ExtensionsExt.jl

A package that defines some commonly used boilerplate using in extensions.

There are different usecases that appeared during the transition to julia 1.9 extensions. We are going to use an hypothetical package, Things.jl, to showcase how we can use ExtensionsExt.jl to address those usecases.

Case 0: only extending new methods, using Requires.jl

This is the mayority of the cases. excluding has_extension and the @require macros, there is not much else to do, other than following the julia documentation. ExtensionsExt.jl helps on reducing the boilerplate that is generated by repeated uses of if isdefined(Base,:get_extension).

In our particular example, Things.jl loaded Unitful.jl via Requires.jl to extend HotThing

• In the Things module:

struct HotThing
    val::Float64
end

if !has_extensions() #if isdefined(Base,:get_extension)
    function __init__()
        @require Unitful="1986cc42-f94f-5a68-af5c-568840ba703d" include("../ext/ThingsUnitfulExt.jl")
    end
end

• In the ThingsUnitfulExt module:

module ThingsUnitfulExt

using ExtensionsExt
@require_using Unitful: ustrip, uprefered,Temperature #if isdefined(Base,:get_extension) ...
@require_import Things: HotThing

HotThing(val::Temperature) = HotThing(ustrip(upreferred(val)))
end #module

Case 1: new extension, with an exported function

Things.jl added a new conditional dependency, Plots.jl into the package, it wants to export a function that depends on Plots.jl being loaded, with ExtensionsExt, you can use this:

• In the Things module:

const PLOT_EXT_ERR = "thingplot requires that Plots is loaded, i.e. `using Plots`"
function thingplot(x::Thing)
    ext = try_extension(Things,:ThingsPlotsExt,err_msg = PLOT_EXT_ERR)
    ext.__thingplot(x)
end

Case 2: dependency to extension, with an exported struct

Things.jl moved Tables.jl from inconditional dependency to Extension. A struct, ThingTable, was exported. On 1.8 and before, Things.jl loaded the methods for ThingTable inconditionally. Now, Tables.jl needs to be loaded before constructing a ThingTable. We want to error on the construction of ThingTable if Tables.jl is not loaded in 1.9, but don't do anything on earlier versions.

• In the Things module:

struct ThingTable
    x::Vector{Thing}
    function ThingTable(x::Vector{Thing})
        #if force == true, assert_extension will fail on julia 1.8 and before.
        assert_extension(Things,:ThingsTablesExt, force = false)
        return new(x)
    end
end

#at the end of the module, we suppose that the ThingsTablesExt module loads Tables.jl
if !has_extensions() && include("../ext/ThingsTablesExt.jl")
export ThingTable