ANN: ScopedSettings - ScopedValues with malleable global values

Package Announcements
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I recently registered ScopedSettings.

Thanks to @vchuravy we now have ScopedValues (natively as Base.ScopedValues since Julia v.11 and via the fallback package ScopedValues for earlier Julia versions).

ScopedValues are a powerful new way to do “dependency injection” in Julia, and handle “preferences” in a scope-dependent way. One issue though is that the global/default value of a ScopedValue is fixed. In the past, we often used a global Ref or so for default settings in packages. Those are easy to change/override, but don’t provide scoping. While ScopedValue doesn’t allow for an easy global change/override.

ScopedSetting builds on ScopedValue to combine these two approaches: The global/default value can be manipulated/overridden (thread-safe), and it can be a fixed value or the result of a function that computes it on the fly. But in scopes, it behaves like a ScopedValue:

using ScopedSettings
some_setting = ScopedSetting(42)
some_setting isa ScopedSetting{Int}

is accessed like a ScopedValue

some_setting[] == 42

it’s global default value can be overridden

some_setting[] = 11
some_setting[] == 11

and can also be restored to the original default value

some_setting[] = nothing
some_setting[] == 42

The global default can also be function (without arguments):

other_setting = ScopedSetting(()->rand())
[other_setting[], other_setting[], other_setting[]] # random values
other_setting[] = 1.2 # override
other_setting[] == 1.2 # no more random values

Like with a ScopedValue, scoped settings can be set to different values for different scopes:

@with some_setting => 33 other_setting => 5.2 begin
    # Within this scope, we have
    some_setting[] == 33 && other_setting[] == 5.2
end

with(some_setting => 33, other_setting => 5.2) do
    # Within this scope, we have
    some_setting[] == 33 && other_setting[] == 5.2
end

# Globally we still have

some_setting[] == 42
other_setting[] == 1.2

ScopedSettings re-exports ScopedValues.@with and ScopedValues.with(...). You can mix ScopedSetting and ScopedValue objects in @with expressions

and with(...) calls.

To base ScopedSetting default values on package preferences and environment variables, ScopedSettings provides GetPreference{T} function objects:

setting_foo = ScopedSetting(GetPreference(SomePackage, "foo", 42))
setting_bar = ScopedSetting(GetPreference(SomePackage, "bar", :green))

setting_foo[] == either_envvar_or_preference_value_or_42
setting_bar[] == either_envvar_or_preference_value_or_green

@with setting_foo => 11 setting_bar => :blue begin
    # Different values within this scope
    setting_foo[] == 11 && setting_bar[] == :blue
end

# Original values outside of the scope
setting_foo[] == either_envvar_or_preference_value_or_42
setting_bar[] == either_envvar_or_preference_value_or_green

In the global scope, the value of setting_foo[] will depend on the LocalPreferences.toml files (if any) in your LOAD_PATH that have entries like

[SomePackage]
foo = 33
bar = "turquoise"

and environment variables like SOMEPACKAGEJL_FOO and SOMEPACKAGEJL_BAR (environment variables take precedence over preferences).

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