Inconsistent eltype between map and broadcast for empty collections

[nalimilan]

On Julia 0.5.0:

julia> h3(x::Float64...) = prod(x)
h3 (generic function with 1 method)

julia> map(h3, Int[])
0-element Array{Any,1}

julia> broadcast(h3, Int[])
0-element Array{Any,1}

On current master:

julia> h3(x::Float64...) = prod(x)
h3 (generic function with 1 method)

julia> map(h3, Int[])
0-element Array{Any,1}

julia> broadcast(h3, Int[])
0-element Array{Union{},1}

I guess returning Union{} is not intended here? Anyway, whatever element type we choose, I think we should be consistent. Cf. old discussion at #7258.

[nalimilan]

I've opened a PR to make broadcast consistent with map: #20049.

Another inconsistency not covered by the PR:

julia> f(x::Float64) = 1
f (generic function with 1 method)

julia> map(f, Any[])
0-element Array{Any,1}

julia> broadcast(f, Any[])
0-element Array{Int64,1}

[martinholters]

Also this:

julia> map((x) -> error(x), 1:0)
0-element Array{Union{},1}

julia> map((x) -> error(x), [])
0-element Array{Any,1}

Looks a bit like mapping over an empty array is the exception to the rule.

[JeffBezanson]

Also this:

julia> map((x::String)->error(), Int[])
0-element Array{Any,1}

julia> map(x->error(), Int[])
0-element Array{Union{},1}

It looks like return_type works differently when no method matches, vs. when a method matches and that method always throws.

[martinholters]

It's not directly return_type; that actually works fine:

julia> Core.Inference.return_type((x) -> error(), Tuple{Int})
Union{}

julia> Core.Inference.return_type((x::String) -> error(), Tuple{Int})
Union{}

But the output element type is not determined with return_type(f, Tuple{eltype(A)}), it goes through some indirections, ending in something like return_type(first, Tuple{typeof(Generator(f,A))}). I think the underlying problem boils down to

julia> foo(f, x) = f(x)
foo (generic function with 1 method)

julia> @code_warntype foo(x->error(), 1)
Variables:
  #self#::#foo
  f::##39#40
  x::Int64

Body:
  begin 
      return (Base.throw)((Base.ErrorException)(((Core.getfield)((Core.getfield)(Base.Main, :Base)::Any, :string)::Any)()::Any)::ErrorException)::Union{}
  end::Union{}

julia> @code_warntype foo((x::String)->error(), 1)
Variables:
  #self#::#foo
  f::##41#42
  x::Int64

Body:
  begin 
      return (f::##41#42)(x::Int64)::Any
  end::Any

which is due to this. Interesting, but probably beyond the scope of this issue.

[nalimilan]

@vtjnash Is there any chance this line could be changed, or should we find a workaround?

[nalimilan]

Now consistent on current master:

# Example 1
julia>  h3(x::Float64...) = prod(x)
h3 (generic function with 1 method)

julia> map(h3, Int[])
0-element Array{Union{},1}

julia> broadcast(h3, Int[])
0-element Array{Union{},1}

# Example 2
julia> f(x::Float64) = 1
f (generic function with 1 method)

julia> map(f, Any[])
0-element Array{Int64,1}

julia> broadcast(f, Any[])
0-element Array{Int64,1}

# Example 3
julia> map((x) -> error(x), 1:0)
0-element Array{Union{},1}

julia> map((x) -> error(x), [])
0-element Array{Union{},1}

# Example 4
julia> map((x::String)->error(), Int[])
0-element Array{Union{},1}

julia> map(x->error(), Int[])
0-element Array{Union{},1}