Creating the instance of a function type

[raminammour]

I will link Jameson’s important comment from that thread, and emphasize below:

@jameson

Most of the “solutions” presented here are wrong: you should never need to access “.instance”. This is an internal API and not stable. The correct way to do this is to store the actual function object itself, and always refer to it by value and don’t try to do this in the type-domain. Adopting the original example, we get the much-shortened:

julia> struct MyStruct{F}
           f::F
       end

julia> ExtractFun(s::MyStruct) = s.f

What is wrong with the solution he presented, where you store the function in the struct and access it by value as above, why does it need to be done in the type domain?