[Haskell-beginners] Problem implementing a stack
chaddai.fouche at gmail.com
Sat Dec 12 06:03:05 EST 2009
On Wed, Dec 9, 2009 at 2:17 AM, Jacob Faren <shujinw at gmail.com> 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 )
> *Main> let st = stack
> *Main> push st 4
> 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)
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