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{-# LANGUAGE ScopedTypeVariables, RankNTypes #-}
-- Helper for simpl009.hs (see comments there)
module Simpl009Help where
import Control.Monad
newtype Parser s a
= Parser (forall res . (a -> [String] -> P s res) -> [String] -> P s res)
data P s res
= Symbol (s -> P s res)
| Fail [String] [String]
| Result res (P s res)
instance Monad (Parser s) where
return a = Parser (\fut -> fut a)
Parser f >>= k =
Parser (\fut -> f (\a -> let Parser g = k a in g fut))
fail s =
Parser (\fut exp -> Fail exp [s])
instance MonadPlus (Parser s) where
mplus = error "urk"
mzero = Parser (\fut exp -> Fail exp [])
lookAhead :: forall s. Parser s s
lookAhead =
Parser (\fut exp -> Symbol (\c ->
feed c (fut c [])
))
where
feed :: forall res. s -> P s res -> P s res
feed c (Symbol sym) = sym c
feed c (Result res fut) = Result res (feed c fut)
feed c p@(Fail _ _) = p
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