Example hnix transformation

IncrementNix.hs

#!/usr/bin/env nix-shell
#!nix-shell -i runhaskell -p "haskellPackages.ghcWithPackages (p: with p; [ hnix ])"
module Main where

import Data.Text.IO as T
import System.Exit

import Nix
import Nix.Atoms
import Nix.Utils (transport)

{-
$ echo "x: x + 1" | ./Increment.hs
x:
  x + 2
-}

main :: IO ()
main = do
    stdin <- T.getContents
    case parseNixText stdin of
      Success expr ->
          let expr' = transport update expr in
          print $ prettyNix expr'
      e -> error (show e)

    where
        update (NConstant (NInt i)) = NConstant (NInt (i+1))
        update x = x