One issue I have with Julia’s type parameters, related to this, is that some of them are part of the public API and some of them are private. For above example A might be public, but B isn’t. Another one is:
julia> typeof(1.0:0.1:0.9)
StepRangeLen{Float64,Base.TwicePrecision{Float64},Base.TwicePrecision{Float64}}
Where the first parameter is public (you would potentially use it for dispatch) but the second two are internal ones (I think calculated from the first somehow but I’m not sure).
It would be nice if it was possible to distinguish between the two somehow and hide the private parameters. In particular when declaring structs with fields for, e.g. float-ranges, one has to handle those private parameters:
struct MyT{F,_F1,_F2}
fr:StepRangeLen{F, Base.TwicePrecision{_F1},Base.TwicePrecision{_F2}}
end
even though one doesn’t care at all about them.
x-ref: https://github.com/JuliaLang/julia/issues/18466#issuecomment-274442844