continue/break-with-value & else clauses in loops

[jakobjpeters]

These are a couple of ideas to extend those proposed in #22891.

• As a natural extension to break-with-value in a loop, I propose continue to append a value to a returned array.

• I had initially thought default would be more descriptive than else and may lessen objections to it (break-with-value and for/else implementation #23260 (comment)). However, @tkluck pointed out (break-with-value and for/else implementation #23260 (comment)) that this would not be backwards compatible because 'default' is not a keyword. What about finally? It makes sense in context with loops; after iteration it is the final block of code to run.

• The return keyword could be used to explicitly exit the finally block.

• Are there any issues with using a type declaration on the assigned variable that would need to be addressed as well?

values = rand(1:100, 10)

evens::Array{Int64} = for i in values # optional return type
    if iseven(i)
        continue i # append to returned array
    elseif i == 13
        continue "not Int64" # throw an exception
    end
finally # or `else`
    newvalues = values * 2
    return newvalues # explicit return
end

P.S. This is my first foray into github, I welcome constructive criticism/feedback :)

[tkluck]

Thanks for sharing the ideas! FYI I'm not a committer to Julia but I'll share some thoughts that are hopefully helpful.

• Backwards compatibility. New syntax has to be a syntax error in current versions of Julia -- that way you can be sure it doesn't already have a meaning. This was the problem with default and it's the same with events::Array{Int64} = for ...: this is currently valid syntax that will throw a runtime error when trying to assign the loop expression's value (nothing) to the variable.

• Selling point. What gap in the language does this fill? For example, for for/else we could find applications in the standard library (break-with-value and for/else implementation #23260 (comment)). Maybe you can do the same? It's up to you to prove that you have "product/market fit". If you're able to point at other languages that have a feature like this, even better.

For comparison here's a way of achieving what you want in current Julia:

mapreduce((a,b) -> push!(a,b...), values, init=Int64[]) do i
    if iseven(i)
        return (i,)
    elseif i == 13
        throw("not Int64")
    else
        return ()
    end
end

It's a little bit more cumbersome, but on the other hand it's able to push several values in a single iteration (which is incidentally a shortcoming of using continue for accumulation).

• finally. I don't get your proposal -- what's the case in which this block would run? There's no break so clearly you mean something different from break-with-value and for/else implementation #23260.

Hope that's helpful! And to set expectations, the chances of something like this being merged are very small -- #23260 also didn't make the triage. You could have a look at good first issue Indicates a good issue for first-time contributors to Julia so you can start with rewarding experiences before sinking your teeth in the big ones :)