Class PMVector overrides `==`

[khinsen]

PMVector is the only class in my Pharo image that overrides ProtoObject >> ==, whose comment clearly states that no class should ever override it.

[SergeStinckwich]

This is part of DHB original code that was copy-paste in PolyMath, so maybe we have to remove it.

[AtharvaKhare]

Solving this issue only involves removing

PolyMath/src/Math-Core/PMVector.class.st

Lines 92 to 96 in b1ec1ef

	{ #category : #comparing }
	PMVector >> == aNumber [
	"Apply == operator to every element of a vector"
	1 to: self size do: [ :n | self at: n put: (self at: n) == aNumber].
	]

running tests, and fix tests that dont?

[SergeStinckwich]

Do you understand what is == method ? and why classes should not reimplement it ?

[AtharvaKhare]

My understanding is that == is used to check if two objects pointers refer to the same location (both objects are same) and it is already defined in ProtoObject, there is no reason to override it.

The way PMVector == is implemented, it compares the contents of the object and not the memory location. Renaming it to something like isEqualTo (or =?) would be better.

Am I correct?

[khinsen]

@AtharvaKhare Your analysis is correct, but I'd go farther in the conclusion: overriding == is not only useless but potentially dangerous because it breaks the semantics of the operation.

You suggest renaming it, but do you actually see any situation in which it might be useful? Note that = is already there (inherited from SequenceableCollection), but does something different from PMVector>>==.

[AtharvaKhare]

@khinsen right, I get it :)

Also, looking at the comment and the syntax == aNumber, I think it is intended to apply equality to every element of the vector with a number, so it is not the same as =, which tests for equal contents between objects.
So I think it's use is like this (when rename to something like isEqualTo:

x := #(1 1 1) asPMVector.
x isEqualToThis: 1
>>> a PMVector(true true true)

I also have a question about the API, why do these arithmetic operators (>, <, isEqualTo) modify the object and not return a new one, unlike +, -, * which return new one?

[khinsen]

My question was rather if you see a situation where one might actually want to use it. Why would I ever want to know if all elements of a vector are one-by-one identical? I don't see a good use case, but obviously the author of that code did, so maybe I am just missing it.

[AtharvaKhare]

I can't figure out how it would be useful for PMVector, but in Dataseries, it can be used for converting binary categories to 0-1s.

[khinsen]

Element-by-element comparison yielding a vector/series of booleans are definitely useful in some circumstances. What I don't see is the utility of testing for element-wise object identity. Identity tests a (==) are rarely used. In Pharo I mostly see them as a first step in equality tests, to avoid doing more expensive comparisons. That scenario never happens for elementwise comparison of vectors. So what would be a situation where elementwise identity matters?

[AtharvaKhare]

I got a bit confused between == and = in my previous reply. I can't think of any application that would need ==.

[SergeStinckwich]

== is used everytime you need to be sure objects are unique, for example in a Set. Look at == senders in your image. There are more than 9000 in my image, so this is used a lot of time actually :-)