6.2. 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. 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

-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

-Wno-compat

Disables all warnings enabled by -Wcompat.

-w

Turns off all warnings, including the standard ones and those that -Wall doesn’t enable.

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.

-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.

-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.

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

Enables warnings when the compiler encounters a -W... flag that is not recognised.

This warning is on by default.

-Wtyped-holes

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

This warning is on by default.

-Wtype-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

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 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 -Wdeferred-out-of-scope-variables.

-Wpartial-type-signatures

Determines whether the compiler reports holes in partial type signatures as warnings. Has no effect unless -XPartialTypeSignatures is enabled, which controls whether errors should be generated for holes in types or not. See 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 and DERIVE.

This option is on by default.

-Wmissed-specialisations
-Wall-missed-specialisations

Emits a warning if GHC cannot specialise an overloaded function, usually because the function needs an INLINABLE 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 INLINABLE (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 -Werror.

These options are both off by default.

-Wwarnings-deprecations

Causes a warning to be emitted when a module, function or type with a WARNING or DEPRECATED pragma is used. See WARNING and DEPRECATED pragmas for more details on the pragmas.

This option is on by default.

-Wamp

This option is deprecated.

Caused a warning to be emitted when a definition was in conflict with the AMP (Applicative-Monad proosal).

-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.

-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 -Wmissing-monadfail-instances.

This option is off by default.

-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 -Wcompat option group.

-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.

  1. Instances of Monoid should also be instances of Semigroup
  2. The Semigroup operator (<>) will be in Prelude, which clashes with custom local definitions of such an operator

Being part of the -Wcompat option group, this warning is off by default, but will be switched on in a future GHC release.

-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 as ccall.

-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.

-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.
-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].

-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 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 -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.

-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.

-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 by -XNoImplicitPrelude or a LANGUAGE NoImplicitPrelude pragma).

Note that no warning is given for syntax that implicitly refers to the Prelude, even if -XNoImplicitPrelude would change whether it refers to the Prelude. For example, no warning is given when 368 means Prelude.fromInteger (368::Prelude.Integer) (where Prelude refers to the actual Prelude module, regardless of the imports of the module being compiled).

This warning is off by default.

-Wincomplete-patterns
-Wincomplete-uni-patterns

The option -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 -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.

The 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
-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.

-Wincomplete-record-updates

The function f below will fail when applied to Bar, 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-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-import-lists

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 -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.

-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 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 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.

-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 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

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 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.

-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.

-Wunsupported-llvm-version

Warn when using -fllvm with an unsupported version of LLVM.

-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 and Succ 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, 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.

-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 report x and y 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 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 }
-Wunused-type-patterns

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 -Wunused-matches, -Wunused-type-variables 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 and c as unused.

-Wwrong-do-bind

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 }
-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

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 in other-modules.

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.)