Shortened lexer

lex-lonlines.hs

-- This lexer is not completely faithful to the Haskell lexical syntax.
-- Current limitations:
--    Qualified names are not handled properly
--    Octal and hexidecimal numerics are not recognized as a single token
--    Comments are not treated properly


lex              :: ReadS String
lex ""           =  [("","")]
lex (c:s)
   | isSpace c   =  lex (dropWhile isSpace s)
lex ('\'':s)     =  [('\'':ch++"'", t) | (ch,'\'':t)  <- lexLitChar s, ch /= "'" ]
lex ('"':s)      =  [('"':str, t)      | (str,t) <- lexString s]
                    where
                    lexString ('"':s) = [("\"",s)]
                    lexString s       = [(ch++str, u) | (ch,t) <- lexStrItem s, (str,u) <- lexString t]

                    lexStrItem ('\\':'&':s)             =  [("\\&",s)]
                    lexStrItem ('\\':c:s) | isSpace c   =  [("\\&",t) | '\\':t <- [dropWhile isSpace s]]
                    lexStrItem s                        =  lexLitChar s

lex (c:s) | isSingle c = [([c],s)]
          | isSym c    = [(c:sym,t)    | (sym,t) <- [span isSym s]]
          | isAlpha c  = [(c:nam,t)    | (nam,t) <- [span isIdChar s]]
          | isDigit c  = [(c:ds++fe,t) | (ds,s)  <- [span isDigit s], (fe,t) <- lexFracExp s]
          | otherwise  = []    -- bad character
             where
              isSingle c =  c `elem` ",;()[]{}_`"
              isSym c    =  c `elem` "!@#$%&*+./<=>?\\^|:-~"
              isIdChar c =  isAlphaNum c || c `elem` "_'"

              lexFracExp ('.':c:cs) | isDigit c  = [('.':ds++e,u) | (ds,t) <- lexDigits (c:cs), (e,u)  <- lexExp t]
              lexFracExp s                       = lexExp s

              lexExp (e:s) | e `elem` "eE"  = [(e:c:ds,u) | (c:t)  <- [s], c `elem` "+-", (ds,u) <- lexDigits t] ++
                                              [(e:ds,t)   | (ds,t) <- lexDigits s]
              lexExp s                      = [("",s)]