Language.C.Inline

Enable painless embedding of C code in Haskell code. If you're interested in how to use the library, skip to the "Inline C" section. To build, read the first two sections.

This module is intended to be imported qualified:

import qualified Language.C.Inline as C

Build process

Each module that uses at least one of the TH functions in this module gets a C file associated to it, where the filename of said file will be the same as the module but with a `.c` extension. This C file must be built after the Haskell code and linked appropriately. If you use cabal, all you have to do is declare each associated C file in the .cabal file.

For example:

executable foo
  main-is:             Main.hs, Foo.hs, Bar.hs
  hs-source-dirs:      src
  -- Here the corresponding C sources must be listed for every module
  -- that uses C code.  In this example, Main.hs and Bar.hs do, but
  -- Foo.hs does not.
  c-sources:           src/Main.c, src/Bar.c
  -- These flags will be passed to the C compiler
  cc-options:          -Wall -O2
  -- Libraries to link the code with.
  extra-libraries:     -lm
  ...

Note that currently cabal repl is not supported, because the C code is not compiled and linked appropriately.

If we were to compile the above manually, we could:

$ ghc -c Main.hs
$ cc -c Main.c -o Main_c.o
$ ghc Foo.hs
$ ghc Bar.hs
$ cc -c Bar.c -o Bar_c.o
$ ghc Main.o Foo.o Bar.o Main_c.o Bar_c.o -lm -o Main

Contexts

data Context

A Context stores various information needed to produce the files with the C code derived from the inline C snippets.

Contexts can be composed with their Monoid instance, where mappend is right-biased -- in mappend x y y will take precedence over x.

Instances

Monoid Context

baseCtx :: Context

Context useful to work with vanilla C. Used by default.

ctxTypesTable: converts C basic types to their counterparts in Foreign.C.Types.

No ctxAntiQuoters.

funCtx :: Context

This Context includes a AntiQuoter that removes the need for explicitely creating FunPtrs, named "fun".

For example, we can capture function f of type CInt -> CInt -> IO CInt in C code using $fun:(int (*f)(int, int)).

When used in a pure embedding, the Haskell function will have to be pure too. Continuing the example above we'll have CInt -> CInt -> IO CInt.

Does not include the baseCtx, since most of the time it's going to be included as part of larger contexts.

vecCtx :: Context

This Context includes two AntiQuoters that allow to easily use Haskell vectors in C.

Specifically, the vec-len and vec-ptr will get the length and the pointer underlying mutable (IOVector) and immutable (Vector) storable vectors.

Note that if you use vecCtx to manipulate immutable vectors you must make sure that the vector is not modified in the C code.

To use vec-len, simply write $vec-len:x, where x is something of type IOVector a or Vector a, for some a. To use vec-ptr you need to specify the type of the pointer, e.g. $vec-len:(int *x) will work if x has type IOVector CInt.

bsCtx :: Context

bsCtx serves exactly the same purpose as vecCtx, but only for ByteString. vec-ptr becomes bs-ptr, and vec-len becomes bs-len. You don't need to specify the type of the pointer in bs-ptr, it will always be unsigned char*.

context :: Context -> DecsQ

Sets the Context for the current module. This function, if called, must be called before any of the other TH functions in this module. Fails if that's not the case.

Inline C

The quasiquoters below are the main interface to this library, for inlining C code into Haskell source files.

In general, quasiquoters are used like so:

[C.XXX| int { <C code> } |]

Where C.XXX is one of the quasi-quoters defined in this section.

This syntax stands for a piece of typed C, decorated with a type:

• The first type to appear (int in the example) is the type of said C code.
• The syntax of the <C code> depends on on the quasi-quoter used, and the anti-quoters available. The exp quasi-quoter expects a C expression. The block quasi-quoter expects a list of statements, like the body of a function. Just like a C function, a block has a return type, matching the type of any values in any return statements appearing in the block.

See also the README.md file for more documentation.

Anti-quoters

Haskell variables can be captured using anti-quoters. inline-c provides a basic anti-quoting mechanism extensible with user-defined anti-quoters (see Language.C.Inline.Context). The basic anti-quoter lets you capture Haskell variables, for example we might say

let x = pi / 3 in [exp| double { cos($(double x)) } |]

Which would capture the Haskell variable x of type CDouble.

In C expressions the $ character is denoted using $$.

Variable capture and the typing relation

The Haskell type of the inlined expression is determined by the specified C return type. The relation between the C type and the Haskell type is defined in the current Context -- see convertCType. C pointers and arrays are both converted to Haskell Ptrs, and function pointers are converted to FunPtrs. Sized arrays are not supported.

Similarly, when capturing Haskell variables using anti-quoting, their type is assumed to be of the Haskell type corresponding to the C type provided. For example, if we capture variable x using double x in the parameter list, the code will expect a variable x of type CDouble in Haskell (when using baseCtx).

Purity

The exp and block quasi-quotes denote computations in the IO monad. pure denotes a pure value, expressed as a C expression.

Safe and unsafe calls

unsafe variants of the quasi-quoters are provided in Language.C.Inline.Unsafe to call the C code unsafely, in the sense that the C code will block the RTS, but with the advantage of a faster call to the foreign code. See https://www.haskell.org/onlinereport/haskell2010/haskellch8.html#x15-1590008.4.3.

Examples

Inline C expression

{-# LANGUAGE QuasiQuotes #-}
import qualified Language.C.Inline as C
import qualified Language.C.Inline.Unsafe as CU
import           Foreign.C.Types

C.include "<math.h>"

c_cos :: CDouble -> IO CDouble
c_cos x = [C.exp| double { cos($(double x)) } |]

faster_c_cos :: CDouble -> IO CDouble
faster_c_cos x = [CU.exp| double { cos($(double x)) } |]

Inline C statements

{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
import qualified Data.Vector.Storable.Mutable as V
import qualified Language.C.Inline as C
import           Foreign.C.Types

C.include "<stdio.h>"

parseVector :: CInt -> IO (V.IOVector CDouble)
parseVector len = do
  vec <- V.new $ fromIntegral len0
  V.unsafeWith vec $ \ptr -> [C.block| void {
    int i;
    for (i = 0; i < $(int len); i++) {
      scanf("%lf ", &$(double *ptr)[i]);
    }
  } |]
  return vec

exp :: QuasiQuoter

C expressions.

pure :: QuasiQuoter

Variant of exp, for use with expressions known to have no side effects.

BEWARE: use this function with caution, only when you know what you are doing. If an expression does in fact have side-effects, then indiscriminate use of pure may endanger referential transparency, and in principle even type safety.

block :: QuasiQuoter

C code blocks (i.e. statements).

include :: String -> DecsQ

Emits a CPP include directive for C code associated with the current module. To avoid having to escape quotes, the function itself adds them when appropriate, so that

include "foo.h" ==> #include "foo.h"

but

include "<foo>" ==> #include <foo>

verbatim :: String -> DecsQ

Emits an arbitrary C string to the C code associated with the current module. Use with care.

Ptr utils

withPtr :: Storable a => (Ptr a -> IO b) -> IO (a, b)

Like alloca, but also peeks the contents of the Ptr and returns them once the provided action has finished.

withPtr_ :: Storable a => (Ptr a -> IO ()) -> IO a

class WithPtrs a where

Type class with methods useful to allocate and peek multiple pointers at once:

withPtrs_ :: (Storable a, Storable b) => ((Ptr a, Ptr b) -> IO ()) -> IO (a, b)
withPtrs_ :: (Storable a, Storable b, Storable c) => ((Ptr a, Ptr b, Ptr c) -> IO ()) -> IO (a, b, c)
...

Minimal complete definition

withPtrs

Associated Types

type WithPtrsPtrs a :: *

Methods

withPtrs :: (WithPtrsPtrs a -> IO b) -> IO (a, b)

withPtrs_ :: (WithPtrsPtrs a -> IO ()) -> IO a

Instances

(Storable a, Storable b) => WithPtrs (a, b)

(Storable a, Storable b, Storable c) => WithPtrs (a, b, c)

(Storable a, Storable b, Storable c, Storable d) => WithPtrs (a, b, c, d)

(Storable a, Storable b, Storable c, Storable d, Storable e) => WithPtrs (a, b, c, d, e)

(Storable a, Storable b, Storable c, Storable d, Storable e, Storable f) => WithPtrs (a, b, c, d, e, f)

(Storable a, Storable b, Storable c, Storable d, Storable e, Storable f, Storable g) => WithPtrs (a, b, c, d, e, f, g)

FunPtr utils

mkFunPtr :: TypeQ -> ExpQ

$(mkFunPtr [t| CDouble -> IO CDouble |] generates a foreign import wrapper of type

(CDouble -> IO CDouble) -> IO (FunPtr (CDouble -> IO CDouble))

And invokes it.

mkFunPtrFromName :: Name -> ExpQ

$(mkFunPtrFromName 'foo), if foo :: CDouble -> IO CDouble, splices in an expression of type IO (FunPtr (CDouble -> IO CDouble)).

peekFunPtr :: TypeQ -> ExpQ

$(peekFunPtr [t| CDouble -> IO CDouble |]) generates a foreign import dynamic of type

FunPtr (CDouble -> IO CDouble) -> (CDouble -> IO CDouble)

And invokes it.

C types re-exports

module Foreign.C.Types