The following is a small Haskell module that exposes a function callable from Mathematica that gets a pair of Ints (as a tuple) and returns their sum to Mathematica:

module Main where

import Foreign.MathLink

addTwo :: ML ()
addTwo = do
  (i1,i2) <- get
  put ((i1 + i2) :: Int)

main = runMathLink [ Function { callPattern = "AddTwo[i_Integer,j_Integer]"
                              , argumentPattern = "{i,j}"
                              , function = addTwo
                              }
                   ]

A function to be exposed to Mathematica has type ML (). In its body it uses the get method to receive a value from Mathematica, performs the desired computation, and sends the result back to Mathematica via the put function.

The types that can be marshaled to/from Mathematica are instances of the Expressible class.

See the examples directory of the source distribution for more.

The author developed the library on a 64-bit Linux platform with GHC 6.10.1 and Mathematica 7. However, among the author's design goals is that the library be useful for:

1. any Haskell system with modest language extensions beyond the 98 standard and implementing the FFI addendum;
2. any version of Mathematica supporting version 3 of the MathLink interface;
3. any 32-bit or 64-bit OS.

Please report any failures in this regard, (or, better yet, send a patch!) and the author will make an effort (time, patience, and resources permitting) to bring the library closer to its stated goal, or at least to explicitly document the limitation in a future release.