Editing Foreign Function Interface (section)

== History ==

=== Header inclusion ===

In old versions of GHC (6.8.3 and earlier), the compiler was able to check the prototypes of the foreign imports by including C header files into the generated C code. For instance, you could write:
<haskell>
{-# INCLUDE <math.h> #-}
</haskell>
or 
<haskell>
foreign import ccall "math.h sin" c_sin :: Double -> Double
</haskell>
to include the "math.h" header.

This is deprecated in GHC. Nevertheless you may still find examples using this syntax so it is good to know that it has been used. Moreover, other compilers may still use this feature.

* Justification of the deprecation from the [http://www.haskell.org/ghc/docs/6.10.1/html/users_guide/ffi-ghc.html#glasgow-foreign-headers GHC 6.10.1 manual]:

"C functions are normally declared using prototypes in a C header file. Earlier versions of GHC (6.8.3 and earlier) #included the header file in the C source file generated from the Haskell code, and the C compiler could therefore check that the C function being called via the FFI was being called at the right type.

GHC no longer includes external header files when compiling via C, so this checking is not performed. The change was made for compatibility with the native code backend (-fasm) and to comply strictly with the FFI specification, which requires that FFI calls are not subject to macro expansion and other CPP conversions that may be applied when using C header files. This approach also simplifies the inlining of foreign calls across module and package boundaries: there's no need for the header file to be available when compiling an inlined version of a foreign call, so the compiler is free to inline foreign calls in any context.

The -#include option is now deprecated, and the include-files field in a Cabal package specification is ignored."

* [http://www.haskell.org/ghc/docs/6.8.3/html/users_guide/ffi-ghc.html#glasgow-foreign-headers Documentation of this feature in the GHC 6.8.3 manual]