SY - a physical units gem

SY is a domain model of physical units (magnitudes, quantities, dimensions...). In SY, I (would like to) emphasize the domain model correctness and ease of use over the ease of implementation. SY can be used in two modes:

• When loaded by require 'sy', Numeric class is extended with methods corresponding to units and their abbreviations.
• When loaded by require 'sy/noinclude', built-in classes are not modified, while physical magnitudes can still be contructed explicitly from the appropriate physical quantities.

Good manners require me to also mention another good library of physical units in Ruby, phys-units.

Upon require 'sy', we can say 5.metre, or Rational( 5, 2 ).metre, and the computer will understand, that these numbers represent magnitudes of the physical quantity SY::Length expressed in the unit SY::METRE. Equally, we can use abbreviations (such as 5.m, 2.5.m), prefixes (such as 5.km, 5.kilometre, 5.km), exponents (such as 5.m² for 5 square metres), and chaining (such as 5.m.s⁻¹ to denote speed of 5 metres per second). Please read also the sy code file for the DSL statements defining default quantities and their units.

Users of sy should learn how to type Unicode exponent characters, such as ², ³, ⁻¹ etc. It is possible to use alterantive syntax, such as 5.m.s(-1) instead of 5.m.s⁻¹, but Unicode exponents should be used everywere except non-reused code. Unicode exponents make the physical models that you will be constructing with SY much more readable. And we know that code is (typically) write once, read many times. So it pays off to type an extra keystroke when writing to increase readability for the many subsequent revisions.

If sy is used in the mode that extends Numeric with unit methods, then since many of these are short and common words, there can be collisions. For example, ActiveSupport already provides handling for time units (hour, minute, second etc.), which would collide with SY methods of the same name. Since SY relies on method_missing, if these methods are already defined for numerics, SY's method_missing will not activate and ActiveSupport methods will be used. In this particular case, SY methods still can be invoked using abbreviations (5.s, 5.h, 5.min).

When noinclude mode is activated by calling require 'sy/noinclude', methods such as 5.metre cannot be used. Instead, the magnitude has to be constructed explicitly:

  mgn = SY::Length.magnitude( 5 )

Another effect of noinclude mode is, that one cannot perform conversions using unit symbols, as in SY::WATTHOUR.in( :J ), but instead one has to write:

  SY::WATTHOUR.in( SY::JOULE )

SY has been written emphasizing the object model over plentitude of defined units. There is plenty of room for defining units thus far not defined by SY. Also, the object model, though advanced, could possibly be refactored, eg. with respect to the way that parametrized descendants of Magnitude are introduced.

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Added some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request