Project Preferences Package
Author JuliaPackaging
33 Stars
Updated Last
4 Months Ago
Started In
September 2020


Continuous Integration Code Coverage

The Preferences package provides a convenient, integrated way for packages to store configuration switches to persistent TOML files, and use those pieces of information at both run time and compile time in Julia v1.6+. This enables the user to modify the behavior of a package, and have that choice reflected in everything from run time algorithm choice to code generation at compile time. Preferences are stored as TOML dictionaries and are, by default, stored within a (Julia)LocalPreferences.toml file next to the currently-active project. If a preference is "exported", it is instead stored within the (Julia)Project.toml instead. The intention is to allow shared projects to contain shared preferences, while allowing for users themselves to override those preferences with their own settings in the LocalPreferences.toml file, which should be .gitignored as the name implies.

Preferences can be set with depot-wide defaults; if package Foo is installed within your global environment and it has preferences set, these preferences will apply as long as your global environment is part of your LOAD_PATH. Preferences in environments higher up in the environment stack get overridden by the more proximal entries in the load path, ending with the currently active project. This allows depot-wide preference defaults to exist, with active projects able to merge or even completely overwrite these inherited preferences. See the docstring for set_preferences!() for the full details of how to set preferences to allow or disallow merging.

Preferences that are accessed during compilation are automatically marked as compile-time preferences, and any change recorded to these preferences will cause the Julia compiler to recompile any cached precompilation .ji files for that module. This allows preferences to be used to influence code generation. When your package sets a compile-time preference, it is usually best to suggest to the user that they should restart Julia, to allow recompilation to occur.

Note that the package can be installed on Julia v1.0+ but is only functional on Julia v1.6+.


Preferences use is very simple; it is all based around four functions (which each have convenience macros): @set_preferences!(), @load_preference(), @has_preference(), and @delete_preferences!().

  • @load_preference(key, default = nothing): This loads a preference named key for the current package. If no such preference is found, it returns default.

  • @set_preferences!(pairs...): This allows setting multiple preferences at once as pairs.

  • @has_preference(key): Returns true if the preference named key is found, and false otherwise.

  • @delete_preferences!(keys...): Delete one or more preferences.

To illustrate the usage, we show a toy module, taken directly from this package's tests:

module UsesPreferences

function set_backend(new_backend::String)
    if !(new_backend in ("OpenCL", "CUDA", "jlFPGA"))
        throw(ArgumentError("Invalid backend: \"$(new_backend)\""))

    # Set it in our runtime values, as well as saving it to disk
    @set_preferences!("backend" => new_backend)
    @info("New backend set; restart your Julia session for this change to take effect!")

const backend = @load_preference("backend", "OpenCL")

# An example that helps us to prove that things are happening at compile-time
function do_computation()
    @static if backend == "OpenCL"
        return "OpenCL is the best!"
    elseif backend == "CUDA"
        return "CUDA; so fast, so fresh!"
    elseif backend == "jlFPGA"
        return "The Future is Now, jlFPGA online!"
        return nothing

# A non-compiletime preference
function set_username(username::String)
    @set_preferences!("username" => username)
function get_username()
    return @load_preference("username")

end # module UsesPreferences

Conditional Loading

To use Preferences with Julia 1.6 and later but falling back to a default value for older Julia versions, you can conditionally load Preferences like this:

@static if VERSION >= v"1.6"
    using Preferences

@static if VERSION >= v"1.6"
    preference = @load_preference("preference", "default")
    preference = "default"

Note that these cannot be merged into a single @static if. Loading the package with using Preferences must be done on its own.

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