module Lexer where

data Token = Plus
           | Minus
           | Multiply
           | Divide
           | Integer Int
           | LBrace
           | RBrace
           deriving Show

lex :: String -> [Token]
lex source = reverse (lex' source [] (length source))

lex' :: String -> [Token] -> Int -> [Token]
lex' "" tokens _ = tokens
lex' (' ':xs) tokens orgLength = lex' xs tokens orgLength
lex' (x:xs) tokens orgLength = case x of
    '+' -> lex' xs (Plus:tokens) orgLength
    '-' -> lex' xs (Minus:tokens) orgLength
    '*' -> lex' xs (Multiply:tokens) orgLength
    '/' -> lex' xs (Divide:tokens) orgLength
    '(' -> lex' xs (LBrace:tokens) orgLength
    ')' -> lex' xs (RBrace:tokens) orgLength
    _ -> lexNumbers (x:xs) tokens orgLength

lexNumbers :: String -> [Token] -> Int -> [Token]
lexNumbers (x:xs) tokens orgLength
    | isDigit x = lex' (dropWhile isDigit (x:xs)) ((Integer(read (takeWhile isDigit (x:xs))::Int)):tokens) orgLength
    | otherwise = error $ "Syntax error: " ++ [x] ++ " (" ++ show (orgLength - (length (x:xs))) ++ ")"

isDigit x = x `elem` "0123456789"