becoming a weakdep of Graphs

[Krastanov]

It would be nice if this functionality is more easily available from Graphs. Maybe something like:

julia> using Graphs

julia> canonize!(randomgraph::SimpleGraph)
MethodError:...
Hint: canonize algorithms are implemented in dependencies like NautyGraphs.jl; Make sure you first import NautyGraphs

julia> using NautyGraphs # which loads extension NautyGraphsExt

julia> canonize!(randomgraph::SimpleGraph) # with default implementation dispatching to `canonize!(randomgraphs, ::AlgNautyGraph)`

I will not immediately have a chance to work on this, but wanted to drop it here in case it can spur discussion or other volunteers.

I am also trying to package the very_nauty package as well canonize!

[mxhbl]

Hi, I think this is a great idea. I'm not sure what specifically I can do to help, but if you can point me in the right direction I'd be happy to work on this.

[Krastanov]

My preferred way (and a relatively standard way) would be to:

1. declare a function without definition in Graphs so that there is a central place for documenting it, e.g. function canonize! end
2. import it in NautyGraphs and create a NautyGraphs specific method canonize!(::NautyGraph)
3. create a package extension in Graphs (a weak-dependency on NautyGraphs, the extension will be loaded only if NautyGraphs is loaded) that has function canonize!(::Graph, ::AlgNautyGraphs) where AlgNautyGraphs is just a placeholder struct that is used to specify an algorithm choice. Internally that function will use canonize!(::NautyGraph) and a conversion
4. add an error hint so that if someone has imported Graphs without having NautyGraphs, the method error for the undefined canonize! contains tips on how to proceed.

Here is an example of 4: https://github.com/QuantumSavory/QuantumSavory.jl/pull/170/files#diff-3015c58a9942fc79861dadb84fd7a7e9110fb7ff82e4395ca2b1fa3b186e6e8dR79-R83 (QuantumSavory is the main package, and has a plotting extension that depends on Makie)

Here are more details on package extensions: https://pkgdocs.julialang.org/v1/creating-packages/#Conditional-loading-of-code-in-packages-(Extensions)

In a few months I might be able to get back to this to implement it myself, but in the meantime I am happy to help with reviews and tips if you feel like tackling it

[mxhbl]

Sounds great. I'll try to get a PR together in the next week or two.

[mxhbl]

See JuliaGraphs/Graphs.jl#418 for continued discussion.