5.2. Warnings and sanity-checking¶
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.
To turn off an individual warning -W<wflag>
, use -Wno-<wflag>
.
To reverse -Werror
, which makes all warnings into errors, use -Wwarn
.
By default, you get a standard set of warnings which are generally likely to indicate bugs in your program. These are:
The following flags are simple ways to select standard “packages” of warnings:
-
-W
¶ Provides the standard warnings plus
-
-Wall
¶ Turns on all warning options that indicate potentially suspicious code. The warnings that are not enabled by
-Wall
are
-
-Weverything
¶ Turns on every single warning supported by the compiler.
-
-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
These options control which warnings are considered fatal and cause compilation to abort.
-
-Werror
¶ 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.
-
-Werror
=⟨wflag⟩
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=compat
has the same effect as-Werror=...
for each warning flag in the-Wcompat
option group.
-
-Wwarn
¶ Warnings are treated only as warnings, not as errors. This is the default, but can be useful to negate a
-Werror
flag.
-
-Wwarn
=⟨wflag⟩
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=compat
has the same effect as-Wwarn=...
for each warning flag in the-Wcompat
option group.
When a warning is emitted, the specific warning flag which controls it is shown.
-
-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-)*
.
-
-Wunrecognised-warning-flags
¶ Default: on Enables warnings when the compiler encounters a
-W...
flag that is not recognised.
-
-Wcompat-unqualified-imports
¶ Warns on qualified imports of core library modules which are subject to change in future GHC releases. Currently the following modules are covered by this warning:
Data.List
due to the future addition ofData.List.singleton
and specialisation of exports to the[]
type. See the mailing list for details.
This warning can be addressed by either adding an explicit import list or using a
qualified
import.
-
-Wtyped-holes
¶ Default: on Determines whether the compiler reports typed holes warnings. Has no effect unless typed holes errors are deferred until runtime. See Typed Holes and Deferring type errors to runtime
-
-Wdeferred-type-errors
¶ Causes a warning to be reported when a type error is deferred until runtime. See Deferring type errors to runtime
This warning is on by default.
-
-fdefer-type-errors
¶ Implies: -fdefer-typed-holes
,-fdefer-out-of-scope-variables
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 Deferring type errors to runtime
-
-fdefer-typed-holes
¶ Defer typed holes errors (errors about names with a leading underscore (e.g., “_”, “_foo”, “_bar”)) until runtime. This will turn the errors produced by typed holes into warnings. Using a value that depends on a typed hole produces a runtime error, the same as
-fdefer-type-errors
(which implies this option). See Typed Holes and Deferring type errors to runtime.Implied by
-fdefer-type-errors
. See also-Wtyped-holes
.
-
-fdefer-out-of-scope-variables
¶ Defer variable out-of-scope errors (errors about names without a leading underscore) until runtime. This will turn variable-out-of-scope errors into warnings. Using a value that depends on an out-of-scope variable produces a runtime error, the same as
-fdefer-type-errors
(which implies this option). See Typed Holes and Deferring type errors to runtime.Implied by
-fdefer-type-errors
. See also-Wdeferred-out-of-scope-variables
.
-
-Wdeferred-out-of-scope-variables
¶ Warn when a deferred out-of-scope variable is encountered.
-
-Wpartial-type-signatures
¶ Determines whether the compiler reports holes in partial type signatures as warnings. Has no effect unless
PartialTypeSignatures
is enabled, which controls whether errors should be generated for holes in types or not. See Partial Type Signatures.This warning is on by default.
-
-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.
-
-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
andDERIVE
.This option is on by default.
-
-Wmissed-specialisations
¶ 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
-Werror
.This option is off by default.
-
-Wall-missed-specialisations
¶ 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
-Werror
.This option is off by default.
-
-Wwarnings-deprecations
¶ Causes a warning to be emitted when a module, function or type with a
WARNING
orDEPRECATED pragma
is used. See WARNING and DEPRECATED pragmas for more details on the pragmas.This option is on by default.
-
-Wdeprecations
¶ Causes a warning to be emitted when a module, function or type with a
WARNING
orDEPRECATED pragma
is used. See WARNING and DEPRECATED pragmas for more details on the pragmas. An alias for-Wwarnings-deprecations
.This option is on by default.
-
-Wnoncanonical-monad-instances
¶ Warn if noncanonical
Applicative
orMonad
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.
- If
-
-Wnoncanonical-monadfail-instances
¶ Warn if noncanonical
Monad
orMonadFail
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
-Wmissing-monadfail-instances
.This option is off by default.
- If
-
-Wnoncanonical-monoid-instances
¶ Warn if noncanonical
Semigroup
orMonoid
instances declarations are detected.When this warning is enabled, the following conditions are verified:
In
Monoid
instances declarations warn if any of the following conditions does not hold:- If
mappend
is defined it must be canonical (i.e.mappend = (Data.Semigroup.<>)
).
Moreover, in
Semigroup
instance declarations:- Warn if
(<>)
is defined backwards (i.e.(<>) = mappend
).
This warning is off by default. However, it is part of the
-Wcompat
option group.- If
-
-Wmissing-monadfail-instances
¶ Warn when a failable pattern is used in a do-block that does not have a
MonadFail
instance.See also
-Wnoncanonical-monadfail-instances
.Being part of the
-Wcompat
option group, this warning is off by default, but will be switched on in a future GHC release, as part of the MonadFail Proposal (MFP).
-
-Wsemigroup
¶ Warn when definitions are in conflict with the future inclusion of
Semigroup
into the standard typeclasses.- Instances of
Monoid
should also be instances ofSemigroup
- The
Semigroup
operator(<>)
will be inPrelude
, which clashes with custom local definitions of such an operator
Being part of the
-Wcompat
option group, this warning is off by default, but will be switched on in a future GHC release.- Instances of
-
-Wdeprecated-flags
¶ Causes a warning to be emitted when a deprecated command-line flag is used.
This option is on by default.
-
-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 asccall
.
-
-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 typet
, whereas the second form declares thatf
itself is a C function with typet
. The first declaration is usually a mistake, and one that is hard to debug because it results in a crash, hence this warning.
-
-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.
-
-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.
- When a datatype
-
-Woverflowed-literals
¶ Causes a warning to be emitted if a literal will overflow, e.g.
300 :: Word8
.
-
-Wempty-enumerations
¶ Causes a warning to be emitted if an enumeration is empty, e.g.
[5 .. 3]
.
-
-Wderiving-defaults
¶ Since: 8.10 Causes a warning when both
DeriveAnyClass
andGeneralizedNewtypeDeriving
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
-
-Wduplicate-constraints
¶ 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
-Wredundant-constraints
.
-
-Wredundant-constraints
¶ Since: 8.0 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 theOrd 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 off
.)
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
-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.
-
-Wduplicate-exports
¶ 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.
-
-Whi-shadowing
¶ 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.
-
-Widentities
¶ Causes the compiler to emit a warning when a Prelude numeric conversion converts a type
T
to the same typeT
; such calls are probably no-ops and can be omitted. The functions checked for are:toInteger
,toRational
,fromIntegral
, andrealToFrac
.
-
-Wimplicit-kind-vars
¶ Have the compiler warn if a kind variable is not explicitly quantified over. For instance, the following would produce a warning:
f :: forall (a :: k). Proxy a
This can be fixed by explicitly quantifying over
k
:f :: forall k (a :: k). Proxy a
-
-Wimplicit-prelude
¶ 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 byNoImplicitPrelude
or aLANGUAGE NoImplicitPrelude
pragma).Note that no warning is given for syntax that implicitly refers to the Prelude, even if
NoImplicitPrelude
would change whether it refers to the Prelude. For example, no warning is given when368
meansPrelude.fromInteger (368::Prelude.Integer)
(wherePrelude
refers to the actual Prelude module, regardless of the imports of the module being compiled).This warning is off by default.
-
-Wincomplete-patterns
¶ The option
-Wincomplete-patterns
warns about places where a pattern-match might fail at runtime. The functiong
below will fail when applied to non-empty lists, so the compiler will emit a warning about this when-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
-W
.
-
-Wincomplete-uni-patterns
¶ The flag
-Wincomplete-uni-patterns
is similar to-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
-
-fmax-pmcheck-models
=⟨n⟩
¶ 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
againstJust 4
, we split each incoming matching model into two uncovered sub-models: One wherex
isNothing
and one wherex
isJust y
buty
is not4
.This can be exponential in the arity of the pattern and in the number of guards in some cases. The
-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.
-
-Wincomplete-record-updates
¶ The function
f
below will fail when applied toBar
, so the compiler will emit a warning about this when-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.
-
-Wmissing-deriving-strategies
¶ Since: 8.8.1 The datatype below derives the
Eq
typeclass, but doesn’t specify a strategy. When-Wmissing-deriving-strategies
is enabled, the compiler will emit a warning about this.data Foo a = Foo a deriving (Eq)
The compiler will warn here that the deriving clause doesn’t specify a strategy. If the warning is enabled, but
DerivingStrategies
is not enabled, the compiler will suggest turning on theDerivingStrategies
extension. This option is not on by default, having to be turned on manually or with-Weverything
.
-
-Wmissing-fields
¶ 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.
-
-Wmissing-export-lists
¶ Since: 8.4.1 This flag warns if you declare a module without declaring an explicit export list. For example
module M where p x = x
The
-Wmissing-export-lists
flag will warn thatM
does not declare an export list. Declaring an explicit export list forM
enables GHC dead code analysis, prevents accidental export of names and can ease optimizations like inlining.
-
-Wmissing-import-lists
¶ This flag warns if you use an unqualified
import
declaration that does not explicitly list the entities brought into scope. For examplemodule M where import X( f ) import Y import qualified Z p x = f x x
The
-Wmissing-import-lists
flag will warn about the import ofY
but notX
If moduleY
is later changed to export (say)f
, then the reference tof
inM
will become ambiguous. No warning is produced for the import ofZ
because extendingZ
‘s exports would be unlikely to produce ambiguity inM
.
-
-Wmissing-methods
¶ 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
MINIMAL
pragma can be used to change which combination of methods will be required for instances of a particular class. See MINIMAL pragma.
-
-Wmissing-signatures
¶ If you would like GHC to check that every top-level function/value has a type signature, use the
-Wmissing-signatures
option. As part of the warning GHC also reports the inferred type. The option is off by default.
-
-Wmissing-exported-sigs
¶ This option is now deprecated in favour of
-Wmissing-exported-signatures
.
-
-Wmissing-exported-signatures
¶ 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
-Wmissing-exported-signatures
option. This option takes precedence over-Wmissing-signatures
. As part of the warning GHC also reports the inferred type. The option is off by default.
-
-Wmissing-local-sigs
¶ This option is now deprecated in favour of
-Wmissing-local-signatures
.
-
-Wmissing-local-signatures
¶ If you use the
-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.
-
-Wmissing-pattern-synonym-signatures
¶ If you would like GHC to check that every pattern synonym has a type signature, use the
-Wmissing-pattern-synonym-signatures
option. If this option is used in conjunction with-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.
-
-Wname-shadowing
¶ 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) }
-
-Worphans
¶ 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 Orphan modules and instance declarations for details.
The flag
-Worphans
warns about user-written orphan rules or instances.
-
-Woverlapping-patterns
¶ 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.
-
-Winaccessible-code
¶ 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 instep2
is inaccessible, because in order forcheckTEQ
to be able to produce aJust
,t ~ u
must hold, but since we’re passingFoo1
andFoo2
here, it follows thatt ~ Char
, andu ~ Int
, and thust ~ u
cannot hold.
-
-Wstar-is-type
¶ Since: 8.6 The use of
*
to denote the kind of inhabited types relies on theStarIsType
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 #30.This warning allows to detect such uses of
*
before the actual breaking change takes place. The recommended fix is to replace*
withType
imported fromData.Kind
.Being part of the
-Wcompat
option group, this warning is off by default, but will be switched on in a future GHC release.
-
-Wstar-binder
¶ Under
StarIsType
, a*
in types is not an operator nor even a name, it is special syntax that stands forData.Kind.Type
. This means that an expression likeEither * Char
is parsed asEither (*) 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 asa (*) 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 toa * b
. With-Wstar-binder
we warn when this special treatment of(*)
takes place.
-
-Wsimplifiable-class-constraints
¶ Since: 8.2 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 forEq [a]
. GHC goes to some efforts to use the former, but if it should use the latter, it would then have an insolubleEq a
constraint. Best avoided by instead writing:f :: Eq a => a -> a
This option is on by default. As usual you can suppress it on a per-module basis with
-Wno-simplifiable-class-constraints
.
-
-Wtabs
¶ Have the compiler warn if there are tabs in your source file.
-
-Wtype-defaults
¶ 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 typeInt
, whereas Haskell 98 and later defaults it toInteger
. This may lead to differences in performance and behaviour, hence the usefulness of being non-silent about this.This warning is off by default.
-
-Wmonomorphism-restriction
¶ 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.
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.
-
-Wunticked-promoted-constructors
¶ 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
andSucc
are not written as'Zero
and'Succ
.This warning is enabled by default in
-Wall
mode.
-
-Wunused-binds
¶ Report any function definitions (and local bindings) which are unused. An alias for
-
-Wunused-top-binds
¶ 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
-
-Wunused-local-binds
¶ 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
-
-Wunused-pattern-binds
¶ 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 Bang patterns and Strict Haskell) is not a no-op, because it forces evaluation, and is useful as an alternative toseq
.
-
-Wunused-imports
¶ 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.
-
-Wunused-matches
¶ Report all unused variables which arise from term-level pattern matches, including patterns consisting of a single variable. For instance
f x y = []
would reportx
andy
as unused. The warning is suppressed if the variable name begins with an underscore, thus:f _x = True
Note that
-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-Wunused-type-patterns
flag.
-
-Wunused-do-bind
¶ Report expressions occurring in
do
andmdo
blocks that appear to silently throw information away. For instancedo { mapM popInt xs ; return 10 }
would report the first statement in thedo
block as suspicious, as it has the typeStackM [Int]
and notStackM ()
, 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 }
-
-Wunused-type-patterns
¶ 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
andy
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
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
andy
as unused on the right hand sideUnlike
-Wunused-matches
,-Wunused-type-patterns
is not implied by-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.
-
-Wunused-foralls
¶ 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
andc
as unused.
-
-Wunused-record-wildcards
¶ Since: 8.10.1 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.
-
-Wredundant-record-wildcards
¶ Since: 8.10.1 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..
.
-
-Wwrong-do-bind
¶ Report expressions occurring in
do
andmdo
blocks that appear to lack a binding. For instancedo { return (popInt 10) ; return 10 }
would report the first statement in thedo
block as suspicious, as it has the typeStackM (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 }
-
-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 How rules interact with INLINE/NOINLINE pragmas.
-
-Wcpp-undef
¶ 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.
-
-Wunbanged-strict-patterns
¶ 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 Unboxed types for information about unlifted types.
-
-Wmissing-home-modules
¶ 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 inother-modules
.
-
-Wpartial-fields
¶ Since: 8.4 The option
-Wpartial-fields
warns about record fields that could fail when accessed via a lacking constructor. The functionf
below will fail when applied toBar
, so the compiler will emit a warning at its definition when-Wpartial-fields
is enabled.The warning is suppressed if the field name begins with an underscore.
data Foo = Foo { f :: Int } | Bar
-
-Wunused-packages
¶ Since: 8.10 The option
-Wunused-packages
warns about packages, specified on command line via-package ⟨pkg⟩
or-package-id ⟨unit-id⟩
, but were not loaded during compilation. Usually it means that you have an unused dependency.You may want to enable this warning on a clean build or enable
-fforce-recomp
in order to get reliable results.
-
-Winvalid-haddock
¶ 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
-haddock
is disabled.
-
-Wderiving-typeable
¶ Since: 7.10 This flag warns when
Typeable
is listed in a deriving clause or derived withStandaloneDeriving
.Since GHC 7.10,
Typeable
is automatically derived for all types. Thus, derivingTypeable
yourself is redundant.
If you’re feeling really paranoid, the -dcore-lint
option is a good choice.
It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC’s
sanity, not yours.)