Feature #7604: Make === comparison operator ability to delegate comparison to an argument

• Status: Assigned
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[prijutme4ty (Ilya Vorontsov)]

=begin
I propose to expand default behaviour of === operator in the following way:
Objects have additional instance method Object#reverse_comparison?(other) which is false by default in all basic classes.
Each class that overrides Object#===(other) should check whether reverse_comparison? is true or false
If it is false, behavior is not changed at all.
If it is true, comparison is delegated to === method of an argument with self as an argument.

This technique can help in constructing RSpec-style matchers for case statement. Example:

usual method call

arr = %w[cat dog rat bat]
puts arr.end_with?(%w[dog bat]) # ==> false
puts arr.end_with?(%w[rat bat]) # ==> true
puts arr.end_with?(%w[bat]) # ==> true

predicate-style case

case %w[cat dog rat bat].end_with?
when %w[dog bat]
puts '..., dog, bat'
when %w[rat bat]
puts '..., rat, bat'
when %w[bat]
puts '..., bat'
else
puts 'smth else'
end

==> ..., rat, bat

Code needed to run this is not very complex:
class Object
def reverse_comparison?(other)
false
end
alias_method :'old===', :'==='
def ===(other)
(other.reverse_comparison?(self) ? (other.send 'old===',self) : (self.send 'old===',other))
end
end

class Predicate
def initialize(&block)
@block = block
end
def reverse_comparison?(other)
true
end
def ===(*args)
@block.call(*args)
end
end

class Array
alias_method :'old===', :'==='
def ===(other)
other.reverse_comparison?(self) ? (other.send('===',self)) : (self.send('old===',other))
end

def end_with?(expected_elements = nil)
  return last(expected_elements.size) == expected_elements  if expected_elements
  Predicate.new{|suffix| last(suffix.size) == suffix }
end

end

This technique looks powerful and beautiful for me. One detail is that obj#reverse_comparison? can distinguish different types of arguments and returns true only for certain types of given object. Also this can be used to prevent double-mirroring (as shown below)

The problem is that many base classes already defined custom === operator, so each of those classes (Fixnum, Float, String, Regexp, Range etc) should be redefined in such a way to make a solution full-fledged.
Another problem is case that both objects defined reverse_comparison? to return true. In my solution Predicate#=== just ignores result of revese_comparison? which is not consistent.
Another possible way is to raise errors on double mirroring:
def reverse_comparison?(other)
raise 'double mirroring' if @__mirroring_started
@__mirroring_started = true
return true unless other.reverse_comparison?(self)
false
ensure
remove_instance_variable :@__mirroring_started
end

My proposal is to add reverse_comparison? method and change base classes operator === to use its result as shown above. May be it's worth also to make a class analogous to Predicate in stdlib.
=end

Your proposal reminds me of trying to extend #coerce behavior. What you call "mirroring", happens with #coerce. "Double mirrorring" is prevented by simply by #coerce being required to return a compatible pair. That being said, I did have times, when I wanted operator-specific #coerce (eg. different physical quantities do not add or compare, but do multiply). Essentially, you are proposing:

(1.) Let us have operator-specific #coerce (for #=== at least).
(2.) Let us have #=== actually using its specific coerce for some chosen argument types.

To me, achieving (1.) is imaginable as either #coerce taking an optional second argument, as in other.coerce( self, :=== ), or as having special #coerce_plus, #coerce_asterisk, #coerce_double_equal_sign, #coerce_triple_equal_sign etc.

Achieving (2.) is more difficult. As you pointed out, many classes have their own #===. But it is a general case that operator methods should be written with #coerce in mind.

Having thus reframed your proposal, let me also express my personal opinion about it: I would be in favor of cautiously implementing (1.), while (2.) means a bit work for everyone. I noticed that Marc Andre was also concerned about #coerce specification.

[prijutme4ty (Ilya Vorontsov)]

boris_stitnicky (Boris Stitnicky) wrote:

Your proposal reminds me of trying to extend #coerce behavior. What you call "mirroring", happens with #coerce. "Double mirrorring" is prevented by simply by #coerce being required to return a compatible pair. That being said, I did have times, when I wanted operator-specific #coerce (eg. different physical quantities do not add or compare, but do multiply). Essentially, you are proposing:

(1.) Let us have operator-specific #coerce (for #=== at least).
(2.) Let us have #=== actually using its specific coerce for some chosen argument types.

To me, achieving (1.) is imaginable as either #coerce taking an optional second argument, as in other.coerce( self, :=== ), or as having special #coerce_plus, #coerce_asterisk, #coerce_double_equal_sign, #coerce_triple_equal_sign etc.

Achieving (2.) is more difficult. As you pointed out, many classes have their own #===. But it is a general case that operator methods should be written with #coerce in mind.

Having thus reframed your proposal, let me also express my personal opinion about it: I would be in favor of cautiously implementing (1.), while (2.) means a bit work for everyone. I noticed that Marc Andre was also concerned about #coerce specification.

I like the idea of #coerce having additional argument(first time I thought whether current behavior of coerce can help me in solving this problem). Coercion implies that code of operators like + or === in built-in should be changed as in (2) case. I think that your solution can be actually much more flexible than mine. Also I can't realize any benefits of (2) over (1).

(2) and (1) are two steps of the same campaign, to make the behavior you described possible, but (1) might be easier and mildly useful on its own. Current #coerce would solve the problem provided that you make it return special objects with customized multiple operator methods, similar to your Predicate. Why not make a coerce-based gem demonstrating this? I would be interested in using it personally. You would have to find and patch those scattered #=== methods, while I am more interested in :+, :-, :*, :/, :**, and :<=>. We could have common special object for all of these.

[prijutme4ty (Ilya Vorontsov)]

boris_stitnicky (Boris Stitnicky) wrote:

(2) and (1) are two steps of the same campaign, to make the behavior you described possible, but (1) might be easier and mildly useful on its own. Current #coerce would solve the problem provided that you make it return special objects with customized multiple operator methods, similar to your Predicate. Why not make a coerce-based gem demonstrating this? I would be interested in using it personally. You would have to find and patch those scattered #=== methods, while I am more interested in :+, :-, :*, :/, :**, and :<=>. We could have common special object for all of these.

I will create a proof-of-concept gem, but not sure that I'll be able to create a native extension. So arithmetical operations can become much slower.

Let me know when you make the first commit.

[ko1 (Koichi Sasada)]

• Category set to core
• Target version set to 2.6

[ko1 (Koichi Sasada)]

• Assignee set to matz (Yukihiro Matsumoto)

[prijutme4ty (Ilya Vorontsov)]

boris_stitnicky (Boris Stitnicky) wrote:

Let me know when you make the first commit.

I released proof-of-concept gem. https://github.com/prijutme4ty/flex_coerce It makes no changes in behavior of base classes, you need to patch only your own class. But actually I didn't found any use-cases of this (e.g. physical quantities more naturally looks if there is a special quantity representing unity). I hope you'll hint me some good applications of this gem.
It's sad but #=== doesn't use coerce so this gem can't help me solve my task. So I'll soon create another gem that patches === method.

@ilya (Ilya Boltnev): I have noticed your post, I'll pay closer attention after next week.

I'have started working on it (first I have to handle switch to 2.0 on my machine).

[headius (Charles Nutter)]

As a feature that affects all Ruby implementations, this should probably move to CommonRuby: https://bugs.ruby-lang.org/projects/common-ruby

[naruse (Yui NARUSE)]

• Target version deleted (2.6)

[hsbt (Hiroshi SHIBATA)]

• Status changed from Open to Assigned