Coercion in Ruby

Transcript

1. Coercion in Ruby
   Between the strong and weak typing
   

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2. Grzegorz Witek
   

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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”
   

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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”
   

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

   amount * value

   end
   end
   

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6. How do I Ruby?
   money = Money.new(3)
   money * 2 # => 6
   2 * money # => ERROR U FAIL
   

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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
   

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8. Bad solution
   Pros: works
   Cons: it’s wrong on as many levels as you can imagine

   

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9. Good solution
   class Money < Struct.new(:amount)

   

   def *(value)

   amount * value

   end
   def coerce(other) 

   [self, other]

   end
   end
   

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10. Good solution
    def coerce(other) 

    [self, other]

    end
    

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11. Good solution
    def coerce(other) 

    [other, amount]

    end
    

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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])
    

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

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14. How does it work?
    Fixnum#(Money)* => omg, what to do?
    Money#coerce => [Money, Fixnum]
    

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15. How does it work?
    Fixnum#*(Money) => omg, what to do?
    Money#coerce => [Money, Fixnum]
    Money#*(Fixnum) => I know how to handle it!
    

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16. Coercion in Ruby
    Thanks!
    Grzegorz Witek

    @arnvald
    

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