Feature #8223

Make Matrix more omnivorous.

Added by Anonymous almost 8 years ago. Updated almost 8 years ago.

Target version:


Let's imagine a class Metre, whose instances represent physical magnitudes in metres.

class Metre
  attr_reader :magnitude
  def initialize magnitude; @magnitude = magnitude end
  def to_s; magnitude.to_s + ".m" end

Let's say that metres can be multiplied by a number:

class Metre
  def * multiplicand
    case multiplicand
    when Numeric then magnitude * multiplicand )
      raise "Metres can only be multiplied by numbers, multiplication by #{multiplicand.class} attempted!"

And that they can be summed up with other magnitudes in metres, but, as a feature,
not with numbers (apples, pears, seconds, kelvins...).

class Metre
  def + summand
    case summand
    when Metre then magnitude + summand.magnitude )
      raise "Metres can only be summed with metres, summation with #{summand.class} attempted!"

Now with one more convenience constructor Numeric#m:

class Numeric
  def m; self end

We can write expressions such as

3.m + 5.m
#=> 8.m
3.m * 2
#=> 6.m

And with defined #coerce:

class Metre
  def coerce other; [ self, other ] end

Also this expression is valid:

2 * 3.m
#=> 6.m

Before long, the user will want to make a matrix of magnitudes:

require 'matrix'
mx = 2, 2 do 1.m end
#=> Matrix[[1.m, 1.m], [1.m, 1.m]]

It works, but the joy does not last long. The user will fail miserably if ze wants to perform matrix multiplication:

cv = Matrix.column_vector [1, 1]
mx * cv
#=> RuntimeError: Metres can only be summed with metres, summation with Fixnum attempted!
# where 2.m would be expected

In theory, everything should be O.K., since Metre class has both metre summation and multiplication by a number defined. The failure happens due to the internal workings of the Matrix class, which assumes that the elements can be summed together with numeric 0. But it is a feature of metres, that they are picky and allow themselves to be summed only with other Metre instances.

In my real physical units library that I have written, I have solved this problem by
defining an über zero object that produces the expected result, when summed with objects, that would otherwise not lend themselves to summation with ordinary numeric 0,
and patching the Matrix class so that it uses this über zero instead of the ordinary one.

But this is not a very systematic solution. Actually, I think that the Matrix class would be more flexible, if, instead of simply using 0, it asked the elements of the matrix what their zero is, as in:

class << Metre
  def zero; new 0 end

But of course, that would also require that ordinary numeric classes can tell what their zero is, as in:

def; 0 end
def; 0.0 end
def; Complex 0.0, 0.0 end
# etc.

I think that this way of doing things (that is, having #zero methods in numeric classes and making Matrix actually require the class of the objects in it to have public class method #zero defined) would make everything more consistent and more algebra-like. I am having this problem for already almost half a year, but I only gathered courage today to encumber you guys with this proposal. Please don't judge me harshly for it. I have actually already seen something like this, in particular with bigdecimal's Jacobian (, which requires that the object from which the Jacobian is computed implements methods #zero, #one, #two etc. Sorry again.

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