preferunits without global state

[sostock]

Currently, preferunits works by changing the values in the global dictionary Unitful.promotion. This causes problems with precompilation (#545, #600).

My suggestion for Unitful v2.0 is to replace the global "unit preference dictionary" (Unitful.promotion) by an object that the user creates and explicitly uses to convert to their preferred units. Instead of

preferunits(cm)
upreferred(1m)

the user would do the following:

my_unit_preference = UnitPreference(cm) # this returns a UnitPreference object, does not alter global state
my_unit_preference(1m)                  # the UnitPreference object is callable

However, this means that automatic promotion would not promote to the user’s preferred units but always to the default units (unless they are using ContextUnits/FixedUnits).

Introducing this new UnitPreference object can be done in a minor release, but removing the possibility of setting a global unit preference would be breaking.

[j-fu]

My initial feeling says that this is the more sane of the two solutions discussed for #545. Howeverthis seems to shift the temptation to create global state to client packages...