Class.hs

-----------------------------------------------------------------------------
-- |
-- Module      :  Text.Trifecta.Parser.Token.Class
-- Copyright   :  (c) Edward Kmett 2011
-- License     :  BSD3
-- 
-- Maintainer  :  ekmett@gmail.com
-- Stability   :  experimental
-- Portability :  non-portable
-- 
-----------------------------------------------------------------------------
module Text.Trifecta.Parser.Token.Class
  ( MonadTokenParser(..)
  ) where

import Control.Applicative
import Control.Monad.Trans.Class
import Control.Monad.Trans.State.Lazy as Lazy
import Control.Monad.Trans.State.Strict as Strict
import Text.Trifecta.Parser.Class

class MonadParser m => MonadTokenParser m where
  -- | Parses any white space. White space consists of /zero/ or more
  -- occurrences of a 'space', a line comment or a block (multi
  -- line) comment. Block comments may be nested. How comments are
  -- started and ended is defined by this method.
  whiteSpace       :: m ()
  
  -- | @lexeme p@ first applies parser @p@ and than the 'whiteSpace'
  -- parser, returning the value of @p@. Every lexical
  -- token (lexeme) is defined using @lexeme@, this way every parse
  -- starts at a point without white space. Parsers that use @lexeme@ are
  -- called /lexeme/ parsers in this document.
  -- 
  -- The only point where the 'whiteSpace' parser should be
  -- called explicitly is the start of the main parser in order to skip
  -- any leading white space.
  --
  -- >    mainParser  = do { whiteSpace
  -- >                     ; ds <- many (lexeme digit)
  -- >                     ; eof
  -- >                     ; return (sum ds)
  -- >                     }
  lexeme :: m a -> m a
  lexeme p = p <* whiteSpace

instance MonadTokenParser m => MonadTokenParser (Lazy.StateT s m) where
  whiteSpace = lift whiteSpace
  lexeme (Lazy.StateT m) = Lazy.StateT $ lexeme . m

instance MonadTokenParser m => MonadTokenParser (Strict.StateT s m) where
  whiteSpace = lift whiteSpace
  lexeme (Strict.StateT m) = Strict.StateT $ lexeme . m