From: "jballanc (Joshua Ballanco)" <jballanc@...>
Date: 2012-11-14T17:55:29+09:00
Subject: [ruby-core:49339] [ruby-trunk - Bug #7342] String#<=> checks for a #to_str method on other but never uses it?


Issue #7342 has been updated by jballanc (Joshua Ballanco).


As the page you linked points out, #to_str is an *implicit* cast. i.e. It should be used internally to retrieve the string representation of an object. I think this is in keeping with all other uses of #to_str in Ruby source.

Another thing to note is that currently in Ruby if you have an object that provides #to_str but *not* #<=>, then it **cannot** be compared to a string object.

class Foo
  def to_str
    "my string"
  end
end

"test" < Foo.new #=> ArgumentError: comparison of String with Foo failed
----------------------------------------
Bug #7342: String#<=> checks for a #to_str method on other but never uses it?
https://bugs.ruby-lang.org/issues/7342#change-32897

Author: jballanc (Joshua Ballanco)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: 2.0.0


=begin
This isn't exactly a bug, as much as a request for clarification. I was looking at the semantics of the (({<=>})) operator and noticed something curious. For most classes, when evaluating (({thing <=> other})), if (({other})) is not of a compatible type, then (({nil})) is returned.

The exceptions (as far as I can find) are String and Time. For the Time class, if (({other})) is not a kind of (({Time})), then the reverse comparison (({other <=> thing})) is tried and the inverse of this result is returned (if not nil). For String, the reverse comparison is only tried IF (({other.respond_to?(:to_str)})), HOWEVER the referenced (({other.to_str})) method is never called. For example:

    class NotAString
      def <=>(other)
        1
      end
      def to_str
        raise "I'm not a string!"
      end
    end
    
    "test" <=> NotAString.new #=> -1

This seems very counterintuitive to me. I would expect that if my class implemented (({to_str})), that the return value of this would be used for comparison.
=end



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