Allow map!(f, array)

[mcabbott]

Edit -- implements #31677, see discussion there.

I didn't find an issue, but did find some tests of non-AbstractArray cases where this is allowed:

julia/test/dict.jl

Lines 1182 to 1183 in 3d08f82

	testdict = Dict(:a=>1, :b=>2)
	map!(v->v-1, values(testdict))

julia> filter(x -> x.nargs==3, collect(methods(map!)))  # on master
[1] map!(f, iter::Base.ValueIterator{<:Dict}) in Base at dict.jl:717
[2] map!(f, iter::Base.ValueIterator{<:WeakKeyDict}) in Base at weakkeydict.jl:140
[3] map!(f, iter::Base.ValueIterator) in Base at abstractdict.jl:571

[fredrikekre]

xref #31677

[mbauman]

Here's the rub:

• map!(f, A, B, C) is kinda like* A .= f.(B, C); map(f, B, C) is like f.(B, C).
• map!(f, A, B) is like A .= f.(B). map(f, B) is like f.(B).
• map!(f, A) would be ???. map(f) is like f.().

^{*ignoring differences between map and broadcast, of course}

[mcabbott]

Sorry I missed the earlier discussion, somehow. Agree it's a bit awkward.

Is the difference between map!'s behaviour of stopping when any collection is exhausted, and broadcast!'s demanding equality (and extending 1) relevant? Maybe not, broadcast!(()->99, zeros(3)) isn't extending anything, unlike broadcast!(_->99, zeros(3), [pi]).

[vtjnash]

Extending from @mbauman...unless we change map(f), then the last row would be:

map(f) is like A -> map(f, A)
and
map!(f, A) may be B -> map!(f, A, B) (I assume, by extension)

[brenhinkeller]

FWIW map and broadcast seem different enough that the parallel structure argument doesn't really seem like a problem to me..

[mbauman]

I'm honestly not sure what I was trying to say there, hah. I think my point was:

• the number of evaluations of f in map(f, args...) is fully defined by the iteration space of args..., and thus if they're missing, it's just a single scalar evaluation.
• the number of evaluations in map(f, arg1, args...) is exactly identical to map!(f, output, arg1, args...)
• the number of evaluations in map(f) would necessarily be different from map!(f, output).

I'm not so sure that's a problem anymore. But we would have to change the documentation. Currently:

map(f, c...) -> collection

Transform collection c by applying f to each element.

map!(function, destination, collection...)

Like map, but stores the result in destination rather than a new collection.

[JeffBezanson]

We now have map!(f, ::ValueIterator) for dicts, so there is a bit of precedent for this.

[LilithHafner]

Triage discussed this (#31677 (comment))

[adienes]

with #52631 merged, maybe this can merge too before feature freeze?

closes #31677

closes #12277, in that no further changes to behavior are planned

[vtjnash]

As a (minor) breaking change, I think #52631 needs to be in at least public release before we can merge this? (which does mean we can merge this on Tuesday, just not today)

[LilithHafner]

IMO it would be possible to keep this even if we revert #52631, so no need to wait a release; but I'm not in a hurry here.