-- Parser for the Lamb programming language
-- Copyright (c) 2013 darkf
-- Licensed under the terms of the zlib license, see LICENSE for details

module Parser where

import Text.Parsec
import Text.Parsec.String
import Text.Parsec.Expr
import qualified Text.Parsec.Token as T
import Text.Parsec.Language
import AST

languageDef = emptyDef {T.commentStart="{-",
						  T.commentEnd="-}",
						  T.commentLine="--",
						  T.nestedComments=True,
						  T.identStart = letter <|> char '_',
						  T.identLetter = alphaNum <|> char '_' <|> char '\'' <|> char '!' <|> char '?',
						  T.reservedNames = ["do", "end"],
						  T.reservedOpNames = ["+", "-", "*", "/", "==", "!=", "<", ">"]}

lexer = T.makeTokenParser languageDef
exprparser = buildExpressionParser ops term <?> "expression"
ops = [ [Infix (reservedOp "*" >> return Mul) AssocLeft]
	  , [Infix (reservedOp "/" >> return Div) AssocLeft]
	  , [Infix (reservedOp "+" >> return Add) AssocLeft]
	  , [Infix (reservedOp "-" >> return Sub) AssocLeft]

	  , [Infix (reservedOp "==" >> return Equals) AssocLeft]
	  , [Infix (reservedOp "!=" >> return NotEquals) AssocLeft]
	  , [Infix (reservedOp "<" >> return LessThan) AssocLeft]
	  , [Infix (reservedOp ">" >> return GreaterThan) AssocLeft]
	  ]

identifier = T.identifier lexer
reserved   = T.reserved   lexer
reservedOp = T.reservedOp lexer
parens     = T.parens     lexer
integer    = T.integer    lexer
semi       = T.semi       lexer
whiteSpace = T.whiteSpace lexer
symbol	   = T.symbol lexer

statement = exprparser

-- http://codereview.stackexchange.com/a/2572
stringChar =
	escaped <|> noneOf "\""
	where
		escaped = char '\\' >> choice (zipWith escapedChar codes replacements)
		escapedChar code replacement = char code >> return replacement
		codes        = ['b',  'n',  'f',  'r',  't',  '\\', '\"', '/']
		replacements = ['\b', '\n', '\f', '\r', '\t', '\\', '\"', '/']

stringLiteral = do
	char '"'
	x <- many stringChar
	char '"'
	whiteSpace
	return x

block = do
	reserved "do"
	lst <- seqStmt
	reserved "end"
	return $ Block lst

listSeq p cons = do
	symbol "["
	lst <- sepBy p (symbol ",")
	symbol "]"
	return $ cons lst

tupleSeq p cons = do
	symbol "("
	lst <- sepBy1 p (symbol ",")
	symbol ")"
	return $ cons lst

emptyTuple cons = do
	symbol "("
	symbol ","
	symbol ")"
	return $ cons []

intPattern = fmap IntP integer
varPattern = fmap VarP identifier
stringPattern = fmap StrP stringLiteral
listPattern = listSeq pattern ListP

consPattern = do
	x <- intPattern <|> varPattern <|> stringPattern <|> try (tupleSeq pattern TupleP)
	symbol "::"
	y <- pattern
	return $ ConsP x y

pattern = try consPattern
		<|> try (emptyTuple TupleP)
		<|> try (tupleSeq pattern TupleP)
		<|> listPattern
	    <|> varPattern
	    <|> intPattern
	    <|> stringPattern

patterns = sepBy pattern (symbol ",")

funDef = do
	name <- identifier
	symbol "("
	pats <- patterns
	let pat = (case pats of
		[] -> UnitP
		[a] -> a
		otherwise -> TupleP pats)
	symbol ")"
	symbol "->"
	body <- exprparser
	return $ Defun name $ Lambda [(pat, body)]

lambda = do
	symbol "\\"
	pats <- patterns
	let pat = (case pats of
		[] -> UnitP
		[a] -> a
		otherwise -> TupleP pats)
	symbol "->"
	body <- exprparser
	return $ Lambda [(pat, body)]

call p argp = do
	lhs <- p
	whiteSpace
	symbol "("
	args <- sepBy argp (symbol ",")
	let arg = (case args of
		[] -> UnitConst
		[a] -> a
		otherwise -> TupleConst args)
	symbol ")"
	return $ Call lhs arg

consExpr = do
	x <- expr3
	symbol "::"
	y <- exprparser
	return $ Cons x y

ifExpr = do
	symbol "if"
	cond <- exprparser
	symbol "then"
	t <- exprparser
	symbol "else"
	e <- exprparser
	return $ IfExpr cond t e

bool = fmap BoolConst $ (symbol "true" >> return True) <|> (symbol "false" >> return False)

def = do
	pat <- pattern
	whiteSpace
	symbol "="
	value <- exprparser
	return $ Def pat value

-- field access
accessOp = do
	symbol "/"
	return Access

expr1 = try block
	 <|> try lambda
	 <|> try def
	 <|> try (emptyTuple TupleConst)
	 <|> try (tupleSeq exprparser TupleConst)
	 <|> parens exprparser
	 <|> listSeq exprparser ListConst
	 <|> try ifExpr
	 <|> try bool
	 <|> fmap Var identifier
	 <|> fmap StrConst stringLiteral
	 <|> fmap IntConst integer

expr2 = try $ chainl1 expr1 accessOp
	  <|> expr1

expr3 = try funDef
	   <|> try (call expr2 exprparser)
	   <|> expr2

term = try consExpr
	<|> expr3

seqStmt = sepBy1 statement semi

program =
	many1 $ do
		whiteSpace
		e <- exprparser
		symbol "."
		return e

parseProgram = parse program "program"