module SimpleLexer where
import Char

data Token = Number Integer
                     | Sep Char
                     | Ident String          deriving (Show)

simpleLexer :: String -> [Token]
simpleLexer (c:cs)
  | isDigit c  =  lexNumber [c] cs
  | isAlpha c  =  lexIdent  [c] cs
  | isSep   c  =  Sep c : simpleLexer cs
  | isSpace c  =  simpleLexer cs
  | otherwise  =  error ("simpleLexer: illegal character: " ++ take 20 (c:cs))
simpleLexer [] = []

lexNumber, lexIdent :: String -> String -> [Token]
lexNumber prefix (c:cs) | isDigit c     =  lexNumber (prefix ++ [c]) cs
lexNumber prefix s                      =  Number (read prefix) : simpleLexer s
lexIdent  prefix (c:cs) | isAlphaNum c  =  lexIdent (prefix ++ [c]) cs
lexIdent  prefix s                      =  Ident prefix : simpleLexer s

isSep c = c `elem` "(){};,+-*/="