break with value + loop else clauses

[StefanKarpinski]

Once upon a time, I proposed giving a meaning to for-else and while-else that would be quite at odds with the meaning given to these constructs in Python. In version 1.19, Rust introduced a different feature which finally made Python's for-else and while-else contructs make sense to me: break with a value. Of course, Python still doesn't have break with a value and Rust doesn't have for-else or while-else, but these features really dovetail nicely. Let me explain by showing how this would work in Julia...

Loops in Julia are expressions like everything, but they evaluate to nothing. If we added break with a value, then a loop that's exited with break x would evaluate to x, allowing one to write something like this:

using Primes

values = rand(1:100, 10)

p = for x in values
    isprime(x) && break x
end

After this executes, p is equal to the first number in values that's prime (of course, we could use findfirst, but bear with me). This code is type-unstable, however: if values happens to contain no primes, then the loop evaluates to nothing. Enter the Python-style else clause on a for loop:

p = for x in values
    isprime(x) && break x
else
    maximum(values) # makes no sense, but whatever
end

Now p is set to the first prime occurring in values, or it is set to the maximum value of values. This is a silly example, but you get the point. This is all non-breaking since all of these syntaxes are currently errors.

[tkluck]

I took a jab at this with the pull request mentioned above; I took it as an opportunity to learn a bit about Julia's internals. Let me know what you think!

I'm not particularly attached to the current implementation; I'll be happy to learn from any suggestions or alternative patches.

[jakobjpeters]

Since this issue is still open but most of the discussion appears to occur in #23260, I posted some ideas #23260 (comment) for if this is ever taken up again.