[Haskell-cafe] Stack ADT?

[michael rice]

Not using Stack for anything, just trying to understand how things can be done in Haskell.

To that end...

What's going on here? I'm not even calling function POP.

Michael

======================

module Data.Stack (Stack, emptyStack, isEmptyStack, push, pop, top) where

newtype Stack a = Stack [a]

emptyStack = Stack []
isEmptyStack (Stack xs) = null xs
push x (Stack xs) = Stack (x:xs)
pop (Stack (_:xs)) = Stack xs
top (Stack (x:_)) = x

======================

[michael at localhost ~]$ ghci Stack.hs
GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Data.Stack       ( Stack.hs, interpreted )
Ok, modules loaded: Data.Stack.
*Data.Stack> let s1 = emptyStack
*Data.Stack> top (push 1 s1)
1
*Data.Stack> top (push 2 s1)
2
*Data.Stack> top (push 3 s1)
3
*Data.Stack> let s2 = pop s1 
*Data.Stack> top s2
*** Exception: Stack.hs:8:0-28: Non-exhaustive patterns in function pop

*Data.Stack> 




--- On Fri, 2/5/10, Casey Hawthorne <caseyh at istar.ca> wrote:

From: Casey Hawthorne <caseyh at istar.ca>
Subject: Re: [Haskell-cafe] Stack ADT?
To: haskell-cafe at haskell.org
Date: Friday, February 5, 2010, 10:36 AM

You could also implement stacks with mutable data structures, e.g.
STArray, etc.

What do you want to use a stack ADT for?

Usually stacks are discussed for pedagogical purposes but usually
recursion is used if you need a stack like operation.
--
Regards,
Casey
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