Question

I need to create stack data type in haskell to be able to write like this:

let a = emptyStack

push 10 a
//[10]

pop a 
[]

I want push to look like

push :: a -> Stack a -> Stack a
push a b = a:b

but I have problems with syntax, exactly how to declare this new data type, so that

let a = emptyStack 
:t a

would show Stack

any hints on syntax

Answer 1

Let’s look at your implementation for push. It uses the operator :. You can find out the type of that operator like this:

ghci> :t (:)
(:) :: a -> [a] -> [a]

So this operator takes an a (which represents an arbitrary type), and a sequence of as, and returns the updated sequence. So your type Stack needs to be a sequence.

type Stack a = [a]

If you then define emptyStack like this:

emptyStack :: Stack a
emptyStack = []

You’ll get the result you’re looking for.

ghci> :t a
a :: Stack a

With that help, I think you’ll be able to figure out how to write pop.