On Wed, Dec 9, 2009 at 2:17 AM, Jacob Faren wrote:

Hello. I'm implement data structures (a stack, here) to get familiar with Haskell. It's not going very well. I have the following code...

    -- test.hs     data Stack a = Top (Stack a) | Layers [a] deriving (Show)

    stack :: Stack a     stack = Top (Layers [])

    push :: Stack a -> a -> Stack a     push (Top (Layers ls)) value = Top (Layers (value:ls))

And I run the following in ghci...

    Prelude> :l test     [1 of 1] Compiling Main             ( test.hs, interpreted )     Ok, modules loaded: Main.     *Main> push stack 4     Top (Layers [4])     *Main> let st = stack     *Main> push st 4     ^CInterrupted.

The output of the last command never resolves. I have to kill it. Might anyone have an idea why the push function is failing with st, but not with stack?

It shouldn't, and it doesn't here, which version of GHC are you using ? By the way, your Stack datatype seems broken to me, what would be the meaning of (Top (Top (Top (Layers []))) ? I do believe that you do not wish to allow anything besides a Layers inside a Top, and your push function supports this interpretation. In this case, your Layers constructor is unnecessary and your datatype declaration should be :

data Stack a = Top [a] deriving (Show)

At which point you may realize that this type is isomorphic to the list type (+ one bottom) and thus is better written : - Either as a type synonym : type Stack a = [a] - Or if you wish to ensure some invariants on your type or add some typeclass instances specific to stacks : newtype Stack a = Top [a] deriving (Show) -- Jedaï