module Unlambda.Interpreter (
    unlambda
    ) where


import Unlambda.Parser




unlambda :: UnlambdaTerm -> IO UnlambdaTerm
unlambda term = eval term



eval :: UnlambdaTerm -> IO UnlambdaTerm
eval term =
    case term of
        App f x -> do
            t <- eval f
            apply t x
        _ -> return term



apply :: UnlambdaTerm -> UnlambdaTerm -> IO UnlambdaTerm
apply firstTerm secondTerm =
    case firstTerm of
        K -> eval secondTerm >>= return . Kpartial

        Kpartial x -> eval secondTerm >> return x

        S -> eval secondTerm >>= return . Spartial

        Spartial x -> eval secondTerm >>= return . (Sapp x)

        Sapp x y -> do
            z <- eval secondTerm
            result <- eval (App (App x z) (App y z))
            return result

        I -> eval secondTerm

        V -> eval secondTerm >> return V

        C -> return I --placeholder

        D -> return (Promise secondTerm)

        Promise x -> do
            y <- eval secondTerm
            result <- eval (App x y)
            return result

        Dot c -> putChar c >> eval secondTerm

        R -> putChar '\n' >> eval secondTerm

        E -> return I --placeholder

        Reed -> return I --placeholder

        Bar -> return I --placeholder

        Compare c -> return I --placeholder

        Continuation cont -> return I --placeholder