addTopDecls example for Template Haskell

AddTopDecls.hs

{-# LANGUAGE TemplateHaskell #-}

module AddTopDecls where

import Language.Haskell.TH
import Language.Haskell.TH.Syntax

importDoubleToDouble :: String -> ExpQ
importDoubleToDouble fname = do
    n <- newName fname
    d <- forImpD CCall unsafe fname n [t|Double -> Double|]
    addTopDecls [d]
    [|$(varE n)|]

Main.hs

{-# LANGUAGE TemplateHaskell #-}

module Main where

import Language.Haskell.TH
import Language.Haskell.TH.Syntax

import AddTopDecls

main :: IO ()
main = do
    print ($(importDoubleToDouble "sin") pi)
    print ($(importDoubleToDouble "cos") pi)