Add conversions for US customary volumes to metric (liters and milliliters)

[sferik]

Unit conversion numbers taken from Wikipedia.

In addition to adding this new functionality (and specs), I also cleaned up a bunch of RSpec warnings about error expectations without a specified error class.

[mhuggins]

This won't work in conjunction with this require approach. Some users do not want to load all unit definitions, but simply the ones they need. If someone loads just us_customary or us_customary/volume definitions, then these definitions won't work.

Perhaps these belong in a separate require, but let me think on it. It might just make sense to expose an interface for adding custom conversions within your own project as well.

[sferik]

My understanding of your approach was to allow people to selectively require source units (i.e. what they were converting from) but, once that was required, to allow conversion to any destination units of the same type (e.g. volume ➡️ volume).

I think it’s quite common to want to convert units across the US/metric boundary and I’d hope this library would allow that functionality, even if that means offering a less granular require options, the value of which is less obvious, at least to me.

[MyklClason]

I've found you can write an initializer that will override the existing definitions. For example:

# config/initializers/ruby_measurement/volume.rb
...
Measurement.define(:'c.') do |unit|
  unit.alias :c, :cup, :cups
  unit.convert_to(:gal) { |value| value / 16.0 }
  unit.convert_to(:qt) { |value| value / 4.0 }
  unit.convert_to(:pt) { |value| value / 2.0 }
  unit.convert_to(:'fl oz') { |value| value * 8.0 }
  unit.convert_to(:tbsp) { |value| value * 16.0 }
  unit.convert_to(:tsp) { |value| value * 48.0 }
  unit.convert_to(:ml) { |value| value * 236.5882365 }
  unit.convert_to(:l) { |value| value * 0.2365882365 }
end
...

Hardly ideal, but might be useful. Perhaps make it easier to monkey patch existing measurements as well. Though perhaps I'm missing something in that regard. Perhaps include an extend method or something that could be used for adding additional aliases and conversions. So that one could simply use:

Measurement.extend(:'c.') do |unit|
  unit.alias :cuppa, :cuppas # Couldn't think of a better example here
  unit.convert_to(:ml) { |value| value * 236.5882365 }
  unit.convert_to(:l) { |value| value * 0.2365882365 }
end

[mhuggins]

@MyklClason That's something I always intended to add, but never got around to. I haven't needed this library in awhile, but I'm open to accepting changes for it!