Re: [Haskell-cafe] Stack ADT?

What's "going on" is that data structures in Haskell are immutable. Thus, when you call "push" on a stack, you get a new stack with the new element pushed onto it, and the original stack is left un-touched. On Fri, Feb 5, 2010 at 10:56 AM, michael rice wrote:

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@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 * wrote:

From: Casey Hawthorne Subject: Re: [Haskell-cafe] Stack ADT? To: haskell-cafe@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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mc/compose?to=Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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