Cabal/src/Distribution/ZinzaPrelude.hs

   1 -- | A small prelude used in @zinza@ generated
   2 -- template modules.
   3 module Distribution.ZinzaPrelude
   4   ( Writer
   5   , execWriter
   6   , tell
   7
   8     -- * Re-exports
   9   , forM_
  10   , Generic
  11   , PackageName
  12   , Version
  13   , prettyShow
  14   ) where
  15
  16 import Distribution.Compat.Prelude
  17 import Prelude ()
  18
  19 import Control.Monad (forM_)
  20 import Distribution.Pretty (prettyShow)
  21 import Distribution.Types.PackageName (PackageName)
  22 import Distribution.Types.Version (Version)
  23
  24 newtype Writer a = W {unW :: ShowS -> (ShowS, a)}
  25
  26 instance Functor Writer where
  27   fmap = liftM
  28
  29 instance Applicative Writer where
  30   pure x = W $ \ss -> (ss, x)
  31   (<*>) = ap
  32
  33 instance Monad Writer where
  34   return = pure
  35   m >>= k = W $ \s1 ->
  36     let (s2, x) = unW m s1
  37      in unW (k x) s2
  38   {-# INLINE (>>=) #-}
  39
  40 execWriter :: Writer a -> String
  41 execWriter w = fst (unW w id) ""
  42
  43 tell :: String -> Writer ()
  44 tell s = W $ \s' -> (s' . showString s, ())