I am trying trying to implement Combinatory Logic in Haskell, and I would like to write to parser for the language. I am having trouble getting a parser to work via Parsec. The basic problem is that I need a way to ensure that the objects returned by the parser are well typed. Does anyone have any creative ideas on how to do this?
{-# Language GeneralizedNewtypeDeriving #-}
import qualified Data.Map as Map
import qualified Text.ParserCombinators.Parsec as P
import Text.Parsec.Token (parens)
import Text.ParserCombinators.Parsec ((<|>))
import Control.Applicative ((<$>), (<*>), (*>), (<*))
data CTree = CApp CTree CTree | CNode String deriving (Eq, Read)
instance Show CTree where
show c@(CApp x y) = showL c
where showL (CApp x' y')= "(" ++ showL x' ++ " " ++ showR y' ++ ")"
showL (CNode s) = s
showR (CApp x' y') = "(" ++ showL x' ++ " " ++ showR y' ++ ")"
showR (CNode s) = s
show (CNode s) = s
-- | Parser
parseC :: String -> Maybe CTree
parseC s = extract$ P.parse expr "combinator_string" s
where extract (Right r) = Just r
extract (Left e) = Nothing
expr :: P.CharParser () CTree
expr = P.try (CApp <$> (CApp <$> term <*> term) <*> expr)
<|> P.try (CApp <$> term <*> term)
<|> term
term = P.spaces *> (node <|> P.string "(" *> expr <* P.string ")")
node :: P.CharParser () CTree
node = CNode <$> (P.many1 $ P.noneOf "() ")
eval (CApp (CNode "I") x) = x
eval (CApp (CApp (CApp (CNode "S") f) g) x) =
(CApp (CApp f x) (CApp g x))
eval (CApp (CApp (CApp (CNode "B") f) g) x) =
(CApp f (CApp g x))
eval (CApp (CApp (CApp (CNode "C") f) g) x) =
(CApp (CApp f x) g)
eval x = x