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module Main where
import Text.ParserCombinators.Parsec (many1, sepBy1, string, noneOf, char, digit, spaces, sepEndBy, Parser, (<|>), parse, letter, oneOf)
--import Relude hiding ((<|>))
import Prelude
import Data.Foldable (foldl1)
import qualified Data.Set as Set
import Control.Monad (forever, void)
import System.IO (hFlush, stdout)
data Term = Var String | App Term Term | Lam String Term deriving (Eq)
instance Show Term where
show (Var x) = x
show (App x y) = show x ++ show y
show (Lam x y) = "(λ"++x++". "++show y++")"
parens p = char '(' *> spaces *> p <* spaces <* char ')'
var = Var <$> many1 letter
lam = do
void $ char '\\'
spaces
vars <- many1 $ (many1 letter) <* spaces
void $ string "."
spaces
body <- term
return (foldr Lam body vars)
app = foldl1 App <$> sepBy1 (var <|> parens term) spaces
term = lam <|> app <|> var
fv :: Term -> Set.Set String
fv (Var x) = Set.singleton x
fv (Lam x m) = x `Set.delete` (fv m)
fv (App m1 m2) = (fv m1) `Set.union` (fv m2)
pickFresh :: Set.Set String -> String -> String
pickFresh s n | n `Set.notMember` s = n
pickFresh s n = pickFresh s $ n <> "'"
subst n x (Var e)
| x == e = n
| otherwise = Var e
subst n x (App m1 m2) = App (subst n x m1) (subst n x m2)
subst n x l@(Lam y m)
| x == y = l
| y `Set.notMember` (fv n) = Lam y (subst n x m)
| otherwise = Lam y' (subst n x (subst (Var y') y m))
where
y' = pickFresh ((fv n) `Set.union` (fv m)) y
normalstep (App (Lam x m) n) = Just (subst n x m)
normalstep (Lam x m) = normalstep m >>= return . Lam x
normalstep (App m n) | normalstep m == Nothing = normalstep n >>= return . App m
normalstep (App m n) = normalstep m >>= return . (`App` n)
normalstep _ = Nothing
repeatedly f x = maybe x (repeatedly f) (f x)
eval = (repeatedly normalstep <$>) . parse term "<stdin>"
main = forever $ do
putStr "Untyped> "
hFlush stdout
contents <- getLine
putStrLn $ (either show show . eval) contents
|
