Why can't I attach a singleton method to another object?

Question

some_string = "i love lamp"
another_string = "i love desk"
def some_string.lovehate
    if match /love/
        sub /love/, "hate"
    elsif match /hate/
        sub /hate/, "love"
    end
end

puts some_string.lovehate # => "i hate lamp"
puts another_string.respond_to? :lovehate # => false
m = some_string.method(:lovehate).unbind
m.bind(another_string).call # fails

This fails with

singleton method called for a different object (TypeError)

So clearly the ruby programmers thought this was a bad idea for some reason. My question is what is stopping me from doing this?

(It might merely be an implementation detail not related to the opinions of "the ruby programmers".) — user2864740, Jun 13 '14 at 21:10

score 2 · Accepted Answer · answered Jun 13 '14 at 21:17

This is consistent with how UnboundMethods normally work. The object you bind them to has to be a kind_of? the class the method came from. another_string is not descended from some_string's singleton class (just like no other object is), so you can't bind the the methods of some_string's singleton class to it.

Why can't I attach a singleton method to another object?

1 Answers1