6.20. Pragmas

GHC supports several pragmas, or instructions to the compiler placed in the source code. Pragmas don’t normally affect the meaning of the program, but they might affect the efficiency of the generated code.

Pragmas all take the form {-# word ... #-} where ⟨word⟩ indicates the type of pragma, and is followed optionally by information specific to that type of pragma. Case is ignored in ⟨word⟩. The various values for ⟨word⟩ that GHC understands are described in the following sections; any pragma encountered with an unrecognised ⟨word⟩ is ignored.

Certain pragmas are file-header pragmas:

  • A file-header pragma must precede the module keyword in the file.
  • There can be as many file-header pragmas as you please, and they can be preceded or followed by comments.
  • File-header pragmas are read once only, before pre-processing the file (e.g. with cpp).
  • The file-header pragmas are: {-# LANGUAGE #-}, {-# OPTIONS_GHC #-}, and {-# INCLUDE #-}.

6.20.1. LANGUAGE pragma

{-# LANGUAGE ⟨ext⟩, ⟨ext⟩, ... #-}
Where:file header

Enable or disable a set of language extensions.

The LANGUAGE pragma allows language extensions to be enabled in a portable way. It is the intention that all Haskell compilers support the LANGUAGE pragma with the same syntax, although not all extensions are supported by all compilers, of course. The LANGUAGE pragma should be used instead of OPTIONS_GHC, if possible.

For example, to enable the FFI and preprocessing with CPP:

{-# LANGUAGE ForeignFunctionInterface, CPP #-}

LANGUAGE is a file-header pragma (see Pragmas).

Every language extension can also be turned into a command-line flag by prefixing it with “-X”; for example -XForeignFunctionInterface. (Similarly, all “-X” flags can be written as LANGUAGE pragmas.)

A list of all supported language extensions can be obtained by invoking ghc --supported-extensions (see --supported-extensions).

Any extension from the Extension type defined in Language.Haskell.Extension may be used. GHC will report an error if any of the requested extensions are not supported.

6.20.2. OPTIONS_GHC pragma

{-# OPTIONS_GHC ⟨flags⟩ #-}
Where:file header

The OPTIONS_GHC pragma is used to specify additional options that are given to the compiler when compiling this source file. See Command line options in source files for details.

Previous versions of GHC accepted OPTIONS rather than OPTIONS_GHC, but that is now deprecated.

OPTIONS_GHC is a file-header pragma (see Pragmas).

6.20.3. INCLUDE pragma

The INCLUDE used to be necessary for specifying header files to be included when using the FFI and compiling via C. It is no longer required for GHC, but is accepted (and ignored) for compatibility with other compilers.

6.20.4. WARNING and DEPRECATED pragmas

{-# WARNING #-}

The WARNING pragma allows you to attach an arbitrary warning to a particular function, class, or type.


A DEPRECATED pragma lets you specify that a particular function, class, or type is deprecated.

There are two ways of using these pragmas.

  • You can work on an entire module thus:

    module Wibble {-# DEPRECATED "Use Wobble instead" #-} where


    module Wibble {-# WARNING "This is an unstable interface." #-} where

    When you compile any module that import Wibble, GHC will print the specified message.

  • You can attach a warning to a function, class, type, or data constructor, with the following top-level declarations:

    {-# DEPRECATED f, C, T "Don't use these" #-}
    {-# WARNING unsafePerformIO "This is unsafe; I hope you know what you're doing" #-}

    When you compile any module that imports and uses any of the specified entities, GHC will print the specified message.

    You can only attach to entities declared at top level in the module being compiled, and you can only use unqualified names in the list of entities. A capitalised name, such as T refers to either the type constructor T or the data constructor T, or both if both are in scope. If both are in scope, there is currently no way to specify one without the other (c.f. fixities Infix type constructors, classes, and type variables).

Also note that the argument to DEPRECATED and WARNING can also be a list of strings, in which case the strings will be presented on separate lines in the resulting warning message,

{-# DEPRECATED foo, bar ["Don't use these", "Use gar instead"] #-}

Warnings and deprecations are not reported for (a) uses within the defining module, (b) defining a method in a class instance, and (c) uses in an export list. The latter reduces spurious complaints within a library in which one module gathers together and re-exports the exports of several others.

You can suppress the warnings with the flag -Wno-warnings-deprecations.

6.20.5. MINIMAL pragma

{-# MINIMAL ⟨name⟩ | ⟨name⟩ , ... #-}
Where:in class body

Define the methods needed for a minimal complete instance of a class.

The MINIMAL pragma is used to specify the minimal complete definition of a class, i.e. specify which methods must be implemented by all instances. If an instance does not satisfy the minimal complete definition, then a warning is generated. This can be useful when a class has methods with circular defaults. For example

class Eq a where
    (==) :: a -> a -> Bool
    (/=) :: a -> a -> Bool
    x == y = not (x /= y)
    x /= y = not (x == y)
    {-# MINIMAL (==) | (/=) #-}

Without the MINIMAL pragma no warning would be generated for an instance that implements neither method.

The syntax for minimal complete definition is:

mindef ::= name
        |  '(' mindef ')'
        |  mindef '|' mindef
        |  mindef ',' mindef

A vertical bar denotes disjunction, i.e. one of the two sides is required. A comma denotes conjunction, i.e. both sides are required. Conjunction binds stronger than disjunction.

If no MINIMAL pragma is given in the class declaration, it is just as if a pragma {-# MINIMAL op1, op2, ..., opn #-} was given, where the opi are the methods that lack a default method in the class declaration (c.f. -Wmissing-methods, Warnings and sanity-checking).

This warning can be turned off with the flag -Wno-missing-methods.

6.20.6. INLINE and NOINLINE pragmas

These pragmas control the inlining of function definitions. INLINE pragma

{-# INLINE ⟨name⟩ #-}
Where:any function definition

Force GHC to inline a value.

GHC (with -O, as always) tries to inline (or “unfold”) functions/values that are “small enough,” thus avoiding the call overhead and possibly exposing other more-wonderful optimisations. GHC has a set of heuristics, tuned over a long period of time using many benchmarks, that decide when it is beneficial to inline a function at its call site. The heuristics are designed to inline functions when it appears to be beneficial to do so, but without incurring excessive code bloat. If a function looks too big, it won’t be inlined, and functions larger than a certain size will not even have their definition exported in the interface file. Some of the thresholds that govern these heuristic decisions can be changed using flags, see -f*: platform-independent flags.

Normally GHC will do a reasonable job of deciding by itself when it is a good idea to inline a function. However, sometimes you might want to override the default behaviour. For example, if you have a key function that is important to inline because it leads to further optimisations, but GHC judges it to be too big to inline.

The sledgehammer you can bring to bear is the INLINE pragma, used thusly:

key_function :: Int -> String -> (Bool, Double)
{-# INLINE key_function #-}

The major effect of an INLINE pragma is to declare a function’s “cost” to be very low. The normal unfolding machinery will then be very keen to inline it. However, an INLINE pragma for a function “f” has a number of other effects:

  • While GHC is keen to inline the function, it does not do so blindly. For example, if you write

    map key_function xs

    there really isn’t any point in inlining key_function to get

    map (\x -> body) xs

    In general, GHC only inlines the function if there is some reason (no matter how slight) to suppose that it is useful to do so.

  • Moreover, GHC will only inline the function if it is fully applied, where “fully applied” means applied to as many arguments as appear (syntactically) on the LHS of the function definition. For example:

    comp1 :: (b -> c) -> (a -> b) -> a -> c
    {-# INLINE comp1 #-}
    comp1 f g = \x -> f (g x)
    comp2 :: (b -> c) -> (a -> b) -> a -> c
    {-# INLINE comp2 #-}
    comp2 f g x = f (g x)

    The two functions comp1 and comp2 have the same semantics, but comp1 will be inlined when applied to two arguments, while comp2 requires three. This might make a big difference if you say

    map (not `comp1` not) xs

    which will optimise better than the corresponding use of comp2.

  • It is useful for GHC to optimise the definition of an INLINE function f just like any other non-INLINE function, in case the non-inlined version of f is ultimately called. But we don’t want to inline the optimised version of f; a major reason for INLINE pragmas is to expose functions in f’s RHS that have rewrite rules, and it’s no good if those functions have been optimised away.

    So GHC guarantees to inline precisely the code that you wrote, no more and no less. It does this by capturing a copy of the definition of the function to use for inlining (we call this the “inline-RHS”), which it leaves untouched, while optimising the ordinarily RHS as usual. For externally-visible functions the inline-RHS (not the optimised RHS) is recorded in the interface file.

  • An INLINE function is not worker/wrappered by strictness analysis. It’s going to be inlined wholesale instead.

GHC ensures that inlining cannot go on forever: every mutually-recursive group is cut by one or more loop breakers that is never inlined (see Secrets of the GHC inliner, JFP 12(4) July 2002). GHC tries not to select a function with an INLINE pragma as a loop breaker, but when there is no choice even an INLINE function can be selected, in which case the INLINE pragma is ignored. For example, for a self-recursive function, the loop breaker can only be the function itself, so an INLINE pragma is always ignored.

INLINE pragmas are a particularly good idea for the then/return (or bind/unit) functions in a monad. For example, in GHC’s own UniqueSupply monad code, we have:

{-# INLINE thenUs #-}
{-# INLINE returnUs #-}

See also the NOINLINE (NOINLINE pragma) and INLINABLE (INLINABLE pragma) pragmas. INLINE pragma effects on various locations

Syntactically, an INLINE pragma for a function can be put anywhere its type signature could be put. This means a INLINE pragma can really be put on any definition site for a binding. This includes top-level, let and where bindings as well as default class methods and instance declarations.

The pragma itself will only have an effect when the RHS of the binding it’s applied to is used. For regular bindings this is straight forward but for class methods and instance definitions this can have surprising ramifications.

If we consider a class definition with two instances like this:

class C a where
    op1 :: a -> a

    op2 :: [a] -> [a]
    op2 xs = reverse (xs ++ xs)
    {-# INLINE op2 #-}

instance C T1 where
    op1 x = ...blah...

instance C T2 where
    {-# INLINE op1 #-}
    op1 x = ...blah...
    op2 xs = ...blah...

Then op2 for the T1 instance will get an implicit INLINE pragma. This is because the RHS of the default method is used for op2 which retains it’s INLINE pragma.

In the T2 instance op1 gets an INLINE pragma and behaves accordingly. However op2 for T2 is not implemented by the default method. This means the pragma in the class definition doesn’t apply to this instance. With no pragma being explicitly applied GHC will then decide on a proper inlining behaviour for T2s op2 method on it’s own. INLINABLE pragma

{-# INLINABLE ⟨name⟩ #-}
Where:any function definition

Suggest that the compiler always consider inlining name.

An {-# INLINABLE f #-} pragma on a function f has the following behaviour:

  • While INLINE says “please inline me”, the INLINABLE says “feel free to inline me; use your discretion”. In other words the choice is left to GHC, which uses the same rules as for pragma-free functions. Unlike INLINE, that decision is made at the call site, and will therefore be affected by the inlining threshold, optimisation level etc.
  • Like INLINE, the INLINABLE pragma retains a copy of the original RHS for inlining purposes, and persists it in the interface file, regardless of the size of the RHS.
  • One way to use INLINABLE is in conjunction with the special function inline (Special built-in functions). The call inline f tries very hard to inline f. To make sure that f can be inlined, it is a good idea to mark the definition of f as INLINABLE, so that GHC guarantees to expose an unfolding regardless of how big it is. Moreover, by annotating f as INLINABLE, you ensure that f’s original RHS is inlined, rather than whatever random optimised version of f GHC’s optimiser has produced.
  • The INLINABLE pragma also works with SPECIALISE: if you mark function f as INLINABLE, then you can subsequently SPECIALISE in another module (see SPECIALIZE pragma).
  • Unlike INLINE, it is OK to use an INLINABLE pragma on a recursive function. The principal reason do to so to allow later use of SPECIALISE

The alternative spelling INLINEABLE is also accepted by GHC. NOINLINE pragma

{-# NOINLINE ⟨name⟩ #-}
Where:any function definition

Instructs the compiler not to inline a value.

The NOINLINE pragma does exactly what you’d expect: it stops the named function from being inlined by the compiler. You shouldn’t ever need to do this, unless you’re very cautious about code size.

NOTINLINE is a synonym for NOINLINE (NOINLINE is specified by Haskell 98 as the standard way to disable inlining, so it should be used if you want your code to be portable). CONLIKE modifier

{-# CONLIKE #-}
Where:modifies INLINE or NOINLINE pragma

Instructs GHC to consider a value to be especially cheap to inline.

An INLINE or NOINLINE pragma may have a CONLIKE modifier, which affects matching in RULEs (only). See How rules interact with CONLIKE pragmas. Phase control

Sometimes you want to control exactly when in GHC’s pipeline the INLINE pragma is switched on. Inlining happens only during runs of the simplifier. Each run of the simplifier has a different phase number; the phase number decreases towards zero. If you use -dverbose-core2core you will see the sequence of phase numbers for successive runs of the simplifier. In an INLINE pragma you can optionally specify a phase number, thus:

  • INLINE[k] f” means: do not inline f until phase k, but from phase k onwards be very keen to inline it.
  • INLINE[~k] f” means: be very keen to inline f until phase k, but from phase k onwards do not inline it.
  • NOINLINE[k] f” means: do not inline f until phase k, but from phase k onwards be willing to inline it (as if there was no pragma).
  • NOINLINE[~k] f” means: be willing to inline f until phase k, but from phase k onwards do not inline it.

The same information is summarised here:

                         -- Before phase 2     Phase 2 and later
{-# INLINE   [2]  f #-}  --      No                 Yes
{-# INLINE   [~2] f #-}  --      Yes                No
{-# NOINLINE [2]  f #-}  --      No                 Maybe
{-# NOINLINE [~2] f #-}  --      Maybe              No

{-# INLINE   f #-}       --      Yes                Yes
{-# NOINLINE f #-}       --      No                 No

By “Maybe” we mean that the usual heuristic inlining rules apply (if the function body is small, or it is applied to interesting-looking arguments etc). Another way to understand the semantics is this:

  • For both INLINE and NOINLINE, the phase number says when inlining is allowed at all.
  • The INLINE pragma has the additional effect of making the function body look small, so that when inlining is allowed it is very likely to happen.

The same phase-numbering control is available for RULEs (Rewrite rules).

6.20.7. OPAQUE pragma

{-# OPAQUE ⟨name⟩ #-}

Instructs the compiler to ensure that every call of name remains a call of name, and not some name-mangled variant.

The OPAQUE pragma is an even stronger variant of the NOINLINE pragma. Like the NOINLINE, named functions annotated with a OPAQUE pragma are not inlined, nor will they be be specialized. Unlike the NOINLINE, named functions annotated with a OPAQUE pragma are left untouched by the Worker/Wrapper transformation. Unlike NOINLINE, OPAQUE has no phase control.

In effect, every call of a named function annotated with an OPAQUE pragma remains a call of that named function, not some name-mangled variant. You shouldn’t ever need to use the OPAQUE pragma, unless you have a reason to care about name-mangling.

6.20.8. LINE pragma

{-# LINE ⟨lineno⟩ "⟨file⟩" #-}

Generated by preprocessors to convey source line numbers of the original source.

This pragma is similar to C’s #line pragma, and is mainly for use in automatically generated Haskell code. It lets you specify the line number and filename of the original code; for example

{-# LINE 42 "Foo.vhs" #-}

if you’d generated the current file from something called Foo.vhs and this line corresponds to line 42 in the original. GHC will adjust its error messages to refer to the line/file named in the LINE pragma.

LINE pragmas generated from Template Haskell set the file and line position for the duration of the splice and are limited to the splice. Note that because Template Haskell splices abstract syntax, the file positions are not automatically advanced.

6.20.9. COLUMN pragma

This is the analogue of the LINE pragma and is likewise intended for use in automatically generated Haskell code. It lets you specify the column number of the original code; for example

foo = do
  {-# COLUMN 42 #-}pure ()
  pure ()

This adjusts all column numbers immediately after the pragma to start at 42. The presence of this pragma only affects the quality of the diagnostics and does not change the syntax of the code itself.

6.20.10. RULES pragma

The RULES pragma lets you specify rewrite rules. It is described in Rewrite rules.

6.20.11. SPECIALIZE pragma

{-# SPECIALIZE ⟨name⟩ :: ⟨type⟩ #-}

Ask that GHC specialize a polymorphic value to a particular type.

(UK spelling also accepted.) For key overloaded functions, you can create extra versions (NB: at the cost of larger code) specialised to particular types. Thus, if you have an overloaded function:

hammeredLookup :: Ord key => [(key, value)] -> key -> value

If it is heavily used on lists with Widget keys, you could specialise it as follows:

{-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
  • A SPECIALIZE pragma for a function can be put anywhere its type signature could be put. Moreover, you can also SPECIALIZE an imported function provided it was given an INLINABLE pragma at its definition site (INLINABLE pragma).

  • A SPECIALIZE has the effect of generating (a) a specialised version of the function and (b) a rewrite rule (see Rewrite rules) that rewrites a call to the un-specialised function into a call to the specialised one. Moreover, given a SPECIALIZE pragma for a function f, GHC will automatically create specialisations for any type-class-overloaded functions called by f, if they are in the same module as the SPECIALIZE pragma, or if they are INLINABLE; and so on, transitively.

  • You can add phase control (Phase control) to the RULE generated by a SPECIALIZE pragma, just as you can if you write a RULE directly. For example:

    {-# SPECIALIZE [0] hammeredLookup :: [(Widget, value)] -> Widget -> value #-}

    generates a specialisation rule that only fires in Phase 0 (the final phase). If you do not specify any phase control in the SPECIALIZE pragma, the phase control is inherited from the inline pragma (if any) of the function. For example:

    foo :: Num a => a -> a
    foo = ...blah...
    {-# NOINLINE [0] foo #-}
    {-# SPECIALIZE foo :: Int -> Int #-}

    The NOINLINE pragma tells GHC not to inline foo until Phase 0; and this property is inherited by the specialisation RULE, which will therefore only fire in Phase 0.

    The main reason for using phase control on specialisations is so that you can write optimisation RULES that fire early in the compilation pipeline, and only then specialise the calls to the function. If specialisation is done too early, the optimisation rules might fail to fire.

  • The type in a SPECIALIZE pragma can be any type that is less polymorphic than the type of the original function. In concrete terms, if the original function is f then the pragma

    {-# SPECIALIZE f :: <type> #-}

    is valid if and only if the definition

    f_spec :: <type>
    f_spec = f

    is valid. Here are some examples (where we only give the type signature for the original function, not its code):

    f :: Eq a => a -> b -> b
    {-# SPECIALISE f :: Int -> b -> b #-}
    g :: (Eq a, Ix b) => a -> b -> b
    {-# SPECIALISE g :: (Eq a) => a -> Int -> Int #-}
    h :: Eq a => a -> a -> a
    {-# SPECIALISE h :: (Eq a) => [a] -> [a] -> [a] #-}

    The last of these examples will generate a RULE with a somewhat-complex left-hand side (try it yourself), so it might not fire very well. If you use this kind of specialisation, let us know how well it works. SPECIALIZE INLINE

{-# SPECIALIZE INLINE ⟨name⟩ :: ⟨type⟩ #-}

A SPECIALIZE pragma can optionally be followed with a INLINE or NOINLINE pragma, optionally followed by a phase, as described in INLINE and NOINLINE pragmas. The INLINE pragma affects the specialised version of the function (only), and applies even if the function is recursive. The motivating example is this:

-- A GADT for arrays with type-indexed representation
data Arr e where
  ArrInt :: !Int -> ByteArray# -> Arr Int
  ArrPair :: !Int -> Arr e1 -> Arr e2 -> Arr (e1, e2)

(!:) :: Arr e -> Int -> e
{-# SPECIALISE INLINE (!:) :: Arr Int -> Int -> Int #-}
{-# SPECIALISE INLINE (!:) :: Arr (a, b) -> Int -> (a, b) #-}
(ArrInt _ ba)     !: (I# i) = I# (indexIntArray# ba i)
(ArrPair _ a1 a2) !: i      = (a1 !: i, a2 !: i)

Here, (!:) is a recursive function that indexes arrays of type Arr e. Consider a call to (!:) at type (Int,Int). The second specialisation will fire, and the specialised function will be inlined. It has two calls to (!:), both at type Int. Both these calls fire the first specialisation, whose body is also inlined. The result is a type-based unrolling of the indexing function.

You can add explicit phase control (Phase control) to SPECIALISE INLINE pragma, just like on an INLINE pragma; if you do so, the same phase is used for the rewrite rule and the INLINE control of the specialised function.


You can make GHC diverge by using SPECIALISE INLINE on an ordinarily-recursive function. SPECIALIZE for imported functions

Generally, you can only give a SPECIALIZE pragma for a function defined in the same module. However if a function f is given an INLINABLE pragma at its definition site, then it can subsequently be specialised by importing modules (see INLINABLE pragma). For example

module Map( lookup, blah blah ) where
  lookup :: Ord key => [(key,a)] -> key -> Maybe a
  lookup = ...
  {-# INLINABLE lookup #-}

module Client where
  import Map( lookup )

  data T = T1 | T2 deriving( Eq, Ord )
  {-# SPECIALISE lookup :: [(T,a)] -> T -> Maybe a

Here, lookup is declared INLINABLE, but it cannot be specialised for type T at its definition site, because that type does not exist yet. Instead a client module can define T and then specialise lookup at that type.

Moreover, every module that imports Client (or imports a module that imports Client, transitively) will “see”, and make use of, the specialised version of lookup. You don’t need to put a SPECIALIZE pragma in every module.

Moreover you often don’t even need the SPECIALIZE pragma in the first place. When compiling a module M, GHC’s optimiser (when given the -O flag) automatically considers each top-level overloaded function declared in M, and specialises it for the different types at which it is called in M. The optimiser also considers each imported INLINABLE overloaded function, and specialises it for the different types at which it is called in M. So in our example, it would be enough for lookup to be called at type T:

module Client where
  import Map( lookup )

  data T = T1 | T2 deriving( Eq, Ord )

  findT1 :: [(T,a)] -> Maybe a
  findT1 m = lookup m T1   -- A call of lookup at type T

However, sometimes there are no such calls, in which case the pragma can be useful.

6.20.12. SPECIALIZE instance pragma

{-# SPECIALIZE instance ⟨instance head⟩ #-}
Where:instance body

Same idea, except for instance declarations. For example:

instance (Eq a) => Eq (Foo a) where {
   {-# SPECIALIZE instance Eq (Foo [(Int, Bar)]) #-}
   ... usual stuff ...

The pragma must occur inside the where part of the instance declaration.

6.20.13. UNPACK pragma

{-# UNPACK #-}
Where:data constructor field

Instructs the compiler to unpack the contents of a constructor field into the constructor itself.

The UNPACK indicates to the compiler that it should unpack the contents of a constructor field into the constructor itself, removing a level of indirection. For example:

data T = T {-# UNPACK #-} !Float
           {-# UNPACK #-} !Float

will create a constructor T containing two unboxed floats. This may not always be an optimisation: if the T constructor is scrutinised and the floats passed to a non-strict function for example, they will have to be reboxed (this is done automatically by the compiler).

Unpacking constructor fields should only be used in conjunction with -O [1], in order to expose unfoldings to the compiler so the reboxing can be removed as often as possible. For example:

f :: T -> Float
f (T f1 f2) = f1 + f2

The compiler will avoid reboxing f1 and f2 by inlining + on floats, but only when -O is on.

Any single-constructor data is eligible for unpacking; for example

data T = T {-# UNPACK #-} !(Int,Int)

will store the two Ints directly in the T constructor, by flattening the pair. Multi-level unpacking is also supported:

data T = T {-# UNPACK #-} !S
data S = S {-# UNPACK #-} !Int {-# UNPACK #-} !Int

will store two unboxed Int#s directly in the T constructor. The unpacker can see through newtypes, too.

See also the -funbox-strict-fields flag, which essentially has the effect of adding {-# UNPACK #-} to every strict constructor field.

[1]In fact, UNPACK has no effect without -O, for technical reasons (see %s5252).

6.20.14. NOUNPACK pragma

{-# NOUNPACK #-}

Instructs the compiler not to unpack a constructor field.

The NOUNPACK pragma indicates to the compiler that it should not unpack the contents of a constructor field. Example:

data T = T {-# NOUNPACK #-} !(Int,Int)

Even with the flags -funbox-strict-fields and -O, the field of the constructor T is not unpacked.

6.20.15. SOURCE pragma

{-# SOURCE #-}
Where:after import statement

Import a module by hs-boot file to break a module loop.

The {-# SOURCE #-} pragma is used only in import declarations, to break a module loop. It is described in detail in How to compile mutually recursive modules.

6.20.16. COMPLETE pragmas

{-# COMPLETE #-}
Where:at top level

Specify the set of constructors or pattern synonyms which constitute a total match.

The COMPLETE pragma is used to inform the pattern match checker that a certain set of patterns is complete and that any function which matches on all the specified patterns is total.

The most common usage of COMPLETE pragmas is with Pattern synonyms. On its own, the checker is very naive and assumes that any match involving a pattern synonym will fail. As a result, any pattern match on a pattern synonym is regarded as incomplete unless the user adds a catch-all case.

For example, the data types 2 * A and A + A are isomorphic but some computations are more naturally expressed in terms of one or the other. To get the best of both worlds, we can choose one as our implementation and then provide a set of pattern synonyms so that users can use the other representation if they desire. We can then specify a COMPLETE pragma in order to inform the pattern match checker that a function which matches on both LeftChoice and RightChoice is total.

data Choice a = Choice Bool a

pattern LeftChoice :: a -> Choice a
pattern LeftChoice a = Choice False a

pattern RightChoice :: a -> Choice a
pattern RightChoice a = Choice True a

{-# COMPLETE LeftChoice, RightChoice #-}

foo :: Choice Int -> Int
foo (LeftChoice n) = n * 2
foo (RightChoice n) = n - 2

COMPLETE pragmas are only used by the pattern match checker. If a function definition matches on all the constructors specified in the pragma then the compiler will produce no warning.

COMPLETE pragmas can contain any data constructors or pattern synonyms which are in scope, but must mention at least one data constructor or pattern synonym defined in the same module. COMPLETE pragmas may only appear at the top level of a module. Once defined, they are automatically imported and exported from modules. COMPLETE pragmas should be thought of as asserting a universal truth about a set of patterns and as a result, should not be used to silence context specific incomplete match warnings.

It is also possible to restrict the types to which a COMPLETE pragma applies by putting a double colon :: after the list of constructors, followed by a result type constructor, which will be used to restrict the cases in which the pragma applies. GHC will compare the annotated result type constructor with the type constructor in the head of the scrutinee type in a pattern match to see if the COMPLETE pragma is meant to apply to it.

This is especially useful in cases that the constructors specified are polymorphic, e.g.:

data Proxy a = Proxy

class IsEmpty a where
  isEmpty :: a -> Bool

class IsCons a where
  type Elt a
  isCons :: a -> Maybe (Elt a, a)

pattern Empty :: IsEmpty a => a
pattern Empty <- (isEmpty -> True)

pattern Cons :: IsCons a => Elt a -> a -> a
pattern Cons x xs <- (isCons -> Just (x,xs))

instance IsEmpty (Proxy a) where
  isEmpty Proxy = True

instance IsEmpty [a] where
  isEmpty = null

instance IsCons [a] where
  type Elt [a] = a
  isCons [] = Nothing
  isCons (x:xs) = Just (x,xs)

{-# COMPLETE Empty :: Proxy #-}
{-# COMPLETE Empty, Cons :: [] #-}

foo :: Proxy a -> Int
foo Empty = 0

bar :: [a] -> Int
bar Empty = 0
bar (Cons _ _) = 1

baz :: [a] -> Int
baz Empty = 0

In this example, foo and bar will not be warned about, as their pattern matches are covered by the two COMPLETE pragmas above, but baz will be warned about as incomplete.


{-# OVERLAPS #-}
Where:on instance head

The pragmas OVERLAPPING, OVERLAPPABLE, OVERLAPS, INCOHERENT are used to specify the overlap behavior for individual instances, as described in Section Overlapping instances. The pragmas are written immediately after the instance keyword, like this:

instance {-# OVERLAPPING #-} C t where ...