Add a method for length(x::Base.SkipMissing)

[bkamins]

This currently fails:

julia> x = [1, 2, 3]
3-element Vector{Int64}:
 1
 2
 3

julia> (length∘skipmissing)(x)
ERROR: MethodError: no method matching length(::Base.SkipMissing{Vector{Int64}})

I propose to add an appropriate method even if this is an O(n) operation since it is quite commonly needed.

CC @nalimilan

[timholy]

Can't this be count(!ismissing, x)

[bkamins]

No, because only x -> count(!ismissing, x) is something you can dispatch on and it is an anonymous function that is dynamically typed as opposed to length∘skipmissing which has a known type.
If we had an option to curry count(!ismissing) then indeed length∘skipmissing would not be needed.

That is why I have written (length∘skipmissing)(x) not length(skipmissing(x)) as it highlights what is required.

[Moelf]

is the idea to add:

Base.length(x::Base.SkipMissing) = count(!ismissing, x.x)

?

If not, since we don't have function-type, my feeling is against a manual sprinkling of specialization here and there for a bunch of different X∘Y∘Z.

[bkamins]

This is the idea. I want to add Base.length(x::Base.SkipMissing) definition although it is O(n).

[Moelf]

I would suggest title to be changed to

Add a method for length(x::Base.SkipMissing)

since you can technically add a method for length∘skipmissing without the above

[bkamins]

Ah - good point since:

julia> typeof(length∘skipmissing)
ComposedFunction{typeof(length), typeof(skipmissing)}

is callable so the title was wrong.

[bkamins]

Actually sometimes one gets y = skipmissing(x) as input, and then there is a question how to count number of elements of y, would count(v -> true, y) be the recommended pattern?