module Lexer where
-- Copyright 1994 by Peter Thiemann
-- Last Modified By: M. Walter
--

import Data.Char

------------------------------------------------------------------------------
--NOW the lexer
------------------------------------------------------------------------------

data Token
	= Ident String | Symbol String | String String | Number String

instance Show Token where
  showsPrec n (Ident s) = showChar '[' . showString s . showString "] "
  showsPrec n (Symbol "\n") = showString "\n"
  showsPrec n (Symbol s) = showChar '<' . showString s . showString "> "
  showsPrec n (String s) = showChar '"' . showString s . showString "\" "
  showsPrec n (Number s) = showChar ' ' . showString s . showChar ' '
  showList [] = id
  showList (x:xs) = shows x . showList xs

isIdChar c = isAlpha c || isDigit c || c == '_' || c == '.' -- 1.01

theSymbols = "!@#$%^&*+./<=>?\\|:"
isSymbolChar c = c `elem` theSymbols

lexer :: String -> [Token]
lexer "" = []
lexer ('"':cs) = String (stchars): lexer srest
	where (stchars, srest) = lexString cs
lexer ('\'':cs) = String (oneChar): lexer srest
	where (oneChar, srest) = lexChar cs
lexer ((c@'\n'):cs) = Symbol [c]: lexer cs
lexer (c:cs)
	   | isNewline c = Symbol [c]: lexer cs
	   | isSpace c = lexer cs
	   | isDot c = Ident (c:idchars): lexer irest    -- 1.01
	   | isAlpha c = Ident (c:idchars): lexer irest
	   | isSymbolChar c = Symbol(c:sychars): lexer srest
	   | isDigit c = Number (c:digits): lexer nrest
	   | otherwise = Symbol([c]): lexer cs
	where (idchars, irest) = span isIdChar cs
	      (sychars, srest) = span isSymbolChar cs
	      (digits, nrest)  = span isDigit cs
	      isNewline	c = c == '\n'
	      isDot c = c == '.'                          -- 1.01

-- preprocessor for EBNF style comments
uncomment :: String -> String
uncomment ""        = ""
uncomment ('#':cs)  = uncomment (dropWhile (/= '\n') cs)
uncomment ('"':cs)  = '"':uncommentString cs
uncomment ('\'':cs) = '\'':uncommentChar cs
uncomment (c:cs)    = c:uncomment cs

uncommentString "" = ""
uncommentString ('\\':c:cs) = '\\':c:uncommentString cs
uncommentString ('"':cs) = '"':uncomment cs
uncommentString (c:cs) = c:uncommentString cs

uncommentChar "" = ""
uncommentChar ('\\':c:cs) = '\\':c:uncommentChar cs
uncommentChar ('\'':cs) = '"':uncomment cs
uncommentChar (c:cs) = c:uncommentChar cs

-- generic lexers
lexChar ('\\':c:'\'':cs) = ([c], cs)
lexChar (c:'\'':cs) = ([c], cs)
lexChar cs = ([], cs)

lexString ('\\':c:cs) = (c:stchars, srest) where (stchars, srest) = lexString cs
lexString ('"':cs) = ("", cs)
lexString ("") = ("","")
lexString (c:cs) = (c:stchars, srest) where (stchars, srest) = lexString cs

isIdent (Ident _ ) = True
isIdent _ = False

getIdent (Ident s) = s

isString (String _) = True
isString _ = False

getString (String s) = s