{-
Like Expr, but using only elementary parser combinators.
-}

module SimpleExpr where

import ParseDecorate
import QTree

data Expr
  = Add Expr Expr
  | Mul Expr Expr
  | Num Int
  deriving (Eq, Show, Read)

pExpr :: Parser Char Expr
pExpr = pSum

pSum :: Parser Char Expr
pSum = pMul
   <|> (\x _ y -> Add x y) <$> pMul <*> symbol '+' <*> pSum

pMul :: Parser Char Expr
pMul = pUnit
   <|> (\x _ y -> Mul x y) <$> pUnit <*> symbol '*' <*> pMul

pUnit :: Parser Char Expr
pUnit = pNum
    <|> (\_ x _ -> x) <$> symbol '(' <*> pExpr <*> symbol ')'

pNum :: Parser Char Expr
pNum = Num . read . (:[]) <$> oneOf "1234567890"

oneOf :: Eq s => [s] -> Parser s s
oneOf = foldl1 (<|>) . map symbol