Jules Bean schrieb:
data Stack a b = Stack { run :: [a] -> (b, [a]) }
Thank you, that does the trick.
The correct types for the other functions are:
push :: a -> Stack a () pop :: Stack a a top :: Stack a a
With those clues I think you will be able to write >>= and return more successfully!
Yes, this was the missing link. Because I thought "Stack a a" could be abbreviated using "Stack a" I run into these problems. This was also the cause that "push" echoed back the pushed value.
There are some other interesting combinators to consider, like:
isolate :: Stack b x -> Stack a x -- runs the computation with an empty stack, therefore -- guaranteeing it does more pushes than pops
Did you mean: isolate :: Stack s1 a -> Stack s2 a isolate stack = Stack f where f xs = ( fst $ run stack [], xs)
and so on.
Yes, I have done: push, pop, top, nop, count, clear, isolate and binop. All pretty easy, once I understand that "Stack a b" thing.