.. _options-sanity:
Warnings and sanity-checking
----------------------------
.. index::
single: sanity-checking options
single: warnings
GHC has a number of options that select which types of non-fatal error
messages, otherwise known as warnings, can be generated during
compilation. By default, you get a standard set of warnings which are
generally likely to indicate bugs in your program. These are:
.. hlist::
:columns: 3
* :ghc-flag:`-Woverlapping-patterns`
* :ghc-flag:`-Wwarnings-deprecations`
* :ghc-flag:`-Wdeprecated-flags`
* :ghc-flag:`-Wunrecognised-pragmas`
* :ghc-flag:`-Wduplicate-constraints`
* :ghc-flag:`-Wduplicate-exports`
* :ghc-flag:`-Woverflowed-literals`
* :ghc-flag:`-Wempty-enumerations`
* :ghc-flag:`-Wmissing-fields`
* :ghc-flag:`-Wmissing-methods`
* :ghc-flag:`-Wwrong-do-bind`
* :ghc-flag:`-Wunsupported-calling-conventions`
* :ghc-flag:`-Wdodgy-foreign-imports`
* :ghc-flag:`-Winline-rule-shadowing`
* :ghc-flag:`-Wunsupported-llvm-version`
* :ghc-flag:`-Wtabs`
* :ghc-flag:`-Wunrecognised-warning-flags`
The following flags are simple ways to select standard "packages" of warnings:
.. ghc-flag:: -W
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:: -Wall
Turns on all warning options that indicate potentially suspicious
code. The warnings that are *not* enabled by :ghc-flag:`-Wall` are
.. hlist::
:columns: 3
* :ghc-flag:`-Wincomplete-uni-patterns`
* :ghc-flag:`-Wincomplete-record-updates`
* :ghc-flag:`-Wmonomorphism-restriction`
* :ghc-flag:`-Wimplicit-prelude`
* :ghc-flag:`-Wmissing-local-signatures`
* :ghc-flag:`-Wmissing-exported-signatures`
* :ghc-flag:`-Wmissing-import-lists`
* :ghc-flag:`-Widentities`
.. ghc-flag:: -Wcompat
Turns on warnings that will be enabled by default in the future, but remain
off in normal compilations for the time being. This allows library authors
eager to make their code future compatible to adapt to new features before
they even generate warnings.
This currently enables
.. hlist::
:columns: 3
* :ghc-flag:`-Wmissing-monadfail-instances`
* :ghc-flag:`-Wsemigroup`
* :ghc-flag:`-Wnoncanonical-monoid-instances`
.. ghc-flag:: -Wno-compat
Disables all warnings enabled by :ghc-flag:`-Wcompat`.
.. ghc-flag:: -w
Turns off all warnings, including the standard ones and those that
:ghc-flag:`-Wall` doesn't enable.
.. ghc-flag:: -Werror
Makes any warning into a fatal error. Useful so that you don't miss
warnings when doing batch compilation.
.. ghc-flag:: -Wwarn
Warnings are treated only as warnings, not as errors. This is the
default, but can be useful to negate a :ghc-flag:`-Werror` flag.
When a warning is emitted, the specific warning flag which controls
it is shown.
.. ghc-flag:: -fshow-warning-groups
When showing which flag controls a warning, also show the
respective warning group flag(s) that warning is contained in.
This option is off by default.
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
Enables warnings when the compiler encounters a ``-W...`` flag that is not
recognised.
This warning is on by default.
.. ghc-flag:: -Wtyped-holes
Determines whether the compiler reports typed holes warnings. Has no
effect unless typed holes errors are deferred until runtime. See
:ref:`typed-holes` and :ref:`defer-type-errors`
This warning is on by default.
.. ghc-flag:: -Wtype-errors
Causes a warning to be reported when a type error is deferred until
runtime. See :ref:`defer-type-errors`
This warning is on by default.
.. ghc-flag:: -fdefer-type-errors
:implies: :ghc-flag:`-fdefer-typed-holes`
Defer as many type errors as possible until runtime. At compile time
you get a warning (instead of an error). At runtime, if you use a
value that depends on a type error, you get a runtime error; but you
can run any type-correct parts of your code just fine. See
:ref:`defer-type-errors`
.. ghc-flag:: -fdefer-typed-holes
Defer typed holes errors until runtime. This will turn the errors
produced by :ref:`typed holes <typed-holes>` into warnings. Using a value
that depends on a typed hole produces a runtime error, the same as
:ghc-flag:`-fdefer-type-errors` (which implies this option). See :ref:`typed-holes`
and :ref:`defer-type-errors`.
Implied by :ghc-flag:`-fdefer-type-errors`. See also :ghc-flag:`-Wtyped-holes`.
.. ghc-flag:: -Wpartial-type-signatures
Determines whether the compiler reports holes in partial type
signatures as warnings. Has no effect unless
:ghc-flag:`-XPartialTypeSignatures` is enabled, which controls whether
errors should be generated for holes in types or not. See
:ref:`partial-type-signatures`.
This warning is on by default.
.. ghc-flag:: -fhelpful-errors
When a name or package is not found in scope, make suggestions for
the name or package you might have meant instead.
This option is on by default.
.. ghc-flag:: -Wunrecognised-pragmas
Causes a warning to be emitted when a pragma that GHC doesn't
recognise is used. As well as pragmas that GHC itself uses, GHC also
recognises pragmas known to be used by other tools, e.g.
``OPTIONS_HUGS`` and ``DERIVE``.
This option is on by default.
.. ghc-flag:: -Wmissed-specialisations
-Wall-missed-specialisations
Emits a warning if GHC cannot specialise an overloaded function, usually
because the function needs an ``INLINEABLE`` pragma. The "all" form reports
all such situations whereas the "non-all" form only reports when the
situation arises during specialisation of an imported function.
The "non-all" form is intended to catch cases where an imported function
that is marked as ``INLINEABLE`` (presumably to enable specialisation) cannot
be specialised as it calls other functions that are themselves not specialised.
Note that these warnings will not throw errors if used with :ghc-flag:`-Werror`.
These options are both off by default.
.. ghc-flag:: -Wwarnings-deprecations
.. index::
pair: deprecations; warnings
Causes a warning to be emitted when a module, function or type with
a ``WARNING`` or ``DEPRECATED pragma`` is used. See
:ref:`warning-deprecated-pragma` for more details on the pragmas.
This option is on by default.
.. ghc-flag:: -Wamp
.. index::
single: AMP
single: Applicative-Monad Proposal
This option is deprecated.
Caused a warning to be emitted when a definition was in conflict with
the AMP (Applicative-Monad proosal).
.. ghc-flag:: -Wnoncanonical-monad-instances
Warn if noncanonical ``Applicative`` or ``Monad`` instances
declarations are detected.
When this warning is enabled, the following conditions are verified:
In ``Monad`` instances declarations warn if any of the following
conditions does not hold:
* If ``return`` is defined it must be canonical (i.e. ``return = pure``).
* If ``(>>)`` is defined it must be canonical (i.e. ``(>>) = (*>)``).
Moreover, in ``Applicative`` instance declarations:
* Warn if ``pure`` is defined backwards (i.e. ``pure = return``).
* Warn if ``(*>)`` is defined backwards (i.e. ``(*>) = (>>)``).
This option is off by default.
.. ghc-flag:: -Wnoncanonical-monadfail-instances
Warn if noncanonical ``Monad`` or ``MonadFail`` 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 ``fail`` is defined it must be canonical
(i.e. ``fail = Control.Monad.Fail.fail``).
Moreover, in ``MonadFail`` instance declarations:
* Warn if ``fail`` is defined backwards
(i.e. ``fail = Control.Monad.fail``).
See also :ghc-flag:`-Wmissing-monadfail-instances`.
This option is off by default.
.. ghc-flag:: -Wnoncanonical-monoid-instances
Warn if noncanonical ``Semigroup`` or ``Monoid`` instances
declarations are detected.
When this warning is enabled, the following conditions are verified:
In ``Monoid`` instances declarations warn if any of the following
conditions does not hold:
* If ``mappend`` is defined it must be canonical
(i.e. ``mappend = (Data.Semigroup.<>)``).
Moreover, in ``Semigroup`` instance declarations:
* Warn if ``(<>)`` is defined backwards (i.e. ``(<>) = mappend``).
This warning is off by default. However, it is part of the
:ghc-flag:`-Wcompat` option group.
.. ghc-flag:: -Wmissing-monadfail-instances
.. index::
single: MFP
single: MonadFail Proposal
Warn when a failable pattern is used in a do-block that does not have a
``MonadFail`` instance.
See also :ghc-flag:`-Wnoncanonical-monadfail-instances`.
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, as part of
the `MonadFail Proposal (MFP)
<https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail>`__.
.. ghc-flag:: -Wsemigroup
.. 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
.. index::
single: deprecated flags
Causes a warning to be emitted when a deprecated command-line flag
is used.
This option is on by default.
.. ghc-flag:: -Wunsupported-calling-conventions
Causes a warning to be emitted for foreign declarations that use
unsupported calling conventions. In particular, if the ``stdcall``
calling convention is used on an architecture other than i386 then
it will be treated as ``ccall``.
.. ghc-flag:: -Wdodgy-foreign-imports
Causes a warning to be emitted for foreign imports of the following
form: ::
foreign import "f" f :: FunPtr t
on the grounds that it probably should be ::
foreign import "&f" f :: FunPtr t
The first form declares that \`f\` is a (pure) C function that takes
no arguments and returns a pointer to a C function with type \`t\`,
whereas the second form declares that \`f\` itself is a C function
with type \`t\`. The first declaration is usually a mistake, and one
that is hard to debug because it results in a crash, hence this
warning.
.. ghc-flag:: -Wdodgy-exports
Causes a warning to be emitted when a datatype ``T`` is exported
with all constructors, i.e. ``T(..)``, but is it just a type
synonym.
Also causes a warning to be emitted when a module is re-exported,
but that module exports nothing.
.. ghc-flag:: -Wdodgy-imports
Causes a warning to be emitted in the following cases:
- When a datatype ``T`` is imported with all constructors, i.e.
``T(..)``, but has been exported abstractly, i.e. ``T``.
- When an ``import`` statement hides an entity that is not
exported.
.. ghc-flag:: -Woverflowed-literals
Causes a warning to be emitted if a literal will overflow, e.g.
``300 :: Word8``.
.. ghc-flag:: -Wempty-enumerations
Causes a warning to be emitted if an enumeration is empty, e.g.
``[5 .. 3]``.
.. ghc-flag:: -Wduplicate-constraints
.. 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
: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.
This option is on by default. As usual you can suppress it on a
per-module basis with :ghc-flag:`-Wno-redundant-constraints`.
Occasionally you may specifically want a function to have a more
constrained signature than necessary, perhaps to leave yourself
wiggle-room for changing the implementation without changing the
API. In that case, you can suppress the warning on a per-function
basis, using a call in a dead binding. For example: ::
f :: Eq a => a -> a -> Bool
f x y = True
where
_ = x == x -- Suppress the redundant-constraint warning for (Eq a)
Here the call to ``(==)`` makes GHC think that the ``(Eq a)``
constraint is needed, so no warning is issued.
.. ghc-flag:: -Wduplicate-exports
.. 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.
This option is on by default.
.. ghc-flag:: -Whi-shadowing
.. 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.
.. ghc-flag:: -Widentities
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-prelude
.. 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 :ghc-flag:`-XNoImplicitPrelude` or a
``LANGUAGE NoImplicitPrelude`` pragma).
Note that no warning is given for syntax that implicitly refers to
the Prelude, even if :ghc-flag:`-XNoImplicitPrelude` would change whether it
refers to the Prelude. For example, no warning is given when ``368``
means ``Prelude.fromInteger (368::Prelude.Integer)`` (where
``Prelude`` refers to the actual Prelude module, regardless of the
imports of the module being compiled).
This warning is off by default.
.. ghc-flag:: -Wincomplete-patterns
-Wincomplete-uni-patterns
.. 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`.
The flag :ghc-flag:`-Wincomplete-uni-patterns` is similar, except that
it applies only to lambda-expressions and pattern bindings,
constructs that only allow a single pattern: ::
h = \[] -> 2
Just k = f y
.. ghc-flag:: -fmax-pmcheck-iterations=<N>
:default: 2000000
Sets how many iterations of the pattern-match checker will perform before
giving up. This limit is to catch cases where pattern-match checking might
be excessively costly (due to the exponential complexity of coverage
checking in the general case). It typically shouldn't be necessary to set
this unless GHC informs you that it has exceeded the pattern match checker's
iteration limit (in which case you may want to consider refactoring your
pattern match, for the sake of future readers of your code.
.. ghc-flag:: -Wincomplete-record-updates
.. 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-fields
.. 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-import-lists
.. 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
.. index::
single: missing methods, warning
single: methods, missing
This option is on by default, and warns you whenever an instance
declaration is missing one or more methods, and the corresponding
class declaration has no default declaration for them.
The warning is suppressed if the method name begins with an
underscore. Here's an example where this is useful: ::
class C a where
_simpleFn :: a -> String
complexFn :: a -> a -> String
complexFn x y = ... _simpleFn ...
The idea is that: (a) users of the class will only call
``complexFn``; never ``_simpleFn``; and (b) instance declarations
can define either ``complexFn`` or ``_simpleFn``.
The ``MINIMAL`` pragma can be used to change which combination of
methods will be required for instances of a particular class. See
:ref:`minimal-pragma`.
.. ghc-flag:: -Wmissing-signatures
.. 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. The
option is off by default.
.. ghc-flag:: -Wmissing-exported-sigs
.. index::
single: type signatures, missing
This option is now deprecated in favour of
:ghc-flag:`-Wmissing-exported-signatures`.
.. ghc-flag:: -Wmissing-exported-signatures
.. 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. This option takes precedence over
:ghc-flag:`-Wmissing-signatures`. As part of the warning GHC also
reports the inferred type. The option is off by default.
.. ghc-flag:: -Wmissing-local-sigs
.. index::
single: type signatures, missing
This option is now deprecated in favour of
:ghc-flag:`-Wmissing-local-signatures`.
.. ghc-flag:: -Wmissing-local-signatures
.. 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
.. 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. This option is off by default.
.. ghc-flag:: -Wname-shadowing
.. 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
.. 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
.. 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.
.. ghc-flag:: -Wtabs
.. index::
single: tabs, warning
Have the compiler warn if there are tabs in your source file.
.. ghc-flag:: -Wtype-defaults
.. 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.
This warning is off by default.
.. ghc-flag:: -Wmonomorphism-restriction
.. 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.
This warning is off by default.
.. ghc-flag:: -Wunsupported-llvm-version
Warn when using :ghc-flag:`-fllvm` with an unsupported version of LLVM.
.. ghc-flag:: -Wunticked-promoted-constructors
.. 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 warning is is enabled by default in :ghc-flag:`-Wall` mode.
.. ghc-flag:: -Wunused-binds
.. 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
.. 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
.. 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
.. index::
single: unused binds, warning
single: binds, unused
Warn if a pattern binding binds no variables at all, unless it is a
lone, possibly-banged, wild-card pattern. For example: ::
Just _ = rhs3 -- Warning: unused pattern binding
(_, _) = rhs4 -- Warning: unused pattern binding
_ = rhs3 -- No warning: lone wild-card pattern
!_ = rhs4 -- No warning: banged wild-card pattern; behaves like seq
The motivation for allowing lone wild-card patterns is they are not
very different from ``_v = rhs3``, which elicits no warning; and
they can be useful to add a type constraint, e.g. ``_ = x::Int``. A
lone banged wild-card pattern is useful as an alternative (to
``seq``) way to force evaluation.
.. ghc-flag:: -Wunused-imports
.. 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
.. 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
.. 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
.. index::
single: unused type patterns, warning
single: type patterns, unused
Report all unused 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. The warning is suppressed if the
type variable name begins with an underscore, like so: ::
type instance F _x _y = []
Unlike :ghc-flag:`-Wunused-matches`, :ghc-flag:`-Wunused-type-variables` 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
.. 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:: -Wwrong-do-bind
.. 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
Warn if a rewrite RULE might fail to fire because the function might
be inlined before the rule has a chance to fire. See
:ref:`rules-inline`.
If you're feeling really paranoid, the :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.)