Section 5.17. Performing Bit-level Operations on Numbers


5.16. Coercing Numeric Values

Coercion can be thought of as another form of implicit conversion. When a method (+ for example) is passed an argument it doesn't understand, it tries to coerce the receiver and the argument to compatible types and then do the addition based on those types. The pattern for using coerce in a class you write is straightforward:

class MyNumberSystem def +(other)    if other.kind_of?(MyNumberSystem)      result = some_calculation_between_self_and_other      MyNumberSystem.new(result)    else      n1, n2 = other.coerce(self)      n1 + n2    end  end end


The value returned by coerce is a two-element array containing its argument and its receiver converted to compatible types.

In this example, we're relying on the type of our argument to perform some kind of coercion for us. If we want to be good citizens, we also need to implement coercion in our class, allowing other types of numbers to work with us. To do this, we need to know the specific types that we can work with directly and convert to those types when appropriate. When we can't do that, we fall back on asking our parent class.

def coerce(other)   if other.kind_of?(Float)     return other, self.to_f   elsif other.kind_of?(Integer)     return other, self.to_i   else     super   end end


Of course, for this to work, our object must implement to_i and to_f.

You can use coerce as part of the solution for implementing a Perl-like autocon-version of strings to numbers:

class String   def coerce(n)     if self['.']       [n, Float(self)]     else       [n, Integer(self)]     end   end end x = 1 + "23"        # 24 y = 23 * "1.23"     # 28.29


We don't necessarily recommend this. But we do recommend that you implement a coerce method whenever you are creating some kind of numeric class.




The Ruby Way(c) Solutions and Techniques in Ruby Programming
The Ruby Way, Second Edition: Solutions and Techniques in Ruby Programming (2nd Edition)
ISBN: 0672328844
EAN: 2147483647
Year: 2004
Pages: 269
Authors: Hal Fulton

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