From: Phillip Gawlowski <cmdjackryan@...>
Date: 2008-05-02T06:10:48+09:00
Subject: Re: Please explain nuances of ||=

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David A. Black wrote:

|> x ||= y expands into x = x || y, since x == 1 expands to x = x + 1, and
|
| I think you mean +=.

Yes, indeed.

|> we have no reason to expect ||= to behave differently (MPLOS).
|
| That's really where this thread always starts, though: why doesn't it
| behave like x = x || y in every case? Anyway, the thing about MPOLS is
| that you and I can't invoke it :-) It was actually Matz who showed me
| the x || x = y expansion (most recently at EuRuKo, where I talked to
| him about it because I wanted to make sure the corrections I made to
| my blog post had made it correct).

It doesn't behave that way because classes can implement and override
behavior. Which can lead to surprising results, and sometimes
pathological cases of Monkeypatching. ;)

Also, the MPLOS helps as a guideline, not as a hard and fast rule. :)


|> And Robert's answer works the way it works because the Hash has a
|> default value set, which doesn't equate to false or nil. If you override
|> the default value by setting the value to false, the assignment works
|> again as expected.
|
| Right, but hashes are allowed to have default values. So an
| explanation that accounts for that case is better than one that
| doesn't. The point is that if you do a drop-in replacement x = x || y
| you get a different result from ||=, and with x || x = y you don't.
| That means, definitively I think, that the latter is a more correct
| expansion. (I'm not sure how it couldn't mean that.)

However, a default value results in the Hash not evaluating to nil or
false. Which is required for || and OR to use their *second* operand
(since it otherwise uses the first operand), so x || x = y cannot work.
Unless we get into precedence issues here, I guess.

| Anyway, this is at least the third thread on this in recent months,
| and I've written my blog post (and correction follow-up :-) about it.
| I think I'm about ||='d out.

So am I.

- --
Phillip Gawlowski
Twitter: twitter.com/cynicalryan
Blog: http://justarubyist.blogspot.com

~   Why do we drink cow's milk? Who was the first guy who first looked at
a cow and said "I think I'll drink whatever comes out of these things
when I squeeze 'em!"?	  -- Calvin
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