module BasicParseLib where

import Char

-- basic type and data definitions

data Expr = Lit Int | Op Ops Expr Expr

deriving (Eq, Show)

data Ops = Add | Sub | Mul | Div | Mod

deriving (Eq, Show)

type Parse a b = [a] -> [(b,[a])]

-- some basic parsers

none :: Parse a b

none inp = []

succeed :: b -> Parse a b

succeed val inp = [(val,inp)]

spot :: (a -> Bool) -> Parse a a

spot p [] = []

spot p (x:xs)

| p x = [(x,xs)]

| otherwise = []

token :: Eq a => a -> Parse a a

token a = spot (==a)

-- combining parsers

alt :: Parse a b -> Parse a b -> Parse a b

alt p1 p2 inp = p1 inp ++ p2 inp

infixr 5 >*>

(>*>) :: Parse a b -> Parse a c -> Parse a (b,c)

(>*>) p1 p2 inp = [((y,z),rem2)|(y,rem1) <- p1 inp, (z,rem2) <- p2 rem1]

build :: Parse a b -> (b -> c) -> Parse a c

build p f inp = [(f x,rem)|(x,rem) <- p inp]

list :: Parse a b -> Parse a [b]

list p = (succeed []) `alt` ((p >*> list p) `build` (uncurry (:)))