setindex! returns the wrong value

[KristofferC]

julia> a = rand(MVector{3});

julia> (c = setindex!(a, 1, 1)); c
1

julia> (c = setindex!(collect(a), 1, 1)); c
3-element Array{Float64,1}:
 1.0
 0.30794928630799046
 0.6842053878737089

[c42f]

Is there an official API for the return value of setindex!? I couldn't find it at https://docs.julialang.org/en/v1/manual/interfaces/#man-interface-array, and indeed the examples there seem to return the newly set value rather than the array. setfield! also returns the field value rather than the mutated object.

On the other hand, Base.Array behaves as you say and seems to be consistent about it. So I'm very happy to change this, though also confused about whether there's a "correct" option.

[tkf]

There was a similar discussion in JuliaLang/julia#31891

It looks like the decision was that setindex! can return anything since the return value of (x[i] = v) is determined by lowering.

[tkf]

(Though I think it would have been great if setindex!(A, ...) === A was the part of the API.)

[KristofferC]

Is there an official API for the return value of setindex!?

The convention is to return the modified object.

Some more examples:

julia> a = rand(1,1);

julia> (c = setindex!(a, 1, 1); c)
1×1 Array{Float64,2}:
 1.0

julia> (c = setindex!(sparse(a), 1, 1); c)
1×1 SparseMatrixCSC{Float64,Int64} with 1 stored entry:
  [1, 1]  =  1.0

julia> (c = setindex!(view(a), 1, 1); c)
0-dimensional view(::Array{Float64,0}) with eltype Float64:
1.0

julia> (c = setindex!(MMatrix{1,1}(a), 1, 1); c)
1

[c42f]

Ok, let's follow what Array does. I made #832.