Use cases for type lower bounds

[Tamas_Papp]

One can specify lower bounds for UnionAll types in v0.7 with >:.

I was trying to think about examples where I would use this feature, but could not come up with any. Looking at the sources in Base, I found this one, the rest of the applications look like compiler internals and tests.

If someone could explain some patterns that one would use >: for, or link some examples, I would be interested. My motivation is just trying to place a new language feature in my toolbox.

[Tamas_Papp]

One use case was mentioned in

X::Vector{Union{Missing, Any}}

Indeed, since (as its name indicates) Any can be any value, including missing. In general, use ::AbstractArray{>:Missing} to define a method which should be called for any array which can contain missing. But note that in many cases you can just define a single generic method, knowing that the compiler will (often) optimize out ismissing(A[i]) calls when the array cannot contain missing.

[nalimilan]

I guess AbstractArray{>:T} would also be useful in all cases where a function stores a value of type T inside an array provided as an argument. I’m not sure how common this is in practice, though.