# Mutating global variable during precompilation

**URL:** <https://discourse.julialang.org/t/mutating-global-variable-during-precompilation/51478>\
**Category:** New to Julia\
**Created:** [December 8, 2020, 7:38pm UTC](https://discourse.julialang.org/t/mutating-global-variable-during-precompilation/51478 "2020-12-08T19:38:12Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Macfli](https://avatars.discourse-cdn.com/v4/letter/m/87869e/32.png) [@Macfli](https://discourse.julialang.org/u/Macfli)\
**Post date:** [December 8, 2020, 7:38pm UTC](https://discourse.julialang.org/t/mutating-global-variable-during-precompilation/51478/1 "2020-12-08T19:38:12Z")

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Hi, I am trying to register items in a top-level global variable from `include()`d files and modules, and am seeing behavior I am not able to explain. Here is a minimal code:

File `main.jl`:

```julia
module MyPkg

items = []

function register(item)
    push!(items, item)
end

include("sub.jl")
println(items)

end

```

File `sub.jl`:

```julia
module Sub

using MyPkg: register

register("foo")

end

```

When running my script with `julia main.jl` I get the output:

```julia
Any["foo"]
Any[]

```

In my understanding, the first output is made during precompilation, and the second at runtime. Unfortunately the registered items are lost at runtime. What is going on exactly?

To be more concrete about my use case, I am trying to build a CLI with sub-commands registering themselves with the top-level command. The list of registered commands is fixed before precompilation and does not need to change at runtime. I would like to avoid having the main module gather all the commands itself, and rather having a nice and simple interface to delegate to sub-modules and let them register their commands themselves. How could I achieve this?

Thanks !

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<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [December 8, 2020, 9:12pm UTC](https://discourse.julialang.org/t/mutating-global-variable-during-precompilation/51478/2 "2020-12-08T21:12:38Z")

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Actually, running this without any modification to `LOAD_PATH` or the current package project results in:

```julia
ERROR: LoadError: LoadError: ArgumentError: Package MyPkg not found in current path:
- Run `import Pkg; Pkg.add("MyPkg")` to install the MyPkg package.

```

because the `using MyPkg: register` tries to load some global package with that name, but none exists. The right way to refer to unambiguously refer to a parent module like this is with the `..MyPkg` syntax, e.g.:

```julia
module Sub

using ..MyPkg: register

register("foo")

end

```

Making that change and running `julia main.jl` prints:

```julia
$ julia main.jl
Any["foo"]

```

By the way, you should change `items = []` to `const items = []`. The `const` label just means you will not change the binding `items` to some other value or type, but it does not stop you from _mutating_ the underlying vector. The `const` marking will just improve performance of any code using that variable (per [Performance Tips · The Julia Language](https://docs.julialang.org/en/v1/manual/performance-tips/#Avoid-global-variables) ).

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<div class="post-metadata">

**Author:** ![Macfli](https://avatars.discourse-cdn.com/v4/letter/m/87869e/32.png) [@Macfli](https://discourse.julialang.org/u/Macfli)\
**Post date:** [December 9, 2020, 7:48am UTC](https://discourse.julialang.org/t/mutating-global-variable-during-precompilation/51478/3 "2020-12-09T07:48:51Z")

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Thanks @rdeits! I indeed observe the same behavior as you when making the change to `using ..MyPkg`.

I am actually using a package project, running my program with `julia --project src/main.jl`. Trying different things, I observe two different behaviors:

- With `using MyPkg: register` (and a package project), I get the original two-lines output, with an empty array the second time
- With `using ..MyPkg: register` (with or without a package project), I get the same output as yours

Can you tell me what the difference is and what happens behind the scene?

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<div class="post-metadata">

**Author:** ![pixel27](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pixel27/32/8902_2.png) [@pixel27](https://discourse.julialang.org/u/pixel27)\
**Post date:** [December 10, 2020, 7:13pm UTC](https://discourse.julialang.org/t/mutating-global-variable-during-precompilation/51478/4 "2020-12-10T19:13:15Z")

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My understanding is that `using MyPkg:register` will actually use MyPkg from the **active** Project.toml file from the `deps` section. While `using ..MyPkg` will use the MyPkg defined in the current running “environment” i.e. from a Julia file loaded via the `include` or the main Julia file.

So the MyPkg in these two situations might not be the same module. You should be able to test this by doing `]` followed by `rm MyPkg` that will remove the package from the Project.toml dependency list then `using MyPkg` should result in a package not found.