Cabal/Distribution/ReadE.hs

   1 -----------------------------------------------------------------------------
   2 -- |
   3 -- Module      :  Distribution.ReadE
   4 -- Copyright   :  Jose Iborra 2008
   5 -- License     :  BSD3
   6 --
   7 -- Maintainer  :  cabal-devel@haskell.org
   8 -- Portability :  portable
   9 --
  10 -- Simple parsing with failure
  11
  12 module Distribution.ReadE (
  13    -- * ReadE
  14    ReadE(..), succeedReadE, failReadE,
  15    -- * Projections
  16    parseReadE, readEOrFail,
  17    readP_to_E
  18   ) where
  19
  20 import Prelude ()
  21 import Distribution.Compat.Prelude
  22
  23 import Distribution.Compat.ReadP
  24
  25 -- | Parser with simple error reporting
  26 newtype ReadE a = ReadE {runReadE :: String -> Either ErrorMsg a}
  27 type ErrorMsg   = String
  28
  29 instance Functor ReadE where
  30   fmap f (ReadE p) = ReadE $ \txt -> case p txt of
  31                                        Right a  -> Right (f a)
  32                                        Left err -> Left err
  33
  34 succeedReadE :: (String -> a) -> ReadE a
  35 succeedReadE f = ReadE (Right . f)
  36
  37 failReadE :: ErrorMsg -> ReadE a
  38 failReadE = ReadE . const . Left
  39
  40 parseReadE :: ReadE a -> ReadP r a
  41 parseReadE (ReadE p) = do
  42   txt <- look
  43   either fail return (p txt)
  44
  45 readEOrFail :: ReadE a -> String -> a
  46 readEOrFail r = either error id . runReadE r
  47
  48 readP_to_E :: (String -> ErrorMsg) -> ReadP a a -> ReadE a
  49 readP_to_E err r =
  50     ReadE $ \txt -> case [ p | (p, s) <- readP_to_S r txt
  51                          , all isSpace s ]
  52                     of [] -> Left (err txt)
  53                        (p:_) -> Right p