@traitfn on callable types

[MilesCranmer]

There is a workaround for this but in case you'd like to support it, the following syntax does not work. My use-case is to build a generic constructor for any type D satisfying some trait:

@traitfn (::Type{D})(args...) where {D; IsMyTrait{D}} = f(args...)

This will raise the following error:

ERROR: syntax: invalid "::" syntax around /Users/mcranmer/.julia/packages/SimpleTraits/WyNKl/src/SimpleTraits.jl:338
Stacktrace:
 [1] top-level scope
   @ REPL[27]:1

The workaround is to pass a symbol:

@traitfn (d::Type{D})(args...) where {D; IsMyTrait{D}} = f(args...)

which works.

[MilesCranmer]

As in #87, maybe I should just avoid doing stuff like this, since it will invalidate functions in Base.

[mauro3]

Yes, this is also a bit iffy. Again, similar reasoning to #87 should mean that it should work in principle (i.e. don't forget to define the Not version, preferably before the not-Not version). But again, I may not have thought this through.

Regarding the syntax error: it would be ok to fix this as it could be use for more benign applications too. PR welcome.

[MilesCranmer]

Yeah it seems impossible with current Julia. I can't see any way around the invalidations from stuff like this. I was trying to think if there is some way to write a AbstractTraitWrapper and have it wrap other abstract types, so that users can do, e.g.,

struct MyFloat <: AbstractTraitWrapper{AbstractFloat}
    x::Float64
end

and somehow have this struct inherit all methods on AbstractFloat (so that users can still use their abstract types while also using traits), as well as enabling, e.g.,

Base.:+(x::T, y::T) where {T<:AbstractTraitWrapper; IsMyFloat{T}} = T(x.x + y.x)

so that it wouldn't invalidate other Base methods. But unless there's a way to modify multiple dispatch itself (throughout Julia) to check the parametric type of AbstractTraitWrapper, it doesn't seem possible...

[mauro3]

I'm not following here:

But unless there's a way to modify multiple dispatch itself (throughout Julia) to check the parametric type of AbstractTraitWrapper, it doesn't seem possible...

You don't refer to above example with that, right?

[MilesCranmer]

Sorry I was thinking of the example in #87, not the one in here.