4. Exposing Haskell functions

So far we've provided the Haskell programmer with ways of importing external functions into the Haskell world. The other half of the FFI coin is how to expose Haskell functionality to the outside world. So, dual to the foreign import declaration is foreign export:

topdecl 
  : ...
  ..
  | 'foreign' 'export' callconv [ext_name] varid :: prim_type

A foreign export declaration tells the compiler to expose a locally defined Haskell function to the outside world, i.e., wrap it up behind a calling interface that's useable from C. It is only permitted at the toplevel, where you have to specify the type at which you want to export the function, along with the calling convention to use. For instance, the following export declaration:

foreign export ccall "foo" bar :: Int -> Addr -> IO Double

will cause a Haskell system to generate the following C callable function:

HsDouble foo(HsInt arg1, HsAddr arg2);

When invoked, it will call the Haskell function bar, passing it the two arguments that was passed to foo().

• The range of types that can be passed as arguments and results is restricted, since varid has got a prim_type.
• It is not possible to directly export operator symbols.
• The type checker will verify that the type given for the foreign export declaration is compatible with the type given to function definition itself. The type in the foreign export may be less general than that of the function itself. For example, this is legal:

     f :: Num a => a -> a
     foreign export ccall "fInt"   f :: Int -> Int
     foreign export ccall "fFloat" f :: Float -> Float
  

  These declarations export two C-callable procedures fInt and fFloat, both of which are implemented by the (overloaded) Haskell function f.
• The foreign exported IO action must catch all exceptions, as the FFI does not address how to signal Haskell exceptions to the outside world.