Why does `Kernel :: String" check the result of `to_str`, and` Kernel :: Integer` does not check the result of `to_int`?

Question

Both Kernel::Integerand Kernel::Stringconvert the argument, first trying to cause "long" method ( to_intand to_str, respectively), followed by "short" method ( to_iand to_str, respectively). Both methods check the result class of the "short" method and raise an error if necessary:

[1] pry(main)> class Dummy
[1] pry(main)*   def to_i
[1] pry(main)*     "42"
[1] pry(main)*   end
[1] pry(main)*   def to_s
[1] pry(main)*     42
[1] pry(main)*   end
[1] pry(main)* end;
[2] pry(main)> Integer(Dummy.new)
TypeError: can't convert Dummy to Integer (Dummy#to_i gives String)
from (pry):9:in `Integer'
[3] pry(main)> String(Dummy.new)
TypeError: can't convert Dummy to String (Dummy#to_s gives Fixnum)

This behavior seems logical, since "short" methods should just give an "idea". On the other hand, “long” methods are supposed to be implemented only when the object in question is, in fact, an integer or a string ( see this answer ).

, "" , :

[4] pry(main)> class Dummy
[4] pry(main)*   def to_int
[4] pry(main)*     "42"
[4] pry(main)*   end
[4] pry(main)*   def to_str
[4] pry(main)*     42
[4] pry(main)*   end
[4] pry(main)* end;
[5] pry(main)> Integer(Dummy.new)
=> "42"
[6] pry(main)> String(Dummy.new)
TypeError: can't convert Dummy to String (Dummy#to_str gives Fixnum)

-?

ruby ​​2.1.2, btw:

[7] pry(main)> RUBY_VERSION
=> "2.1.2"

Answer 1

, Kernel#Integer, aka rb_f_integer:

• rb_f_integer rb_convert_to_integer
• rb_convert_to_integer convert_type(val, "Integer", "to_int", FALSE)
• convert_type val.to_int , .

• rb_convert_to_integer :

  tmp = convert_type(val, "Integer", "to_int", FALSE);
  if (NIL_P(tmp)) { // checks if val.to_int is nil. this is the line that causes this
      return rb_to_integer(val, "to_i");
  }
  return tmp;
  

, to_int, , , . , , , . to_i rb_to_integer ( to_int to_int ), to_int . :

class X
  def to_int
    nil
  end

  def to_i
    42
  end
end

class Y
  def to_int
    false
  end

  def to_i
    42
  end
end

p Integer(X.new) #=> 42
p Integer(Y.new) #=> false