A precise definition of what it means to be an instance of an abstract type

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I’m not sure what you mean by “value vy of y” in your definition. If it’s saying that y is an abstract type that implements vy in the way that Int64 is a concrete type that implements 1, then that’s impossible because by definition, only concrete types implement. It looks fine if that detail is omitted: in more familiar terms, x isa y if and only if typeof(x) <: y.

Couple things:

  • isconcretetype(aType) can be used to distinguish a concrete type and an abstract type. Ptr is definitely abstract.
  • Ptr{Float64} is a subtype, not an instance, of Ptr.
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