Eltypes of `view`s

Let’s say I have a vector t = [1, 2, missing]. And I take sub_t = view(t, 1:2). I know that no elements sub_t contain missing values.

Is there any way to make the eltype of sub_t be Int instead of Union{Int, Missing}?

since compiler can’t know, maybe you are looking for a way to remove Missing from the Union type returned by eltype?

Yeah. It’s definitely lying about the internal memory of the object, since the underlying vector still has room for the Missing mask.

I’m thinking of scenarios where functions have overly-restrictive type parameters

foo(x::AbstractVector{Int})
t = [1, 2, missing]
sub_t = view(t, 1:2)
foo(sub_t)

I’m not sure if just overloading the eltype is enough, right? It has to actually be in the type parameter.

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the thing though is that, as I imagine you know:

t = [1,2,missing]
sub_t = view(t, 1:2)  # does not contain missing
t[1] = missing
sub_t[1] === missing

we had a similar issue with ScientificTypes but for slices (so not views), where if you take t[1:2] it would not be of eltype Int. For this you can apply identity.(v) to create a “tight” copy of it something like

t = [1,2,missing]
w = t[1:2]  # has missing type still
z = (eltype(w) >: Missing && findfirst(ismissing, w) === nothing) ?
       identity.(w) : w

not a view though…

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Since a view is quite literally a window into another objects’ memory and the original object can’t loose the ability to hold mixed types for arbitrary indices (it’s a property of the whole object after all), you won’t get rid of this without copying. Setting an index of a view means setting another index in the original object - the view is only there as a thin passthrough and index calculation.

If it were a property of each position however, it would be possible.

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Yeah. That’s true.

If I were to implement my own type for this it would maybe disallow inputting missing values.

But it sounds like re-implementing SubArray or something similar would be the way to go for this, which is probably more work than I would want to go for at the moment.

You could also check every covered index for missing and only allow construction if truly all elements are non-missing. That would allow type stability, but doesn’t change the fact that the underlying array still has the Union.

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Yeah that’s exactly what I’m doing, so I’m 100% sure there aren’t any missing values in the Array. This is like what skipmissing does here.

IteratorEltype(::Type{SkipMissing{T}}) where {T} = IteratorEltype(T)
eltype(::Type{SkipMissing{T}}) where {T} = nonmissingtype(eltype(T))

This works, but the view is lost?

foo(x::AbstractVector{Int}) = dot(x,x)^3   # some example
t = [1, 2, missing]
sub_t = view(t, 1:2)
foo(sub_t)          # Error
foo(Int.(sub_t))    # OK = 125

No. That creates a copy. The point of using a view is that I don’t have to make a copy of the data.

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If you really wanted, you could probably get access to the data array via a pointer. Then you’d just take a view of the data array, as you don’t really care what values live in the masked segments. There isn’t a blessed way to do this (Feature request: convert between Array{Union{T, Missing}, N} and Array{T, N} without copying · Issue #26681 · JuliaLang/julia · GitHub).

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