Stream: helpdesk (published)

Topic: Should Structs have functions?

Davi Sales Barreira (Jan 10 2022 at 13:10):

I know that in Julia the multiple-dispatch makes it (almost?) obsolete the creation of structs with functions. But I was wondering if for my case it would be useful. Consider that I have a plotting package that receives a m::Marker and draws it. Now, I wish that users of the package could implement their own markers, which would be drawn according to a function drawmarker(m::Marker). The thing is, for the set of markers I've implemented, I have the multiple-dispatch setup, i.e.

drawmarker(m::Circle)
drawmarker(m::Bar)

But since I wish a user can create his own marker, I was wondering if it would be better for the marker struct to have a drawmarker as a method. Any thoughts? Should I just make the user pass a drawmarker function as an argument?

Andrey Oskin (Jan 10 2022 at 14:11):

To clarify, what is wrong with user-defined drawmarker(m::CustomMarker)?

Davi Sales Barreira (Jan 10 2022 at 15:34):

Nothing really wrong about. Mostly about design. I mean, I'd have something like:

plot(m::Marker)

which my package provides. Now, the plot function calls drawmarker inside of it. But requiring the user to write a drawmarker function, it would also require that he passed his function to the plot function, and I'd need to check whether or not he passed the function to run it instead of the already defined inside the package. What I mean, is, if the drawmarker was a method from the marker, then I could keep the plot(m::Marker). Otherwise, I need something like:

function plot(m::Marker; drawfunction::Function = nothing)
  if drawfunction === nothing
     drawmarker(m)
 else
    drawfunction(m)
 end

end

Davi Sales Barreira (Jan 10 2022 at 15:37):

So, I'm not saying that it cannot be done. Just that It looks like an uglier solution than just defining the drawmarker as a method of the marker. Hence, I was wondering if it was still advisable not to create the method for the marker.

Felix Kastner (Jan 10 2022 at 15:55):

I think, what Andrey meant is for the user to write something like

import Package: drawmarker
struct MyCustomMarker end

function drawmarker(m::MyCustomMarker)
    # specify how to draw it
end

plot(m::MyCustomMarker)

Davi Sales Barreira (Jan 10 2022 at 20:02):

Hmm, this would work out of the box? I thought since the function with dispatch for the custom marker is not in my package, it would not work. If it indeed works, then all my problems are resolved :)

James Fairbanks (Jan 10 2022 at 22:04):

The import Package: drawmarker line is what makes it work

Kyle Daruwalla (Jan 11 2022 at 02:39):

See here too: https://docs.julialang.org/en/v1/manual/modules/#using-and-import-with-specific-identifiers,-and-adding-methods

Brenhin Keller (Jan 11 2022 at 04:27):

Yeah, this is such a beautiful thing about Julia’s multiple dispatch

Last updated: Oct 02 2023 at 04:34 UTC