{-# LANGUAGE UnicodeSyntax #-}
module ParserDet where

import Prelude.Unicode
import Control.Monad.State


type Parser = State String

runParser ∷ Parser a → String → a
runParser = evalState

anyChar ∷ Parser Char
anyChar = do
	input ← get
	if null input
		then error "EOF"
		else do
			put $ tail xs
			return x

char ∷ Char → Parser Char
char c = satisfy (c ≡) anyChar

satisfy ∷ (a → Bool) → Parser a → Parser a
satisfy pred p = do
	tok ← p
	if pred tok then return tok else error "predicate not met"

string ∷ String → Parser ()
string str = mapM char str >> return ()

(<|>) ∷ Parser a → Parser a → Parser a
p <|> q = do
	tok ← p
	if failed p then return tok else q
	where failed = undefined
	-- the Parser type is not adequate for intercepting a failed parser, and
	-- therefore not sufficient to implement the choice operator.