How can I express the abstract type of a multi-parameter type that only matches the second parameter

General Usage
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1

If I have a type VS_Data{DIM,NDF} and a instance a::VS_Data{3,2}. Then we have isa(a,VS_Data{3})=true but isa(a,VS_Data{2})=false, which implies that VS_Data{DIM,NDF}<:VS_Data{DIM}. My question is how can I express the abstract type that only matches the parameter NDF?

2

As you discovered, supplying type parameters generally works from left to right.

To work around that, the general syntax applied to your example is

VS_Data{DIM,2} where DIM
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3

That’s what I want. Thanks a lot!

5

I wonder if it would be possible to support “currying” of underscores in situations like this:

VS_Data{_, 2}

There has been a long discussion on supporting this for function arguments (e.g. f(_, x)), and this would be a nice parallel to that. But perhaps the same problems show up here, like where the scope of _ should end?

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6

Would it be possible to write a MWE to make it less abstract?

7

Although this is not really more ergonomic in this simple example, for more complex cases it can be convenient to define an alias with type parameters swapped (or generally, re-ordered) like:

const VS_DATA_swapped{NDF,DIM} = VS_Data{DIM,NDF} where {DIM,NDF}

Edit: for actual usecases const is probably a good idea.