.. _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:`-Wgadt-mono-local-binds`
* :ghc-flag:`-Wtype-equality-requires-operators`
* :ghc-flag:`-Wtype-equality-out-of-scope`
* :ghc-flag:`-Wbadly-staged-types`
* :ghc-flag:`-Winconsistent-flags`
* :ghc-flag:`-Wnoncanonical-monoid-instances`
* :ghc-flag:`-Wnoncanonical-monad-instances`
* :ghc-flag:`-Wdata-kinds-tc`
.. 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-record-selectors`
* :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:`-Wderiving-typeable`
.. 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:`-Wimplicit-rhs-quantification`
* :ghc-flag:`-Wdeprecated-type-abstractions`
.. 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: *(deprecated)*
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
This warning is deprecated. It no longer has any effect since GHC 9.12.
.. 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 <typed-holes>` errors are
:ref:`deferred until runtime <defer-type-errors>`. 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
<defer-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 <defer-type-errors>`.
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. ``(*>) = (>>)``).
.. 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``).
.. 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: *(deprecated)*
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
This warning is deprecated. It no longer has any effect since GHC 9.8.
It was used during the transition period of the semigroup proposal,
to warn when an instance of ``Monoid`` was not an instance of ``Semigroup``,
or when a custom local operator ``(<>)`` could clash with `(<>)`,
now exported from ``Prelude``.
.. 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 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:: -Wincomplete-record-selectors
:shortdesc: warn when a record selector application could fail
:type: dynamic
:reverse: -Wno-incomplete-record-selectors
:category:
:since: 9.10
.. index::
single: incomplete record selectors, warning
single: record selectors, incomplete
When a record selector is applied to a constructor that does not
contain that field, it will produce an error. For example ::
data T = T1 | T2 { x :: Int }
f :: T -> Int
f a = x a -- `f T1` will fail
g1 :: HasField "x" t Int => t -> Int
g1 a = 1 + getField @"x" a
g2 :: T -> Int
g2 a = g1 a + 2 -- `g2 T1` will fail as well
The warning warns about cases like that. It also takes into account
previously pattern-matched cases, for example ::
d :: T -> Int
d T1 = 0
d a = x a -- would not warn
.. ghc-flag:: -Wmissing-deriving-strategies
:shortdesc: warn when deriving without mentioning 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. The suggested fix will show which deriving strategies were
assumed.
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 <standalone-kind-signatures>` or a :ref:`CUSK <complete-kind-signatures>`, 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 <standalone-kind-signatures>`
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 <orphan-modules>`
: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
<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0143-remove-star-kind.rst>`__.
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, of the constructors of a
data type, as record selection and update will be partial. For example, when
:ghc-flag:`-Wpartial-fields` is enabled the compiler will emit a warning at
the definition of ``Foo`` below: ::
data Foo = Foo { f :: Int } | Bar
The warning is suppressed if the field name begins with an underscore. ::
data Foo = Foo { _f :: Int } | Bar
Related warnings are :ghc-flag:`-Wincomplete-record-selectors` and
:ghc-flag:`-Wincomplete-record-updates`,
which warn at use sites rather than definition sites.
.. 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 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0229-whitespace-bang-patterns.rst>`__::
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
<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0366-no-ambiguous-field-access.rst>`_
and the documentation on :extension:`DuplicateRecordFields` for further details.
This warning has no effect when :extension:`DuplicateRecordFields` is
disabled.
.. ghc-flag:: -Wforall-identifier
:shortdesc: *(deprecated)* Does nothing
:type: dynamic
:since: 9.4
This warning is deprecated. It no longer has any effect since GHC 9.10.
In the past, GHC used to accept ``forall`` as a term-level identifier:
::
-- from constraints-0.13
forall :: forall p. (forall a. Dict (p a)) -> Dict (Forall p)
forall d = ...
In accordance with `GHC Proposal #281 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0281-visible-forall.rst>`__,
this is no longer possible, as ``forall`` has become a proper keyword.
:ghc-flag:`-Wforall-identifier` was used in the migration period before the
breaking change took place.
.. 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
<https://www.unicode.org/reports/tr9/#Bidirectional_Character_Types>`_
.. 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) <https://www.microsoft.com/en-us/research/publication/outsideinx-modular-type-inference-with-local-assumptions/>`__
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
:default: on
In accordance with `GHC Proposal #371
<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0371-non-magical-eq.md>`__,
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``.
.. 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
<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0371-non-magical-eq.md>`__,
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: *(deprecated)* warn when creating potentially-loopy superclass constraint evidence
:type: dynamic
:reverse: -Wno-loopy-superclass-solve
:since: 9.6.1
This warning is deprecated. It no longer has any effect since GHC 9.10.
In the past, :extension:`UndecidableInstances` allowed potentially
non-terminating evidence for certain superclass constraints.
This is no longer allowed, as explained in :ref:`undecidable-instances`.
This warning was used during the transition period.
.. ghc-flag:: -Wterm-variable-capture
:shortdesc: warn when an implicitly quantified type variable captures a term's name
:type: dynamic
:since: 9.8.1
Under :extension:`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 -- NoRequiredTypeArguments: The ‘a’ is implicitly quantified
-- RequiredTypeArguments: The ‘a’ refers to the term-level binding
When :ghc-flag:`-Wterm-variable-capture` is enabled, GHC warns against implicit quantification
that would stop working under :extension:`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 <role-annotations>`, 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
<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0425-decl-invis-binders.rst>`__,
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:: -Wdeprecated-type-abstractions
:shortdesc: warn when type abstractions in constructor patterns are used without enabling :extension:`TypeApplications`
:type: dynamic
:reverse: -Wno-deprecated-type-abstractions
:category:
:since: 9.10.1
:default: off
Type abstractions in constructor patterns allow binding existential type variables: ::
import Type.Reflection (Typeable, typeRep)
data Ex = forall e. (Typeable e, Show e) => MkEx e
showEx (MkEx @e a) = show a ++ " :: " ++ show (typeRep @e)
Note the pattern ``MkEx @e a``, and specifically the ``@e`` binder.
Support for this feature was added to GHC in version 9.2, but instead of getting
its own language extension the feature was enabled by a combination of
:extension:`TypeApplications` and :extension:`ScopedTypeVariables`.
As per `GHC Proposal #448
<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0448-type-variable-scoping.rst>`__
and its amendment `#604 <https://github.com/ghc-proposals/ghc-proposals/pull/604>`__
we are now transitioning towards guarding this feature behind :extension:`TypeAbstractions` instead.
As a compatibility measure, GHC continues to support old programs that use type abstractions
in constructor patterns without enabling the appropriate extension :extension:`TypeAbstractions`,
but it will stop doing so in a future release.
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
<https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0134-deprecating-exports-proposal.rst>`__,
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:: -Wbadly-staged-types
:shortdesc: warn when type binding is used at the wrong TH stage.
:type: dynamic
:reverse: -Wno-badly-staged-types
:since: 9.10.1
Consider an example: ::
tardy :: forall a. Proxy a -> IO Type
tardy _ = [t| a |]
The type binding ``a`` is bound at stage 1 but used on stage 2.
This is badly staged program, and the ``tardy (Proxy @Int)`` won't produce
a type representation of ``Int``, but rather a local name ``a``.
.. 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:: -Wdata-kinds-tc
:shortdesc: warn when an illegal use of a type or kind without
:extension:`DataKinds` is caught by the typechecker
:type: dynamic
:reverse: -Wno-data-kinds-tc
:since: 9.10.1
Introduced in GHC 9.10.1, this warns when an illegal use of a type or kind
(without having enabled the :extension:`DataKinds` extension) is caught in
the typechecker (hence the ``-tc`` suffix). These warnings complement the
existing :extension:`DataKinds` checks (that have existed since
:extension:`DataKinds` was first introduced), which result in errors
instead of warnings.
This warning is scheduled to be changed to an error in a future GHC
version, at which point the :ghc-flag:`-Wdata-kinds-tc` flag will be
removed. Users can enable the :extension:`DataKinds` extension to avoid
issues (thus silencing the warning).
.. ghc-flag:: -Wdefaulted-exception-context
:shortdesc: warn when an :base-ref:`Control.Exception.Context.ExceptionContext`
implicit parameter is defaulted to
:base-ref:`Control.Exception.Context.emptyExceptionContext`.
:type: dynamic
:reverse: -Wnop-defaulted-exception-context
:since: 9.10.1
Introduced in GHC 9.10.1 with the introduction of an implicit
:base-ref:`Control.Exception.Context.ExceptionContext`` context to
:base-ref:`Control.Exception.SomeException`. To preserve compatibility
with earlier compilers, this constraints is implicitly defaulted to
:base-ref:`Control.Exception.Context.emptyExceptionContext` when no other
evidence is available. As this behavior may result in dropped exception context
this warning is provided to give notice when defaulting occurs.
.. ghc-flag:: -Wview-pattern-signatures
:shortdesc: warn when a view pattern is used with type signature without
explicit parens
:type: dynamic
:reverse: -Wno-view-pattern-signatures
:since: 9.12.1
Introduced in GHC 9.12.1 in accordance with `GHC Proposal #281 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0281-visible-forall.rst>`__,
this flag controls the warning about unparenthesized occurrences of pattern
signatures on the RHS of view patterns. Example: ::
f (g -> p :: t) = ()
The way it is parsed will change in a future GHC release: ::
f (g -> (p :: t)) = () -- current (legacy) parse
f ((g -> p) :: t) = () -- future parse
Unless ``g`` is an endofuction (i.e. its input and output have the same
type), this will break the program.
To make the code forwards-compatible and silence the warning, users are
advised to add parentheses manually.
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.)