Set multiple fields at once in some sort of pipe?

[pdeffebach]

It would be nice to be able to do

n = (a = 1, b  = 2, c = 3)
n2 = @set n begin 
    a = 4
    b = 100
end

[jw3126]

Yes, would be nice to have this, see also jw3126/Setfield.jl#80

[aplavin]

This is already (partially) supported, by the way:

julia> @set n[(:a, :b)] = (4, 100)
(a = 4, b = 100, c = 3)

Also see setall(), but that goes without syntactic sugar.

[wolthom]

Also just stumbled over this. Has there been any work on this feature?

[jw3126]

Not really, for me personally setproperties is almost always good enough.

[aplavin]

Agree, setproperties turns out to be enough more often than I originally expected :)
Sometimes I need to set multiple properties/indices in a nested structure (so setproperties isn't most convenient), but then it basically always is a (named)tuple - so @set n[(:a, :b)] = (4, 100) works.

Aside from the potential implementation itself, it's not obvious what the general interface should look like. Stuff like "set x.a, x.a.c.d, and x.b.c", or "set f(x).b, g(x.a) and h(x.a).b" within a single operation. Is it even possible to make easy and convenient?

[aplavin]

Depending on the specific usecase, a recent feature of AccessorsExtra can be useful:

julia> o = @optic₊ (_.a, _.b)

julia> set(n, o, (4, 100))
(a = 4, b = 100, c = 3)

Isn't much of a win for single-use (is it even possible to make shorter syntax?), but can help when reusing the optic in multiple occasions.

[aplavin]

This seems to be the oldest open issue on the topic, best to consolidate any potential discussion in one place. Others are #119 and #82.

A rough summary of the current understanding:

• Would be useful to have the ability to set/update several optics at once, and still have the target object constructed only once (not N times = number of optics).
• This is challenging to implement generically, aside from the most basic cases already covered by setproperties. See also Kaleido.jl for earlier attempts to do that based on Setfields.jl.

More practically, for now:

• If you want an optic to refer to several parts of the object, use optic ++ optic ++ ... or @optics ... from AccessorsExtra.jl. Setting it (with setall()) reconstructs the object multiple times, but should be free/cheap for small structs. This is used in packages like PlutoTables and AccessibleOptimization, can be upstreamed into Accessors proper within a reasonable time frame.
• Closer to the original topic of this issue, "Set multiple fields at once in some sort of pipe" – there are already convenient piping solutions orthogonal to Accessors
  ([Feature request] Set multiple fields at once #119 (comment)):

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

[tpapp]

I am wondering what the recommended workaround is when the constructor of a struct insists that two values should be of the same type, so a pipe does not work. MWE:

using Accessors

struct Foo{T}
    a::T
    b::T
end

function bump(foo::Foo, a::S, b::S) where S
    @reset foo.a = a
    @reset foo.b = b
end

foo = Foo(1, 2)
bump(foo, 1.0, 2.0)             # errors

(Assume that the user does not want to define a promoting constructor)

[jw3126]

I just use setproperties for this.

[cstjean]

Something like this should work:

using Accessors

struct Foo{T}
    a::T
    b::T
end

ab(::Foo) = (a, b)
Accessors.set(f::Foo, ::typeof(ab), (new_a, new_b)) = Foo(new_a, new_b)

function bump(foo::Foo, a::S, b::S) where S
    @set ab(foo) = (a,b)
end

(details are off the top of my head, so probably not correct)