-- LexerExpr.hs
--
-- Lexer Module for the 15 minute example.
--

module LexerExpr where

-- needed for isChar, isAlpha, isSpace, isDigit
import Data.Char


-- Token datatype.
data Token
      = TokenPlus
      | TokenMult
      | TokenMinus
      | TokenAnd
      | TokenNot
      | TokenNUM Int
      | UnknownChar Char
      deriving (Show,Eq)

-----------------------------------------------------------------
-- Function that performs lexical analysis.
lexer :: String -> [Token]
lexer [] = []
lexer ('+':cs) = TokenPlus      : lexer cs
lexer ('-':cs) = TokenMinus     : lexer cs
lexer ('*':cs) = TokenMult      : lexer cs
lexer ('!':cs) = TokenNot       : lexer cs
lexer ('&':'&':cs) = TokenAnd   : lexer cs

lexer (c:cs) 
      | isSpace c = lexer cs
      | isDigit c = lexNum (c:cs)
      | True = UnknownChar c : lexer cs

lexNum cs = TokenNUM (read num) : lexer rest
      where (num,rest) = span isDigit cs