module Gen where

import Language.Haskell.TH


f :: (Q Exp -> Q Exp) -> Q Exp
{-
f g = [| \x -> let y = $(g [|x|])
               in $(g [|y|])
      |]

-}
f g = do
       x <- newName "x"
       y <- newName "y"
       rhs1 <- g (return (VarE x))
       let dec = ValD (VarP y) (NormalB rhs1) []
       rhs2 <- g (return (VarE y))
       return $ LamE [VarP x] (LetE [dec] rhs2)

h :: Int -> Int
h x = x*x

use :: Q Exp
use = [| $(f (\x -> [| h $x |])) |]

{-
*Gen> $use 3
Loading package haskell98-1.0 ... linking ... done.
Loading package template-haskell-1.0 ... linking ... done.
<interactive>:1:0:
    <interactive>:1:0-3: Splicing expression
        use ======> \ x[a2wK] -> let y[a2wM] = Gen.h x[a2wK] in Gen.h y[a2wM]
    In a 'do' expression: it <- $[splice]use 3
<interactive>:1:0:
    <interactive>:1:0-3: Splicing expression
        use ======> \ x[a2x2] -> let y[a2x4] = Gen.h x[a2x2] in Gen.h y[a2x4]
    In the definition of `it': it = $[splice]use 3
81
-}