Coercion in Ruby

Transcript

1. 1.

   Coercion in Ruby Between the strong and weak typing

2. 2.

   Grzegorz Witek

3. 3.

   Strong vs. weak typing $> 3 + “a” Python: unsupported

   operand type(s) for +: 'int' and 'str' Ruby: String can't be coerced into Fixnum Javascript: “3a”
4. 4.

   Strong vs. weak typing $> 3 + “a” Python: unsupported

   operand type(s) for +: 'int' and 'str' Ruby: String can't be coerced into Fixnum Javascript: “3a”
5. 5.

   Coercion in Ruby class Money < Struct.new(:amount) def *(value)
 amount

   * value
 end end
6. 6.

   How do I Ruby? money = Money.new(3) money * 2

   # => 6 2 * money # => ERROR U FAIL
7. 7.

   Bad solution class Fixnum
 alias :old_multiply :* def *(val)
 if

   defined?(Money) && val.is_a?(Money)
 return self * val.amount
 else
 old_multiply(val)
 end
 end
 end
8. 8.

   Bad solution Pros: works Cons: it’s wrong on as many

   levels as you can imagine

9. 9.

   Good solution class Money < Struct.new(:amount)
 
 def *(value)
 amount

   * value
 end def coerce(other) 
 [self, other]
 end end
10. 10.

    Good solution def coerce(other) 
 [self, other]
 end

11. 11.

    Good solution def coerce(other) 
 [other, amount]
 end

12. 12.

    How does it work? Short answer: when Ruby can’t handle

    the param type, it calls arg.coerce(self) it gets 2 elements array, and calls array[0].method(array[1])
13. 13.

    How does it work? Fixnum#*(Money) => omg, what to do?

14. 14.

    How does it work? Fixnum#(Money)* => omg, what to do?

    Money#coerce => [Money, Fixnum]
15. 15.

    How does it work? Fixnum#*(Money) => omg, what to do?

    Money#coerce => [Money, Fixnum] Money#*(Fixnum) => I know how to handle it!
16. 16.

    Coercion in Ruby Thanks! Grzegorz Witek
 @arnvald