a long time since I did some Haskell and my brain has reverted to its OO dogma...
{-# LANGUAGE GADTs, TypeFamilies, TypeOperators, MultiParamTypeClasses, FlexibleInstances, UndecidableInstances, FlexibleContexts #-}
consider
data Next input output where End :: Next () () Next :: (input -> output) -> Next input output
this is a bit like curried function... I can construct
y :: Next Integer (Next String (Next () ())) y = Next (\x -> if (x == 1) then Next (\y -> if (y == "s") then End else End) else Next (\y -> if (y == "t") then End else End))
which is a bit like "Integer->String->()" I can execute it...
exe :: Next input output -> input -> output exe (Next f) i = f i exe End () = ()
p :: Next String (Next () ()) p = exe y 1
ok.... but how do I constrain all the outputs to be of type Next? I don't want this to be valid
z :: Next Integer Integer z = Next (\x -> x + 1)
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