How to check if a method is defined for an instance? · helpdesk (published)

Stream: helpdesk (published)

Topic: How to check if a method is defined for an instance?

Júlio Hoffimann (Feb 17 2021 at 13:12):

Say I have an instance obj how do I check if there is a method size(obj) defined?

Júlio Hoffimann (Feb 17 2021 at 13:13):

I would like to write a function that behaves differently depending on this availability of size. I know this is a trait, but I wonder if there is a simple way of checking this without relying on packages.

Felix Cremer (Feb 17 2021 at 13:22):

I think, you can use hasmethod like this:

julia> hasmethod(size, (typeof(obj),))

Felix Cremer (Feb 17 2021 at 13:22):

But I am not sure, whether this is the best way to do that.

Andrey Oskin (Feb 17 2021 at 14:00):

Sorry, just to avoid XY problem. How is it possible that method size for object is not defined? I mean, it's you as a developer who is writing code and you know exactly whether you wrote this method or not. Why determine it in runtime?

Júlio Hoffimann (Feb 17 2021 at 14:35):

I would like to check if the user provided type has support for size, and if it has I can specialize the implementation. This is pretty much a trait but since Julia doesn't support traits as a built-in I think the hasmethod is enough in this case.

Fredrik Bagge Carlson (Feb 17 2021 at 19:37):

Mason Protter (Feb 17 2021 at 19:48):

Note that hasmethod is slow for basically ideological reasons. If you want it to be fast, Tricks.jl has static_hasmethod.

Mason Protter (Feb 17 2021 at 19:49):

The readme even has an example of using it to define traits: https://github.com/oxinabox/Tricks.jl#uses

Júlio Hoffimann (Feb 17 2021 at 22:24):

Thank you all for the tips. Very welcome.

Last updated: Oct 02 2023 at 04:34 UTC