Ticket #2814: test.hs

import System
import Char

data V = Fun (V -> V)
       | Val Int
       | Str String

app :: V -> V -> V
app (Fun f) v = f v

unchurch :: V -> Int
unchurch c = i where
        Val i = c `app` Fun inc `app` Val 0
        inc (Val x) = Val (x+1)
church i = Fun (\f -> Fun (\x -> iterate (app f) x !! i))

-- \l . l (\a b i.unchurch a : unlist b) ""
unlist :: V -> String
unlist l = s where Str s = unlist' l
unlist' :: V -> V
unlist' l = l `app` Fun walk `app` Str "" where
        walk a = Fun (\b -> Fun (\i-> Str (chr (unchurch a) : unlist b)))

cons a b = Fun (\x -> x `app` a `app` b)
nil = Fun (\a -> Fun (\b -> b) )
tolist "" = nil
tolist (x:xs) = cons (church (ord x)) (tolist xs)

main= interact (\i->unlist (((Fun (\v0 -> (v0 `app` v0)) `app` Fun (\v0 -> (v0 `app` v0))) `app` (Fun (\v0 -> (v0 `app` v0)) `app` Fun (\v0 -> (v0 `app` v0)))) `app` tolist i))