-- #hide
-----------------------------------------------------------------------------
-- |
-- Module      :  Language.Haskell.Lexer
-- Copyright   :  (c) The GHC Team, 1997-2000
-- License     :  BSD-style (see the file libraries/base/LICENSE)
-- 
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  portable
--
-- Lexer for Haskell.
--
-----------------------------------------------------------------------------

-- ToDo: Introduce different tokens for decimal, octal and hexadecimal (?)
-- ToDo: FloatTok should have three parts (integer part, fraction, exponent) (?)
-- ToDo: Use a lexical analyser generator (lx?)

module Language.Haskell.Lexer (Token(..), lexer) where

import Language.Haskell.ParseMonad

import Data.Char	(isAlpha, isLower, isUpper, toLower,
			 isDigit, isHexDigit, isOctDigit, isSpace,
			 ord, chr, digitToInt)
import qualified Data.Char (isSymbol)
import Data.Ratio

data Token
        = VarId String
        | QVarId (String,String)
	| ConId String
        | QConId (String,String)
        | VarSym String
        | ConSym String
        | QVarSym (String,String)
        | QConSym (String,String)
	| IntTok Integer
	| FloatTok Rational
	| Character Char
        | StringTok String

-- Symbols

	| LeftParen
	| RightParen
	| SemiColon
        | LeftCurly
        | RightCurly
        | VRightCurly			-- a virtual close brace
        | LeftSquare
        | RightSquare
	| Comma
        | Underscore
        | BackQuote

-- Reserved operators

	| DotDot
	| Colon
	| DoubleColon
	| Equals
	| Backslash
	| Bar
	| LeftArrow
	| RightArrow
	| At
	| Tilde
	| DoubleArrow
	| Minus
	| Exclamation

-- Reserved Ids

	| KW_Case
	| KW_Class
	| KW_Data
	| KW_Default
	| KW_Deriving
	| KW_Do
	| KW_Else
	| KW_Foreign
	| KW_If
	| KW_Import
	| KW_In
	| KW_Infix
	| KW_InfixL
	| KW_InfixR
	| KW_Instance
	| KW_Let
	| KW_Module
	| KW_NewType
	| KW_Of
	| KW_Then
	| KW_Type
	| KW_Where

-- Special Ids

	| KW_As
	| KW_Export
	| KW_Hiding
	| KW_Qualified
	| KW_Safe
	| KW_Unsafe

        | EOF
        deriving (Eq,Show)

reserved_ops :: [(String,Token)]
reserved_ops = [
 ( "..", DotDot ),
 ( ":",  Colon ),
 ( "::", DoubleColon ),
 ( "=",  Equals ),
 ( "\\", Backslash ),
 ( "|",  Bar ),
 ( "<-", LeftArrow ),
 ( "->", RightArrow ),
 ( "@",  At ),
 ( "~",  Tilde ),
 ( "=>", DoubleArrow )
 ]

special_varops :: [(String,Token)]
special_varops = [
 ( "-",  Minus ),			--ToDo: shouldn't be here
 ( "!",  Exclamation )		--ditto
 ]

reserved_ids :: [(String,Token)]
reserved_ids = [
 ( "_",         Underscore ),
 ( "case",      KW_Case ),
 ( "class",     KW_Class ),
 ( "data",      KW_Data ),
 ( "default",   KW_Default ),
 ( "deriving",  KW_Deriving ),
 ( "do",        KW_Do ),
 ( "else",      KW_Else ),
 ( "foreign",	KW_Foreign ),
 ( "if",    	KW_If ),
 ( "import",    KW_Import ),
 ( "in", 	KW_In ),
 ( "infix", 	KW_Infix ),
 ( "infixl", 	KW_InfixL ),
 ( "infixr", 	KW_InfixR ),
 ( "instance",  KW_Instance ),
 ( "let", 	KW_Let ),
 ( "module", 	KW_Module ),
 ( "newtype",   KW_NewType ),
 ( "of", 	KW_Of ),
 ( "then", 	KW_Then ),
 ( "type", 	KW_Type ),
 ( "where", 	KW_Where )
 ]

special_varids :: [(String,Token)]
special_varids = [
 ( "as", 	KW_As ),
 ( "export", 	KW_Export ),
 ( "hiding", 	KW_Hiding ),
 ( "qualified", KW_Qualified ),
 ( "safe",	KW_Safe ),
 ( "unsafe", 	KW_Unsafe )
 ]

isIdent, isSymbol :: Char -> Bool
isIdent  c = isAlpha c || isDigit c || c == '\'' || c == '_'
isSymbol c = c `elem` ":!#%&*./?@\\-" || (Data.Char.isSymbol c && not (c `elem` "(),;[]`{}_\"'"))

matchChar :: Char -> String -> Lex a ()
matchChar c msg = do
	s <- getInput
	if null s || head s /= c then fail msg else discard 1

-- The top-level lexer.
-- We need to know whether we are at the beginning of the line to decide
-- whether to insert layout tokens.

lexer :: (Token -> P a) -> P a
lexer = runL $ do
	bol <- checkBOL
	bol <- lexWhiteSpace bol
	startToken
	if bol then lexBOL else lexToken

lexWhiteSpace :: Bool -> Lex a Bool
lexWhiteSpace bol = do
	s <- getInput
	case s of
	    '{':'-':_ -> do
		discard 2
		bol <- lexNestedComment bol
		lexWhiteSpace bol
	    '-':'-':rest | all (== '-') (takeWhile isSymbol rest) -> do
		lexWhile (== '-')
		lexWhile (/= '\n')
		s' <- getInput
		case s' of
		    [] -> fail "Unterminated end-of-line comment"
		    _ -> do
			lexNewline
			lexWhiteSpace True
	    '\n':_ -> do
		lexNewline
		lexWhiteSpace True
	    '\t':_ -> do
		lexTab
		lexWhiteSpace bol
	    c:_ | isSpace c -> do
		discard 1
		lexWhiteSpace bol
	    _ -> return bol

lexNestedComment :: Bool -> Lex a Bool
lexNestedComment bol = do
	s <- getInput
	case s of
	    '-':'}':_ -> discard 2 >> return bol
	    '{':'-':_ -> do
		discard 2
		bol <- lexNestedComment bol	-- rest of the subcomment
		lexNestedComment bol		-- rest of this comment
	    '\t':_    -> lexTab >> lexNestedComment bol
	    '\n':_    -> lexNewline >> lexNestedComment True
	    _:_       -> discard 1 >> lexNestedComment bol
	    []        -> fail "Unterminated nested comment"

-- When we are lexing the first token of a line, check whether we need to
-- insert virtual semicolons or close braces due to layout.

lexBOL :: Lex a Token
lexBOL = do
	pos <- getOffside
	case pos of
	    LT -> do
                -- trace "layout: inserting '}'\n" $
        	-- Set col to 0, indicating that we're still at the
        	-- beginning of the line, in case we need a semi-colon too.
        	-- Also pop the context here, so that we don't insert
        	-- another close brace before the parser can pop it.
		setBOL
		popContextL "lexBOL"
		return VRightCurly
	    EQ ->
                -- trace "layout: inserting ';'\n" $
		return SemiColon
	    GT ->
		lexToken

lexToken :: Lex a Token
lexToken = do
    s <- getInput
    case s of
        [] -> return EOF

	'0':c:d:_ | toLower c == 'o' && isOctDigit d -> do
			discard 2
			n <- lexOctal
			return (IntTok n)
		  | toLower c == 'x' && isHexDigit d -> do
			discard 2
			n <- lexHexadecimal
			return (IntTok n)

	c:_ | isDigit c -> lexDecimalOrFloat

	    | isUpper c -> lexConIdOrQual ""

	    | isLower c || c == '_' -> do
		ident <- lexWhile isIdent
		return $ case lookup ident (reserved_ids ++ special_varids) of
			Just keyword -> keyword
			Nothing -> VarId ident

	    | isSymbol c -> do
		sym <- lexWhile isSymbol
		return $ case lookup sym (reserved_ops ++ special_varops) of
			Just t  -> t
			Nothing -> case c of
			    ':' -> ConSym sym
			    _   -> VarSym sym

	    | otherwise -> do
		discard 1
		case c of

		    -- First the special symbols
		    '(' ->  return LeftParen
		    ')' ->  return RightParen
		    ',' ->  return Comma
		    ';' ->  return SemiColon
		    '[' ->  return LeftSquare
		    ']' ->  return RightSquare
		    '`' ->  return BackQuote
		    '{' -> do
			    pushContextL NoLayout
			    return LeftCurly
		    '}' -> do
			    popContextL "lexToken"
			    return RightCurly

		    '\'' -> do
			    c2 <- lexChar
			    matchChar '\'' "Improperly terminated character constant"
			    return (Character c2)

		    '"' ->  lexString

		    _ ->    fail ("Illegal character \'" ++ show c ++ "\'\n")

lexDecimalOrFloat :: Lex a Token
lexDecimalOrFloat = do
	ds <- lexWhile isDigit
	rest <- getInput
	case rest of
	    ('.':d:_) | isDigit d -> do
		discard 1
		frac <- lexWhile isDigit
		let num = parseInteger 10 (ds ++ frac)
		    decimals = toInteger (length frac)
		exponent <- do
			rest2 <- getInput
			case rest2 of
			    'e':_ -> lexExponent
			    'E':_ -> lexExponent
			    _     -> return 0
		return (FloatTok ((num%1) * 10^^(exponent - decimals)))
	    e:_ | toLower e == 'e' -> do
		exponent <- lexExponent
		return (FloatTok ((parseInteger 10 ds%1) * 10^^exponent))
	    _ -> return (IntTok (parseInteger 10 ds))

    where
	lexExponent :: Lex a Integer
	lexExponent = do
		discard 1	-- 'e' or 'E'
		r <- getInput
		case r of
		    '+':d:_ | isDigit d -> do
			discard 1
			lexDecimal
		    '-':d:_ | isDigit d -> do
			discard 1
			n <- lexDecimal
			return (negate n)
		    d:_ | isDigit d -> lexDecimal
		    _ -> fail "Float with missing exponent"

lexConIdOrQual :: String -> Lex a Token
lexConIdOrQual qual = do
	con <- lexWhile isIdent
	let conid | null qual = ConId con
		  | otherwise = QConId (qual,con)
	    qual' | null qual = con
		  | otherwise = qual ++ '.':con
	just_a_conid <- alternative (return conid)
	rest <- getInput
	case rest of
	  '.':c:_
	     | isLower c || c == '_' -> do	-- qualified varid?
		discard 1
		ident <- lexWhile isIdent
		case lookup ident reserved_ids of
		   -- cannot qualify a reserved word
		   Just _  -> just_a_conid
		   Nothing -> return (QVarId (qual', ident))

	     | isUpper c -> do		-- qualified conid?
		discard 1
		lexConIdOrQual qual'

	     | isSymbol c -> do	-- qualified symbol?
		discard 1
		sym <- lexWhile isSymbol
		case lookup sym reserved_ops of
		    -- cannot qualify a reserved operator
		    Just _  -> just_a_conid
		    Nothing -> return $ case c of
			':' -> QConSym (qual', sym)
			_   -> QVarSym (qual', sym)

	  _ ->	return conid -- not a qualified thing

lexChar :: Lex a Char
lexChar = do
	r <- getInput
	case r of
		'\\':_	-> lexEscape
		c:_	-> discard 1 >> return c
		[]	-> fail "Incomplete character constant"

lexString :: Lex a Token
lexString = loop ""
    where
	loop s = do
		r <- getInput
		case r of
		    '\\':'&':_ -> do
				discard 2
				loop s
		    '\\':c:_ | isSpace c -> do
				discard 1
				lexWhiteChars
				matchChar '\\' "Illegal character in string gap"
				loop s
			     | otherwise -> do
				ce <- lexEscape
				loop (ce:s)
		    '"':_ -> do
				discard 1
				return (StringTok (reverse s))
		    c:_ -> do
				discard 1
				loop (c:s)
		    [] ->	fail "Improperly terminated string"

	lexWhiteChars :: Lex a ()
	lexWhiteChars = do
		s <- getInput
		case s of
		    '\n':_ -> do
			lexNewline
			lexWhiteChars
		    '\t':_ -> do
			lexTab
			lexWhiteChars
		    c:_ | isSpace c -> do
			discard 1
			lexWhiteChars
		    _ -> return ()

lexEscape :: Lex a Char
lexEscape = do
	discard 1
	r <- getInput
	case r of

-- Production charesc from section B.2 (Note: \& is handled by caller)

		'a':_		-> discard 1 >> return '\a'
		'b':_		-> discard 1 >> return '\b'
		'f':_		-> discard 1 >> return '\f'
		'n':_		-> discard 1 >> return '\n'
		'r':_		-> discard 1 >> return '\r'
		't':_		-> discard 1 >> return '\t'
		'v':_		-> discard 1 >> return '\v'
		'\\':_		-> discard 1 >> return '\\'
		'"':_		-> discard 1 >> return '\"'
		'\'':_		-> discard 1 >> return '\''

-- Production ascii from section B.2

		'^':c:_		-> discard 2 >> cntrl c
		'N':'U':'L':_	-> discard 3 >> return '\NUL'
		'S':'O':'H':_	-> discard 3 >> return '\SOH'
		'S':'T':'X':_	-> discard 3 >> return '\STX'
		'E':'T':'X':_	-> discard 3 >> return '\ETX'
		'E':'O':'T':_	-> discard 3 >> return '\EOT'
		'E':'N':'Q':_	-> discard 3 >> return '\ENQ'
		'A':'C':'K':_	-> discard 3 >> return '\ACK'
		'B':'E':'L':_	-> discard 3 >> return '\BEL'
		'B':'S':_	-> discard 2 >> return '\BS'
		'H':'T':_	-> discard 2 >> return '\HT'
		'L':'F':_	-> discard 2 >> return '\LF'
		'V':'T':_	-> discard 2 >> return '\VT'
		'F':'F':_	-> discard 2 >> return '\FF'
		'C':'R':_	-> discard 2 >> return '\CR'
		'S':'O':_	-> discard 2 >> return '\SO'
		'S':'I':_	-> discard 2 >> return '\SI'
		'D':'L':'E':_	-> discard 3 >> return '\DLE'
		'D':'C':'1':_	-> discard 3 >> return '\DC1'
		'D':'C':'2':_	-> discard 3 >> return '\DC2'
		'D':'C':'3':_	-> discard 3 >> return '\DC3'
		'D':'C':'4':_	-> discard 3 >> return '\DC4'
		'N':'A':'K':_	-> discard 3 >> return '\NAK'
		'S':'Y':'N':_	-> discard 3 >> return '\SYN'
		'E':'T':'B':_	-> discard 3 >> return '\ETB'
		'C':'A':'N':_	-> discard 3 >> return '\CAN'
		'E':'M':_	-> discard 2 >> return '\EM'
		'S':'U':'B':_	-> discard 3 >> return '\SUB'
		'E':'S':'C':_	-> discard 3 >> return '\ESC'
		'F':'S':_	-> discard 2 >> return '\FS'
		'G':'S':_	-> discard 2 >> return '\GS'
		'R':'S':_	-> discard 2 >> return '\RS'
		'U':'S':_	-> discard 2 >> return '\US'
		'S':'P':_	-> discard 2 >> return '\SP'
		'D':'E':'L':_	-> discard 3 >> return '\DEL'

-- Escaped numbers

		'o':c:_ | isOctDigit c -> do
					discard 1
					n <- lexOctal
					checkChar n
		'x':c:_ | isHexDigit c -> do
					discard 1
					n <- lexHexadecimal
					checkChar n
		c:_ | isDigit c -> do
					n <- lexDecimal
					checkChar n

		_		-> fail "Illegal escape sequence"

    where
	checkChar n | n <= 0x10FFFF = return (chr (fromInteger n))
	checkChar _		    = fail "Character constant out of range"

-- Production cntrl from section B.2

	cntrl :: Char -> Lex a Char
	cntrl c | c >= '@' && c <= '_' = return (chr (ord c - ord '@'))
	cntrl _                        = fail "Illegal control character"

-- assumes at least one octal digit
lexOctal :: Lex a Integer
lexOctal = do
	ds <- lexWhile isOctDigit
	return (parseInteger 8 ds)

-- assumes at least one hexadecimal digit
lexHexadecimal :: Lex a Integer
lexHexadecimal = do
	ds <- lexWhile isHexDigit
	return (parseInteger 16 ds)

-- assumes at least one decimal digit
lexDecimal :: Lex a Integer
lexDecimal = do
	ds <- lexWhile isDigit
	return (parseInteger 10 ds)

-- Stolen from Hugs's Prelude
parseInteger :: Integer -> String -> Integer
parseInteger radix ds =
	foldl1 (\n d -> n * radix + d) (map (toInteger . digitToInt) ds)
