{-
    Title: Little Parsec Lexer for expression tokens
           (See Parsec manual ...)
    Author: Robin Cockett
    Date: 10 Jan 2006
-}

 module Exlexer (lexer,Tok,Token(ADD,MUL,LPAR,RPAR,NUM)) where
 import Text.ParserCombinators.Parsec

 data Token = ADD|MUL|LPAR|RPAR|NUM Integer
          deriving (Show,Eq)

 type Tok = (SourcePos,Token)


 lexer:: Parser [Tok]
 lexer = do space                --  eat spaces
            ts <- lexer
            return ts
         <|> 
         do p <- getPosition
            n <- num             -- eat numbers
            ts <- lexer 
            return ((p,n):ts)
         <|> 
         do p <- getPosition
            n <- add             -- eat and tokenize addition symbol
            ts <- lexer 
            return ((p,n):ts)
         <|> 
         do p <- getPosition
            n <- mul             -- eat and tokenize multiplication symbol
            ts <- lexer 
            return ((p,n):ts)
         <|> 
         do p <- getPosition
            n <- lpar            -- eat and tokenize left parenthesis symbol
            ts <- lexer 
            return ((p,n):ts)
         <|> 
         do p <- getPosition
            n <- rpar            -- eat and tokenize right parenthesis symbol
            ts <- lexer 
            return ((p,n):ts)
         <|> 
         do eof
            return [] where       
  num :: Parser Token
  num = do     d <- digit
               ds <- digit_more
               return (NUM (read (d:ds)))
        <?> "number"
  digit_more :: Parser String
  digit_more = do d <- digit
                  ds <- digit_more
                  return (d:ds)
           <|> return []
  add = do char '+'
           return ADD
  mul = do char '*'
           return MUL
  lpar= do char '('
           return LPAR
  rpar= do char ')'
           return RPAR