`continue` within `@tasks`

[Moelf]

I think this should be fine, but I guess the @tasks transformation made the Julia parsers unhappy:

function main(tree)
    btag_distribution = Hist1D(; bins = -2:100:2)
    @tasks for evt in tree
        @set scheduler=:static
        if !(evt.trigPassed_HLT_g35_tight_3j25_pf_ftf_PhysicsTLA_L1EM22VHI)
            continue
        end
        btag_score = D_b(evt)
        push!(btag_distribution, btag_score)
    end
end

[carstenbauer]

@tasks doesn't produce a for loop but maps to the higher order function API, tforeach in your case, which doesn't support continue or break. As for continue, we can maybe support it explicitly (in some cases), e.g. replace it by return nothing. break on the other hand doesn't make sense conceptually (I think). After all, the "loop" runs in parallel so it would lead to non deterministic behavior.

[Moelf]

Yeah continue works for @threads so it would be nice to have support for that

[MasonProtter]

return inside of @tasks (or tmapreduce) does what continue does in a loop.

julia> @tasks for i ∈ 1:10
           if isodd(i)
               return
           end
           println(i)
       end
8
2
6
4
10

[carstenbauer]

I guess the question is whether we should replace continue → return automatically inside of @tasks.

[MasonProtter]

I'd lean towards no, because I don't want to be responsible for correctly parsing and understanding arbitrary user code. e.g. if they write

@tasks for x in v
    for i in 1:x
        if cond
            continue
        end
    end
end

then we'd actually want to leave that use of continue alone, lest we break their code in confusing and hard to debug ways.

---

break is supported in Transducers.jl based stuff via their early termination interface. We probably could support it too, but it'd be a little tricky. It conceptually does make sense in some circumstances. For example, if you had a parallelized any function or a parallelized all function you'd want to break out of those as soon as you find a true or false respectively.

The way Transducers does it is if one task signals that it encountered the early termination signal, then it basically just abandons the other tasks and never fetches from them, signalling to them that they shouldn't spawn more tasks in the future, and then eagerly returns the final result.

[carstenbauer]

I've added a documentation label because we should at least mention this (continue in particular) somewhere in the docs/docstrings.

[carstenbauer]

I've created a new issue for early termination support (e.g. break within @tasks) because the original example here is about continue.

[carstenbauer]

Another potential way to support continue:

julia> @tasks for i in 1:4
           for i in i # trivial loop to support continue (could be auto inserted)
               if i > 2
                   continue
               end
               println(i)
           end
       end
2
1

julia> tforeach(1:4) do i
           for i in i # trivial loop to support continue (could be auto inserted)
               if i > 2
                   continue
               end
               println(i)
           end
       end
2
1

[Moelf]

can we insert for i in i automatically? it's quite confusing for users to look at a loop with a bunch of returns and try to tell them they are not return just continue