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Docs of construction undef arrays with missing #31091

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Oct 30, 2020
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Docs of construction undef arrays with missing #31091

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Docs of construction undef arrays with missing
bkamins Feb 16, 2019
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change the description
bkamins Feb 17, 2019
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Update doc/src/manual/missing.md
bkamins Oct 24, 2020
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Update doc/src/manual/missing.md
bkamins Oct 25, 2020
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8 changes: 8 additions & 0 deletions doc/src/manual/missing.md
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Expand Up @@ -254,6 +254,14 @@ julia> Array{Union{Missing, String}}(missing, 2, 3)
missing missing missing
```

!!! note
Currently for `T` that is bits type (`isbitstype(T)` returns `true`),
`Array{Union{Missing, T}}(undef, dims)` creates an array filled with
missing values. Also calling `similar` to create an unitialized array that
has element type of the form `Union{Missing, T}` creates an array filled with
`missing` if `T` is bits type. This behavior is an implementation detail
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"Implementation detail that may change in the future" might be a bit strong, as it makes it sound like the public API cannot be relied on. Maybe just recommend using Array{Union{Missing, T}}(missing, dims), noting that it works for all types? Maybe also remove the leading "currently".

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I know it is strong, but from the discussion I understood that we want to be discouraging. The question is if in e.g. Julia 1.7 we will guarantee this behavior (I know we rely on it internally, and the probability that this will change is minimal, but theoretically it is possible). In general - I understand that this is only the current behavior but it is not a part of a public API.

Actually, I have written this PR is exactly to clarify: do we want to make it a public API or just describe the current behavior and warn users that this is not a part of public API?

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AFAIK, that's part of the public API (anyway, any behavior that is stable like this will de facto end up as being part of the API since lots of code will rely on it). We can mark this for triage to make sure that's the case.

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OK - can you please mark it?

any behavior that is stable like this will de facto end up as being part of the API since lots of code will rely on it

Yes- but I would prefer an explicit decision 😄.

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@JeffBezanson JeffBezanson Mar 7, 2019 •
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If you have Union{SingletonType, NonSingletonType} you are guaranteed to get the singleton type (e.g. Missing), and that won't change. For other combinations it's less predictable.

that may change in the future and should not be relied upon.

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An array allowing for `missing` values but which does not contain any such value
can be converted back to an array which does not allow for missing values using
[`convert`](@ref). If the array contains `missing` values, a `MethodError` is thrown
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