for-else and while-else

[StefanKarpinski]

Allow and else clause on for and while loops that executes if the loop never executes.

[quinnj]

Bump. Can this be 0.4?

[mlhetland]

I guess you've decided on this behavior—just thought I'd pipe in with an opinion. I find Python's behavior really useful here, where the else clause is executed if the loop has not been terminated early by break. The assumption is that you're looking for something, and the else clause is for if you don't find it. Maybe less seemingly logical than the suggested behavior, but I've had use for it many times, at least. In Python, it's also consistent with the else clauses when catching exceptions (try/catch/else), where the else clause is executed if there was no exception. The two seem quite analogous, and the else clause for try statements is also something I think would be really useful. I guess one could still have that even with the suggested behavior here, of course.

[StefanKarpinski]

That Python for-else behavior has always struck me as very unintuitive, honestly. Mostly it's the name, not the functionality itself. The functionality can be emulated with gotos, but it is kind of awkward.

[elextr]

Whilst there is nothing against this, at least in my experience the thing that is needed most often is to know if the loop was breaked (broken?) or if it completed.

But I have never found a "nice" way of specifying the required combination(s) of the three conditions, that the loop never ran, shortcut (break), or completed.

[mlhetland]

Yeah, @StefanKarpinski, I guess it's mainly a matter of recycling keywords in Python. I think the naming is less awkward/surprising for try, though, if you see it as an alternative to the catch part. But, sure, the naming is more logical in the original feature spec. I can't recall having needed that functionality, but if it existed, I surely would ;-)

[ivarne]

Two different suggestions for what one rarely used syntax should mean, seems like a bad sign.

[StefanKarpinski]

Well, one would never guess at the Python meaning without already knowing it, whereas I think the meaning of executing if the main body never executes is pretty intuitive, but yes, that's still a point.

[prcastro]

I know that it's a keyword not yet used on Julia, but I feel that the then keyword is more intuitive when doing what @mlhetland suggested. What do you think?

[mlhetland]

Actually, @StefanKarpinski, I hadn't noticed there were gotos in Julia; saw them in the NEWS.mdfile for 0.3.0 now. Good enough for me—just a @goto instead of a break, and a @label instead of an else (though placed differently, of course). Better than, say, lots of Boolean flags and conditionals. Speaking of macros (and loops), would it be totally insane to adapt something like the Lisp LOOP macro for Julia? (Possibly in a separate library.) Not sure how useful it'd be. Just saw it mentioned in a (much-publicized) blog post on Julia vs Lisp, and implementing it (or something like it) sounded like fun ;-)

[wizzat]

This is a super useful feature. Has a resolution for this been decided or is everyone going to use goto?

[tonyhffong]

using @goto will trigger a lint warning, fyi.