How to subclass OrderedCollection in GNU Smalltalk?

Question

I'm reading a book using VisualWorks and I try to write the code in GNU Smalltalk. I have this:

OrderedCollection subclass: Stack [
    push: anObject [
         self addLast: anObject.
    ]

    pop [
        self isEmpty
           ifTrue: [^nil]
           ifFalse: [^self removeLast].
    ]
]

| st |
st := Stack new.
st push: 'a'.
Transcript show: st pop.

but it doesn't work. Can someone please explain me what am I doing wrong?

Can you be more specific about what is not working and how exactly? — Uko, Jan 29 '14 at 21:30
I got Object: Stack error: should not be implemented in this class, use #basicNew instead. Stuart's response was what I needed. In the future I will try to be more specific and also add the messages i get. — caisah, Jan 30 '14 at 09:45

score 3 · Accepted Answer · edited May 23 '17 at 12:06

3

I'm assuming you're getting Object: Stack error: should not be implemented in this class, use #basicNew instead?

If so, then it looks like you need to add <shape: inherit> in the body of your subclass.

See:

That seems like a bit of a leaky abstraction to me - but I guess it is what it is.

edited May 23 '17 at 12:06

Community

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1

answered Jan 30 '14 at 02:44

Stuart Herring

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That was it . Can you please explain to me what does this exactly do? I don't quite understand from the documentation. Why don't we use it on all the classes we create by inheriting? – caisah Jan 30 '14 at 09:47
1

the pragmas are a GNU Smalltalk specific thing, and I'm not very familiar with it. But I suspect it's just a design tradeoff - it will be providing special information to the compiler to tell it that it's a special type of class - and given that it's pretty rare that you want to subclass one of those types it was probably considered a reasonable tradeoff to simplify the implementation. – Stuart Herring Feb 01 '14 at 23:02

score 0 · Answer 2 · answered Jan 30 '14 at 17:07

0

The primary problem is a design one. When learning smalltalk and object orientation, you will be happier if you choose composition rather than inheritance by default. A stack has a very limited interface

push: anElement
pop
isEmpty

so why not make a class Stack with an instance variable stackData

Stack>>initialize
    stackData := OrderedCollection new

An OrderedCollection has a very wide interface, and this way you get a Stack that only responds to a very narrow interface. That makes it much easier to understand and use.

answered Jan 30 '14 at 17:07

Stephan Eggermont

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I understand what you mean, and I've heard about: "Always favour composition over inheritance". This is my first oo language and with time, I guess I will sharpen my design skills. But the snippet above was just a simple/dummy example and it was never intended to be used in real applications. – caisah Jan 30 '14 at 17:41

score 0 · Answer 3 · answered Oct 13 '16 at 23:16

There is nothing wrong with your code. It's GNU-ST which does wierd things.

In other Smalltalks, this works like a charm. Here is what I typed into a Smalltalk/X workspace, and it does the expected (shows 'a' on the Transcript):

OrderedCollection 
    subclass: #MyStack
    instanceVariableNames:''
    classVariableNames:''
    poolDictionaries:''.

MyStack compile:'
    push: anObject
         self addLast: anObject.
'.

MyStack compile:'
    pop
        self isEmpty
           ifTrue: [^nil]
           ifFalse: [^self removeLast].
'.

| st |

st := XStack new.
st push: 'a'.
Transcript show: st pop.

How to subclass OrderedCollection in GNU Smalltalk?

3 Answers3