-flag:: -Woperator-whitespace :shortdesc: warn on prefix, suffix, and tight infix uses of infix operators :type: dynamic :reverse: -Wno-operator-whitespace :category: :since: 9.2 There are four types of infix operator occurrences, as defined by `GHC Proposal #229 `__:: a ! b -- a loose infix occurrence a!b -- a tight infix occurrence a !b -- a prefix occurrence a! b -- a suffix occurrence A loose infix occurrence of any operator is always parsed as an infix operator, but other occurrence types may be assigned a special meaning. For example, a prefix ``!`` denotes a bang pattern, and a prefix ``$`` denotes a :extension:`TemplateHaskell` splice. This warning encourages the use of loose infix occurrences of all infix operators, to prevent possible conflicts with future language extensions. .. ghc-flag:: -Wauto-orphans :shortdesc: *(deprecated)* Does nothing :type: dynamic :since: 7.4 Does nothing. .. ghc-flag:: -Wmissing-space-after-bang :shortdesc: *(deprecated)* Does nothing :type: dynamic :since: 8.8 Does nothing. .. ghc-flag:: -Wderiving-typeable :shortdesc: warn when Typeable is derived :type: dynamic :reverse: -Wno-deriving-typeable :category: :since: 7.10 This flag warns when ``Typeable`` is listed in a deriving clause or derived with :extension:`StandaloneDeriving`. Since GHC 7.10, ``Typeable`` is automatically derived for all types. Thus, deriving ``Typeable`` yourself is redundant. .. ghc-flag:: -Wambiguous-fields :shortdesc: warn about ambiguous field selectors or updates :type: dynamic :category: :since: 9.2 When :extension:`DuplicateRecordFields` is enabled, the option :ghc-flag:`-Wambiguous-fields` warns about occurrences of fields in selectors or updates that depend on the deprecated mechanism for type-directed disambiguation. This mechanism will be removed in a future GHC release, at which point these occurrences will be rejected as ambiguous. See the proposal `DuplicateRecordFields without ambiguous field access `_ and the documentation on :extension:`DuplicateRecordFields` for further details. This warning has no effect when :extension:`DuplicateRecordFields` is disabled. .. ghc-flag:: -Wforall-identifier :shortdesc: warn when ``forall`` is used as an identifier (at definition sites) :type: dynamic :reverse: -Wno-forall-identifier :since: 9.4 In a future GHC release, ``forall`` will become a keyword regardless of enabled extensions. This will make definitions such as the following illegal:: -- from constraints-0.13 forall :: forall p. (forall a. Dict (p a)) -> Dict (Forall p) forall d = ... Library authors are advised to use a different identifier, such as ``forAll``, ``forall_``, or ``for_all``:: forall_ :: forall p. (forall a. Dict (p a)) -> Dict (Forall p) forall_ d = ... The warning is only triggered at definition sites where it can be addressed by using a different name. Users of a library that exports ``forall`` as an identifier cannot address the issue themselves, so the warning is not reported at use sites. .. ghc-flag:: -Wunicode-bidirectional-format-characters :shortdesc: warn about the usage of unicode bidirectional layout override characters :type: dynamic :category: :since: 9.0.2 Explicit unicode bidirectional formatting characters can cause source code to be rendered misleadingly in many viewers. We warn if any such character is present in the source. Specifically, the characters disallowed by this warning are those which are a part of the 'Explicit Formatting` category of the `Unicode Bidirectional Character Type Listing `_ .. ghc-flag:: -Wgadt-mono-local-binds :shortdesc: warn when pattern matching on a GADT without MonoLocalBinds :type: dynamic :reverse: -Wno-gadt-mono-local-binds :since: 9.4.1 This warning is triggered on pattern matching involving GADTs, if :extension:`MonoLocalBinds` is disabled. Type inference can be fragile in this case. See the `OutsideIn(X) `__ paper (section 4.2) and :ref:`mono-local-binds` for more details. To resolve this warning, you can enable :extension:`MonoLocalBinds` or an extension implying it (:extension:`GADTs` or :extension:`TypeFamilies`). The warning is also triggered when matching on GADT-like pattern synonyms (i.e. pattern synonyms containing equalities in provided constraints). In previous versions of GHC (9.2 and below), it was an error to pattern match on a GADT if neither :extension:`GADTs` nor :extension:`TypeFamilies` were enabled. .. ghc-flag:: -Wtype-equality-out-of-scope :shortdesc: warn when type equality ``a ~ b`` is used despite being out of scope :type: dynamic :reverse: -Wno-type-equality-out-of-scope :since: 9.4.1 In accordance with `GHC Proposal #371 `__, the type equality syntax ``a ~ b`` is no longer built-in. Instead, ``~`` is a regular type operator that can be imported from ``Data.Type.Equality`` or ``Prelude``. To minimize breakage, a compatibility fallback is provided: whenever ``~`` is used but is not in scope, the compiler assumes that it stands for a type equality constraint. The warning is triggered by any code that relies on this fallback. It can be addressed by bringing ``~`` into scope explicitly. The likely culprit is that you use :extension:`NoImplicitPrelude` and a custom Prelude. In this case, consider updating your custom Prelude to re-export ``~`` from ``Data.Type.Equality``. Being part of the :ghc-flag:`-Wcompat` option group, this warning is off by default, but will be switched on in a future GHC release. .. ghc-flag:: -Wtype-equality-requires-operators :shortdesc: warn when type equality ``a ~ b`` is used despite being out of scope :type: dynamic :reverse: -Wno-type-equality-requires-operators :since: 9.4.1 In accordance with `GHC Proposal #371 `__, the type equality syntax ``a ~ b`` is no longer built-in. Instead, ``~`` is a regular type operator that requires the :extension:`TypeOperators` extension. To minimize breakage, ``~`` specifically (unlike other type operators) can be used even when :extension:`TypeOperators` is disabled. The warning is triggered whenever this happens, and can be addressed by enabling the extension. .. ghc-flag:: -Wloopy-superclass-solve :shortdesc: warn when creating potentially-loopy superclass constraint evidence :type: dynamic :reverse: -Wno-loopy-superclass-solve :since: 9.6.1 As explained in :ref:`undecidable-instances`, when using :extension:`UndecidableInstances` it is possible for GHC to construct non-terminating evidence for certain superclass constraints. This behaviour is scheduled to be removed in a future GHC version. In the meantime, GHC emits this warning to inform users of potential non-termination. Users can manually add the required constraint to the context to avoid the problem (thus silencing the warning). .. ghc-flag:: -Wterm-variable-capture :shortdesc: warn when an implicitly quantified type variable captures a term's name :type: dynamic :since: 9.8.1 In accordance with `GHC Proposal #281 `__, a new extension ``RequiredTypeArguments`` will be introduced in a future GHC release. Under ``RequiredTypeArguments``, implicit quantification of type variables does not take place if there is a term variable of the same name in scope. For example: :: a = 15 f :: a -> a -- Does ‘a’ refer to the term-level binding -- or is it implicitly quantified? When :ghc-flag:`-Wterm-variable-capture` is enabled, GHC warns against implicit quantification that would stop working under ``RequiredTypeArguments``. .. ghc-flag:: -Wmissing-role-annotations :shortdesc: warn when type declarations don't have role annotations :type: dynamic :reverse: -Wno-role-annotations-signatures :category: :since: 9.8.1 :default: off .. index:: single: roles, missing If you would like GHC to check that every data type definition has a :ref:`role annotation `, use the :ghc-flag:`-Wmissing-role-annotations` option. You can specify the role via :extension:`RoleAnnotations`. GHC will not warn about type class definitions with missing role annotations, as their default roles are the strictest: all nominal. In other words the type-class role cannot be accidentally left representational or phantom, which could affected the code correctness. .. ghc-flag:: -Wimplicit-rhs-quantification :shortdesc: warn when type variables on the RHS of a type synonym are implicitly quantified :type: dynamic :reverse: -Wno-implicit-rhs-quantification :category: :since: 9.8.1 :default: off In accordance with `GHC Proposal #425 `__, GHC will stop implicitly quantifying over type variables that occur free on the right-hand side of a type synonym but are not mentioned on the left-hand side. Type synonym declarations that rely on this form of quantification should be rewritten with invisible binders. For example: :: type T1 :: forall a . Maybe a type T1 = 'Nothing :: Maybe a -- old type T1 @a = 'Nothing :: Maybe a -- new This warning detects code that will be affected by this breaking change. .. ghc-flag:: -Wincomplete-export-warnings :shortdesc: warn when some but not all of exports for a name are warned about :type: dynamic :reverse: -Wno-incomplete-export-warnings :since: 9.8.1 Ino accordance with `GHC Proposal #134 `__, it is now possible to deprecate certain exports of a name without deprecating the name itself. As explained in :ref:`warning-deprecated-pragma`, when a name is exported in several ways in the same module, but only some of those ways have a warning, it will not end up deprecated when imported in another module. For example: :: module A (x) where x :: Int x = 2 module M ( {-# WARNING x "deprecated" #-} x module A ) import A When :ghc-flag:`-Wincomplete-export-warnings` is enabled, GHC warns about exports that are not deprecating a name that is deprecated with another export in that module. .. ghc-flag:: -Winconsistent-flags :shortdesc: warn when command line options are inconsistent in some way. :type: dynamic :reverse: -Wno-inconsistent-flags :since: 9.8.1 :default: on Warn when command line options are inconsistent in some way. For example, when using GHCi, optimisation flags are ignored and a warning is issued. Another example is :ghc-flag:`-dynamic` is ignored when :ghc-flag:`-dynamic-too` is passed. .. ghc-flag:: -Winconsistent-flags :shortdesc: warn when command line options are inconsistent in some way. :type: dynamic :reverse: -Wno-inconsistent-flags :since: 9.8.1 :default: on Warn when command line options are inconsistent in some way. For example, when using GHCi, optimisation flags are ignored and a warning is issued. Another example is :ghc-flag:`-dynamic` is ignored when :ghc-flag:`-dynamic-too` is passed. If you're feeling really paranoid, the :ghc-flag:`-dcore-lint` option is a good choice. It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's sanity, not yours.)