The State monad parse

parse.hs

type ParseState = State String

parse :: String -> ParseState Expr
parse (ch:chs)
  | isAlpha(ch) = do
    put chs
    return (Var ch)
  | isDigit(ch) = do
    put chs
    return (N (fromEnum ch - fromEnum '0'))
  | ch == '(' = do
    e1 <- parse(chs)
    rest1 <- get
    let (op:rest2) = rest1
    e2 <- parse(rest2)
    rest3 <- get
    let (')':restFinal) = rest3
    put restFinal
    return $ apply op e1 e2
  | ch =='<' = do
    let (v:':':rest) = chs
    e <- parse(rest)
    restFinal <- get
    put restFinal
    return (Assign v e)
      where apply '+' = Add
            apply '-' = Sub