Question

I’m unable to determine why these two functions behave differently. I could just use symbols or my own constants, but I have a deep desire to know what’s going on here (and if I’m doing something bad).

def convert(value, type)
    case type
    when Integer
        value.to_i
    when String
        value.to_s
    else
        value
    end
end

def convert_with_if(value, type)
    if (type == Integer)
        value.to_i
    elsif (type == String)
        value.to_s
    else
        value
    end
end

n = 4.4
p convert(n, Integer)  #=> 4.4
p convert_with_if(n, Integer) #=> 4

Answer 1

case calls ===, the case equality operator. Module#===, and by extension Class#===, actually tests if the given argument’s class is the receiver or is one of its descendants. String === object is practically equivalent to object.kind_of? String.

convert 1, String

Would be equivalent to:

case String
  when Integer
    1.to_i
  when String
    1.to_s
end

That’s like asking is the String class an Integer?, or is the String class a String?. The answer to both questions is No, it is a Class..

In terms of code, String.class returns Class, which is not related in any way to Integer or even String itself. If you introduced a when Class or when Module clause, it would be executed every time.