# JLD and package extensions

**URL:** <https://discourse.julialang.org/t/jld-and-package-extensions/124656>\
**Category:** General Usage\
**Tags:** jld, package-extensions\
**Created:** [January 10, 2025, 8:12pm UTC](https://discourse.julialang.org/t/jld-and-package-extensions/124656 "2025-01-10T20:12:18Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![GHTaarn](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ghtaarn/32/216007_2.png) [@GHTaarn](https://discourse.julialang.org/u/GHTaarn)\
**Post date:** [January 10, 2025, 8:12pm UTC](https://discourse.julialang.org/t/jld-and-package-extensions/124656/1 "2025-01-10T20:12:18Z")

</div>

I would like to make a package extension that loads if `JLD.jl` is loaded. I got the code below to work in the REPL, but when I make `Abc` the main package and `JLDExt` the extension (and remove the dots in front of the package names) I get the warning “type JLDExt.JLDAb not present in workspace; reconstructing” and the deserialization fails.

```julia
module Abc

export Ab

struct Ab
    a
end

end # module Abc

#----------------------

module JLDExt

using ..Abc, JLD
import JLD.writeas
import JLD.readas

export JLDAb

struct JLDAb
    a
end

JLD.writeas(x::Ab) = writeas(JLDAb(x.a))
JLD.readas(x::JLDAb) = Ab(x.a)

end

#--------------------

using Test, .Abc, .JLDExt, JLD

@testset "JLD" begin
    x = Ab(1)
    fname = "/tmp/JLD_test.jld"
    save(fname, "x", x)
    @test load(fname)["x"] == x
end

```

The precise code with all the files involved is at [GitHub - GHTaarn/Abc.jl: For a post on https://discourse.julialang.org](https://github.com/GHTaarn/Abc.jl/)

---

<div class="post-metadata">

**Author:** ![GHTaarn](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ghtaarn/32/216007_2.png) [@GHTaarn](https://discourse.julialang.org/u/GHTaarn)\
**Post date:** [January 11, 2025, 12:09pm UTC](https://discourse.julialang.org/t/jld-and-package-extensions/124656/2 "2025-01-11T12:09:00Z")

</div>

I worked on this some more and it appears that new symbols defined in extensions are not normally visible from outside the extension. I have gotten things to work with an ugly solution at [GitHub - GHTaarn/Abc.jl at fix\_alpha](https://github.com/GHTaarn/Abc.jl/tree/fix_alpha) that has the following Julia files:

```julia
module Abc

export Ab, JLDExt

struct Ab
    a
end

function set_jldext()
    global JLDExt
    pause = 0.01
    for t in 0:pause:2
        JLDExt = Base.get_extension(@ __MODULE__ , :JLDExt)
        !isnothing(JLDExt) && break
        sleep(pause)
    end
    @info "JLDExt set to '$JLDExt'"
end

end # module Abc

```

```julia
module JLDExt

using Abc, JLD
import JLD.writeas
import JLD.readas

struct JLDAb
    a
end

JLD.writeas(x::Ab) = writeas(JLDAb(x.a))
JLD.readas(x::JLDAb) = Ab(x.a)

function __init__ ()
    @async Abc.set_jldext()
    @info "Loaded JLDExt"
end

end

```

```julia
using Test, Abc, JLD

@testset "JLD" begin
    sleep(0.1)
    x = Ab(1)
    fname = "/tmp/JLD_test.jld"
    save(fname, "x", x)
    @test load(fname)["x"] == x
end

```

Any suggestions on how to avoid the `sleep`s would be appreciated. Is there a callback for when a module is loaded or something that `wait` can be called on that triggers when a module is loaded?

---

<div class="post-metadata">

**Author:** ![GHTaarn](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ghtaarn/32/216007_2.png) [@GHTaarn](https://discourse.julialang.org/u/GHTaarn)\
**Post date:** [January 12, 2025, 8:05am UTC](https://discourse.julialang.org/t/jld-and-package-extensions/124656/3 "2025-01-12T08:05:46Z")

</div>

After a good nights sleep, I discovered that the solution is really simple:

```julia
module Abc

export Ab, JLDExt

struct Ab
    a
end

function set_jldext(m)
    global JLDExt = m
    @info "JLDExt set to '$JLDExt'"
end

end # module Abc

```

```julia
module JLDExt

using Abc, JLD
import JLD.writeas
import JLD.readas

struct JLDAb
    a
end

JLD.writeas(x::Ab) = writeas(JLDAb(x.a))
JLD.readas(x::JLDAb) = Ab(x.a)

function __init__ ()
    Abc.set_jldext(JLDExt)
    @info "Loaded JLDExt"
end

end

```

```julia
using Test, Abc, JLD

@testset "JLD" begin
    x = Ab(1)
    fname = "/tmp/JLD_test.jld"
    save(fname, "x", x)
    @test load(fname)["x"] == x
end

```

Full code available at [GitHub - GHTaarn/Abc.jl at good\_fix](https://github.com/GHTaarn/Abc.jl/tree/good_fix)