example.cabal

cabal-version: 3.0
name: example
build-type: Simple
version: 1.0.0

executable example
  main-is: Main.hs
  build-depends: base, ghc-prim
  default-language: Haskell2010
  ghc-options: -ddump-cmm
  c-sources: example.cmm

example.cmm

#include "Cmm.h"

example (W_ n) {
  entry:
    return (n+1);
}

Main.hs

{-# LANGUAGE ForeignFunctionInterface #-}
{-# LANGUAGE GHCForeignImportPrim #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnliftedFFITypes #-}

module Main where

import GHC.Prim
import GHC.Word

foreign import prim "example" example_cmm :: Word# -> Word#

example :: Word64 -> Word64
example (W64# n) = W64# (example_cmm n)

main :: IO ()
main = print (example 10)