Turtle Script for linking ghc from HVRs repository

lnghc.cabal

name:                lnghc
version:             0.1.0.0
synopsis:            easily change ghc-versions for hvr's ppa repository
license:             BSD3
license-file:        LICENSE
author:              Martin Heuschober (TSA)
maintainer:          martin.heuschober@t-systems.at
copyright:           2015 Martin Heuschober
category:            Console App
build-type:          Simple
-- extra-source-files:
cabal-version:       >=1.22

executable lnghc
  hs-source-dirs:      app
  main-is:             Main.hs
  ghc-options:         -threaded -rtsopts -with-rtsopts=-N
  build-depends:       base >= 4.7 && < 5
                     , turtle
                     , foldl
                     , text
                     , system-filepath
  default-language:    Haskell2010

Main.hs

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Turtle
import Prelude hiding (FilePath)
import Data.Either (rights)
import qualified Control.Foldl as F
import qualified Data.Text as T
import Data.Text()


main :: IO ()
main = sh $ do
          args <- arguments
          available_ghcs <- map filename <$> (fold (ls "/opt/ghc/") F.list)
          Just home <- need "HOME"
          case args of
            (a:_) -> if (fromText a) `elem` available_ghcs
                        then do _ <- unlink  $ home <> "/bin/ghc"
                                _ <- symlink ("/opt/ghc/" <> a <> "/bin") $ home <> "/bin/ghc"
                                echo $ "...successfully changed GHC ["<>a<>"]"
                        else do echo $ "'"<> a <>"' is no valid GHC version"
                                list available_ghcs
            _ -> do echo "USAGE: lnghc ${GHC VERSION}"
                    list available_ghcs
    where unlink trgt = proc "unlink" [trgt] ""
          symlink src trgt = proc "ln" ["-s",src,trgt] ""


list :: [FilePath] -> Shell ()
list versions = do current_ghc <- lastMay . T.words <$> inproc "ghc" ["--version"] ""
                   echo "available GHC versions: "
                   mapM_ (echo . highlight current_ghc) $ rights $ map toText versions



highlight :: Maybe Text -> Text -> Text
highlight hi str = if hi == Just str
                     then "[" <> str <> "]"
                     else " " <> str <> " "

lastMay :: [a] -> Maybe a
lastMay [] = Nothing
lastMay xx = Just $ last xx