Add append(_, _) and ++

[jariji]

Draft for #53040

Adds append(_, _) and ++ to concatenate two sequences.

[LilithHafner]

Tagging triage to discuss a substantial new feature

[adienes]

how does this differ from Iterators.flatten ?

[jariji]

how does this differ from Iterators.flatten ?

Iterators.flatten(::Iterator{Iterator})::Iterator always returns an Iterator. append(::T, ::T) -> T returns something like its input type.

[aplavin]

It would be nice indeed to have a function that just concatenates two collections! Such a simple building block was surprisingly missing from Julia afaik.

[adienes]

Such a simple building block was surprisingly missing from Julia

well there are cat, stack, union, join, collect ∘ flatten etc..

I am not sure that I am convinced there needs to exist a singular overarching stick-em-together function if the value-add is primarily "magic"ness vs using a more bespoke function for the object.

and to that end I still don't really get what this would actually mean semantically. for example append! demands that the collections be ordered, but the linked issue describes append(::Set, ::Set) as being synonymous to union

and similarly states

for Matrix arguments it's hcat

which I don't really get because I thought this was supposed to be like a type-stable flatten, and that wouldn't care about dimensions?

anyway I do not actually feel too strongly about the addition of a function like this or not; I would just hope it's added because it's filling a real need rather than just tweaking some of the conventions chosen for the existing functions (e.g. the concern about cat accepting scalars, while I agree that's unfortunate, is pretty much just cosmetic IMO)

I would definitely definitely say that ++ should not be used for this though.

[aplavin]

well there are cat, stack, union, join, collect ∘ flatten etc..

Neither of them does "concatenate two collections" I think. Even when we limit ourselves to just two common Base types, tuples and vectors.

[adienes]

tuples and vectors (any 1d array type, wouldn't have to be necessarily Vector) definitely make sense to me as the primary use case. I'm just trying to understand what the function means in a generic sense.

the PR as written seems to be saying append(x::T, y::T) where T = T(collect(Iterators.flatten((x, y)))) which makes total sense to me. I'm getting stuck on understanding what's in store for multidimensional arrays or promotion rules, but maybe that's no longer in the scope of the PR

[mcabbott]

The big question seems to be: In what context would you have code which wants to be generic over things this function accepts, rather than just the things accepted by vcat or join or merge.

There is some code generic over Union{AbstractVector, Tuple} in base, is that it?

What's downstream of this has to do something beyond iterating, else Iterators.flatten((a, b)) seems fine... what is it doing, in these contexts, that wants String and Vector and Tuple?

Then are lots of detail questions, like is append(x::T, y::T) where T intended to be literally the code, or is this just a sketch? That's a place it differs from vcat... e.g. what do append(1:3, 4:5) and append(1:3, 5:6) and append(1:3, 4:5.0) return, which are errors?

[jishnub]

Should probably be copymutable?