[resu_nimda]

As demonstrated in the postscript, it can still be accomplished when the symbols are all the same. So that difference is not the key to the hack. As ronaldx noted, the key is that a variable can change value in between two equivalence evaluations within a single statement.

[philip_roberts]

Arguably the thing that tripped me up for the majority of the time was latching on to the fact that == and === are different, and trying to _always_ satisfy the two conditions, rather than just spotting that I can change the reference to b from within the first valueOf call. I'm not sure how much I'd read into the "if I was more of a mathematician and less of a dumb programmer this would have been solved trivially" argument.

[nmrm]

If a == b iff a === b, then == ~ ===. If == and === are different (which they presumable are, because otherwise we would only use one symbol) then by definition there exist counter-examples of the thing you were trying to _always_ satisfy.

In other words, it would never even occur to a mathematician to make your mistake...

More to the point, a mathematician wouldn't assume that some arbitrary predicate that you are allowed to define at will behaves the same way as a system-defined equivalence operator.

Edit: Added last paragraph and readability changes. Also, ~ is another symbol for equality, this time equality over binary relations.

[nmrm]

The postscript is irrelevant to the claim made in the original post on this thread. It's a conflated example and not indicative of real programming.

if "get" returns different values when called multiple times within a single expression, one of two things must be true: * Someone's overriding "get" with a mutation to the variable. In all but the very rarest of circumstances, that someone should be publicly shamed. It's a nice hack for the purpose of the article, but not real code. * there's some very fine-grained concurrency. This case is irrelevant to the OP's claim because concurrency of this sort is very likely a fundamental characteristic of the system at hand, and never an artifact of the language.

[resu_nimda]

Which claim is that, exactly? And are you implying that the author's original solution is more indicative of real programming?

Both solutions work the same way - when accessing the value of "b" for the first check, b is mutated such that the next check will return a different value. Both of them are equally bad practices.

The == vs. === issue is the part that's actually irrelevant, it served as a red herring.

[nmrm]

> Which claim is that, exactly? A

"This difference in how syntax is understood actually presents a barrier to programming for modestly trained mathematicians, who would otherwise be expected to excel."

> And are you implying that the author's original solution is more indicative of real programming?

Well, neither is... but yes, overriding == or .equals is somewhat common.

> when accessing the value of "b" for the first check, b is mutated such that the next check will return a different value.

The difference is that you could easily imagine a sane example where x == y != x === y, whereas it's hard to imagine a situation where I would want to override get (or ==) with a method that mutates the requested variable.

> The == vs. === issue is the part that's actually irrelevant, it served as a red herring.

For the claim re: syntax and compatibility with mathematician's expectation, it's exactly the other way around.

FWIW I feel the quoted claim re: syntax claim is interesting, whereas the contents of the article (summarily, "you can override == or .get and cause crazy stuff to happen") is unsurprising and kind of silly to most programmers even moderately experienced with OOP in dynamic languages (for the reasons given in this post and others -- everyone knows about it, it shouldn't take 11 days to figure it out, and in practice no one does purposefully obscure stuff like mutating values when == or .get is called).

edit: grammar