lspitzner

-- a djinn environment that encodes an instance of the
-- post correspondence problem. it requires recursive data
-- types.

data Foo a b x y = Foo (Bar a b (a x) (b (a (a y))))
                       (Bar a b (a (b x)) (a (a y)))
                       (Bar a b (b (b (a x))) (b (b y)))
 
-- we need duplicate to enforce N>=1
data Bar a b x y = Bar (Bar a b (a x) (b (a (a y))))

lspitzner

{-# LANGUAGE Arrows, TemplateHaskell #-}


module Main where


import Prelude hiding (id, init, (.))
import Control.Arrow
import Control.Category