Reskin

ghc-reskin is a tool based on ghc-exactprint which allows quick experimentation with the Haskell language syntax. It provides a single executable, ghc-reskin, which accepts the following arguments:

• <source file>: The name of the source file it is processing.
• <input file>: Where to read the source file from.
• <output file>: Where to write its output to.
• Extensions: After the first three positional arguments, ghc-reskin accepts extension flags which change the syntax of the source language.

ghc-reskin is meant to be used as a preprocessor with GHC via the -F -pgmF flags. To do, provide arguments to GHC, as in the following example:

$ ghc Test.hs -F -pgmF ghc-reskin -optF -XArgumentBlock

This can also be used in a pragma, as in the examples below.

Reskin Installation

To install ghc-reskin, clone the repository:

git clone https://github.com/gibiansky/ghc-reskin.git

Enter the repository directory and run stack install:

cd ghc-reskin
stack install

If you encounter issues with the parser being generated (or get errors about Parser.hs not being found), run stack clean and then cabal configure.

Reskin Extensions

Currently supported extensions include:

• -XArgumentBlock: This extension allows lambdas, case, and do arguments to functions to be provided without a $. For example, the following function is valid:

{-# OPTIONS_GHC -F -pgmF ghc-reskin -optF -XArgumentBlock #-}
module Main where

import System.Environment (getArgs)
import Control.Monad

main :: IO ()
main = do
  args <- getArgs

  -- No $ before the lambda
  forM_ args \arg ->
    putStrLn arg

  putStrLn case args of
    [] -> "Empty argument list..."
    xs -> "Num arguments: " ++ show (length xs)

  -- No $ before do
  when (null args) do
    putStrLn "No arguments"
    putStrLn "Why didn't you argue?"