8.12. 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 (silently) 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.

8.12.1. LANGUAGE pragma

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 Section 8.12, “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-languages (see Section 5.4, “Modes of operation”).

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.

8.12.2. OPTIONS_GHC pragma

The OPTIONS_GHC pragma is used to specify additional options that are given to the compiler when compiling this source file. See Section 5.1.2, “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 Section 8.12, “Pragmas”).

8.12.3. INCLUDE pragma

The INCLUDE pragma is for specifying the names of C header files that should be #include'd into the C source code generated by the compiler for the current module (if compiling via C). For example:

{-# INCLUDE "foo.h" #-}
{-# INCLUDE <stdio.h> #-}

INCLUDE is a file-header pragma (see Section 8.12, “Pragmas”).

An INCLUDE pragma is the preferred alternative to the -#include option (Section 5.10.5, “Options affecting the C compiler (if applicable)”), because the INCLUDE pragma is understood by other compilers. Yet another alternative is to add the include file to each foreign import declaration in your code, but we don't recommend using this approach with GHC.

8.12.4. DEPRECATED pragma

The DEPRECATED pragma lets you specify that a particular function, class, or type, is deprecated. There are two forms.

  • You can deprecate an entire module thus:

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

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

  • You can deprecate a function, class, type, or data constructor, with the following top-level declaration:

       {-# DEPRECATED f, C, T "Don't use these" #-}

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

    You can only deprecate entities declared at top level in the module being compiled, and you can only use unqualified names in the list of entities being deprecated. 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 deprecate one without the other (c.f. fixities Section 8.4.2, “Infix type constructors, classes, and type variables”).

Any use of the deprecated item, or of anything from a deprecated module, will be flagged with an appropriate message. However, deprecations are not reported for (a) uses of a deprecated function within its defining module, and (b) uses of a deprecated function 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 -fno-warn-deprecations.

8.12.5. INLINE and NOINLINE pragmas

These pragmas control the inlining of function definitions. INLINE pragma

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. Normally, if GHC decides a function is “too expensive” to inline, it will not do so, nor will it export that unfolding for other modules to use.

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

key_function :: Int -> String -> (Bool, Double)

{-# INLINE key_function #-}

(You don't need to do the C pre-processor carry-on unless you're going to stick the code through HBC—it doesn't like INLINE pragmas.)

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.

Syntactically, an INLINE pragma for a function can be put anywhere its type signature could be put.

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 pragma (Section, “NOINLINE pragma”). NOINLINE pragma

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). 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'll 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).

  • "INLINE[~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 (Section 8.13, “Rewrite rules ”).

8.12.6. LINE pragma

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.

8.12.7. RULES pragma

The RULES pragma lets you specify rewrite rules. It is described in Section 8.13, “Rewrite rules ”.

8.12.8. SPECIALIZE pragma

(UK spelling also accepted.) For key overloaded functions, you can create extra versions (NB: more code space) 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.

A SPECIALIZE has the effect of generating (a) a specialised version of the function and (b) a rewrite rule (see Section 8.13, “Rewrite rules ”) that rewrites a call to the un-specialised function into a call to the specialised one.

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.

A SPECIALIZE pragma can optionally be followed with a INLINE or NOINLINE pragma, optionally followed by a phase, as described in Section 8.12.5, “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.

Warning: you can make GHC diverge by using SPECIALISE INLINE on an ordinarily-recursive function.

Note: In earlier versions of GHC, it was possible to provide your own specialised function for a given type:

{-# SPECIALIZE hammeredLookup :: [(Int, value)] -> Int -> value = intLookup #-}

This feature has been removed, as it is now subsumed by the RULES pragma (see Section 8.13.4, “Specialisation ”).

8.12.9. SPECIALIZE instance pragma

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.

Compatible with HBC, by the way, except perhaps in the placement of the pragma.

8.12.10. UNPACK pragma

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, 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.

If a field cannot be unpacked, you will not get a warning, so it might be an idea to check the generated code with -ddump-simpl.

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