Learn some template haskell

Test.hs

{-# LANGUAGE TemplateHaskell #-}
import TH

fstNTest1 = $(fstN 3) (1, 2, 4)
fstNTest2 = $(fstN 4) (2, 3, 4, 5)


applyAdd2 = $(apply 'add2 2) 1 2
applyAdd3 = $(apply 'add3 3) 1 2 3

applyAnd  = $(apply 'and 4) True True True True

add2 [x, y] = x + y
add3 [x, y, z] = x + y + z

add4 x y z = x + y + z

x = $(applyToList 'add4 3) [1, 2, 3]

main = undefined

TH.hs

{-# LANGUAGE TemplateHaskell #-}
module TH where

import Language.Haskell.TH
import Control.Monad (replicateM)

-- f :: Int -> Int -> Int -> Int
-- $(apply 'f n)  :: [Int] -> Int
applyToList f n = do
    names <- replicateM n (newName "a")
    return $ LamE [ListP $ map VarP names]
               (curry' (VarE f) (map VarE names))
  where
    curry' f []  = f
    curry' f [x] = AppE f x
    curry' f (x:xs) = curry' (AppE f x) xs

apply f n = do
    xs <- replicateM n (newName "x")
    return $ LamE (map VarP xs) (AppE (VarE f) (ListE $ map VarE xs))

fstN :: Int -> Q Exp
fstN n = do
    x <- newName "x"
    return $ LamE [ TupP $ VarP x : replicate (n - 1) WildP] (VarE x)