How to get rid of TypeError

Question

I have my class

class A
  def to_s
    'string'
  end
  def inspect
    'string'
  end
end

and trying to do something like this

a = A.new
'somestring' + a

Error is "in `+': can't convert A into String (TypeError)"

Is there any way to fix it?

P.S. using 'somestring' + a.to_s isn't an option.

Answer found. to_str method needed.

Answer 1

#to_s is for representing an object as a String. In your case, you need to convert the object to a String, because String#+ only deals with Strings. In Ruby, type conversions are done with the three letter form of the to_X methods, i.e. to_int, to_ary, to_hash (someone can't count to three ...), to_str and so on.

So, you need to implement to_str, in order to make that work.

However, you should only implement to_str, if your object actually is a string! If it's something else, you must not implement to_str and rather have the clients explicitly represent it as a string using to_s.

Basically, implementing to_str in Ruby is like inheriting from String in Java: it means that A IS-A String.

As an example, look at which classes in the Ruby core library actually implement the type conversion methods:

• the only class that implements to_ary, is Array itself,
• the only class that implements to_hash, is Hash itself and
• the only class that implements to_str, is String itself.

This should show you that implementing to_str is a very serious matter that should not be undertaken lightly.

The only conversion method that does not have only a trivial no-op implementation, is to_int, which is also implemented by Float and Numeric. And actually, I think that is a mistake, since there is an infinite number of Floats which aren't Integers. And Numeric is a superclass of Integer, so saying that every Numeric IS-A Integer is even more strange.