module Parser
    ( parseProgram
    ) where

import Text.Parsec
import Text.Parsec.String (Parser)

import Core

lexeme :: Parser a -> Parser a
lexeme p = p <* spaces

symbol :: String -> Parser String
symbol = lexeme . string

keyword :: String -> Parser ()
keyword s = try (string s *> notFollowedBy alphaNum) *> spaces

parens :: Parser a -> Parser a
parens = between (symbol "(") (symbol ")")

parseTerm :: Parser Term
parseTerm = parseIf
        <|> parseSucc
        <|> parsePred
        <|> parseIsZero
        <|> parseTrue
        <|> parseFalse
        <|> parseZero
        <|> parens parseTerm

parseTrue :: Parser Term
parseTrue = TmTrue <$ keyword "true"

parseFalse :: Parser Term
parseFalse = TmFalse <$ keyword "false"

parseZero :: Parser Term
parseZero = TmZero <$ symbol "0"

parseIf :: Parser Term
parseIf = do
    keyword "if"
    t1 <- parseTerm
    keyword "then"
    t2 <- parseTerm
    keyword "else"
    t3 <- parseTerm
    return $ TmIf t1 t2 t3

parseSucc :: Parser Term
parseSucc = do
    keyword "succ"
    t <- parseTerm
    return $ TmSucc t

parsePred :: Parser Term
parsePred = do
    keyword "pred"
    t <- parseTerm
    return $ TmPred t

parseIsZero :: Parser Term
parseIsZero = do
    keyword "iszero"
    t <- parseTerm
    return $ TmIsZero t

parseProgram :: Parser Term
parseProgram = spaces *> parseTerm <* eof