Package extensions for Julia < 1.9

General Usage
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Wow, that’s a really helpful guide — the merge commit diff is the most readable there: Merge pull request #856 from SciML/weakdeps · SciML/DiffEqBase.jl@c2de920 · GitHub

My quick takeaways:

  • Use both Requires (pre-1.9) and Extensions, using isdefined(Base, :get_extension) to determine which system to use
  • Put the conditional dependencies in [weakdeps] and [extras] and [test] sections
  • Extensions go in ext/ExtensionExt.jl, with a structure like:
    module UnitfulExt

import DiffEqBase
isdefined(Base, :get_extension) ? (import Unitful) : (import ..Unitful)
# ... extensions here
end
    (note that Requires plops the dependencies into the parent module, so you use import ..Dependency, whereas the builtin extensions support just makes it happen)
  • Requires support for older versions has this __init__ for the parent package:
    function __init__()
    @static if !isdefined(Base, :get_extension)
        @require Unitful="1986cc42-f94f-5a68-af5c-568840ba703d" begin
             include("../ext/UnitfulExt.jl")
         end
    end
end
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