[Feature request] Set multiple fields at once

[Vilin97]

Setting several fields at once would reduce the number of times the immutable struct is copied. For example,

nt = (a=1, b=2)
@reset nt.a=2, nt.b=3

instead of

nt = (a=1, b=2)
@reset nt.a=2
@reset nt.b=3

[aplavin]

Are you mostly looking for performance or convenience? If the former, then "copying immutable struct" should be free in most cases:

julia> nt = (a=1, b=2, c=3)
julia> @btime let
       x = @set $nt.a = 10
       y = @set x.b = 20
       end
  1.333 ns (0 allocations: 0 bytes)
(a = 10, b = 20, c = 3)

If convenience, then:

• For setting properties at the top level, there's setproperties(nt, (a=10, b=20)).
• Setting arbitrary optics at once would be useful and there are multiple related features requests, see eg "Concat lens": combine multiple lenses side by side #82. But implementing it so that the operation is really done with a single call when possible is complicated, and the proper interface is not obvious.

[Vilin97]

I am mostly looking for performance.
How can constructing a new struct be free? If a struct has n fields, how can creating a new struct not be O(n) time?

[aplavin]

Often, immutable structs are not really "created" in this sense, they don't physically exist in memory.

[ronisbr]

Yes! I can confirm @aplavin statement. In all cases I used Accessors.jl, I saw no performance penalty by setting multiple parameters at once with ConstructionBase.setproperties ou using multiple @resets. However, I think something like this would be less verbose:

@resets nt begin
    nt.a = 1
    nt.b = 2
    nt.c = 3
    ....
end

This idea is to replace all expressions with nt.a to @reset nt.a inside this block.

[aplavin]

That's how I tend to write sequences of Accessors (or any other!) operations:

julia> using Accessors, DataPipes

julia> nt = (a=1, b=2)

julia> @p let
       nt
       @set __.b = 3
       @set __.a += 4
       @insert __.c = 9
       @set √(last(__)) += 1
   end
(a = 5, b = 3, c = 16.0)

# or as a part of a larger pipeline:
julia> nts = [nt, nt]

julia> @p let
       nts
       map() do __
           @insert __.c = 9
           @set __.a += 4
       end
   end
2-element Vector{@NamedTuple{a::Int64, b::Int64, c::Int64}}:
 (a = 5, b = 2, c = 9)
 (a = 5, b = 2, c = 9)

Looking at these examples, not sure if the @resets macro from your example would noticeably improve them...
These flexible building blocks really play nicely together in Julia! :)

[aplavin]

Closing as duplicate of #15