From: ara.t.howard@...
Date: 2006-03-19T11:25:04+09:00
Subject: Re: can you enable coercion between types?

On Sun, 19 Mar 2006, Trans wrote:

>
> ara.t.howard@noaa.gov wrote:
>> try this:
>>
>>      harp:~ > cat a.rb
>>      class Duration
>>        def initialize seconds
>>          @seconds = seconds
>>        end
>>        def coerce other
>>          if other.class == @seconds.class
>>            [@seconds, other]
>>          else
>>            [Float(@seconds), Float(other)]
>>          end
>>        end
>>      end
>
> Could you explain what this is doing? I don't get the effect.

my understanding of coerce is that it's used to produce a pair of operands -
myself and other - which can be used to perform operations.

so, in the case of Fixnum#+ we might see

   def + other
     x, y = other.coerce self
     x + y
   end

which is to say Fixnum need not know about how to add every possible class -
rather it leaves that knowledge up to those classes to export via thier coerce
method.  as i understand it that method should return two new objects which
interoperate so, in the case above (which is made up) we might have

   duration = Duration::new 42.0

   42 + duration

here would call duration(42) and expect back an x and y which could be added
together.  so, if @seconds happens to be a Fixnum the result will be another
Fixnum.  if @seconds happens to a Float the result would be a Float.

i admit that i'm not crystal clear on this, but it sure seems to work:


     harp:~ > cat a.rb
     class Duration
       def initialize seconds
         @seconds = seconds
       end
       def coerce other
         if other.class == @seconds.class
           [@seconds, other]
         else
           [Float(@seconds), Float(other)]
         end
       end
       def + other
         x, y = coerce(other)
         x + y
       end
     end
     def Duration(*a, &b); Duration::new(*a, &b); end

     p( 40 + Duration(2.0) )   # Float 42.0
     p( 40 + Duration(2) )     # Fixnum 42
     p( 40.0 + Duration(2.0) ) # Float 42.0
     p( 40.0 + Duration(2) )   # Float 42.0
     p( Duration(40) + 2 )     # Fixnum 42
     p( Duration(40) + 2.0 )   # Float 42.0
     p( Duration(40.0)  + 2)   # Float 42.0
     p( Duration(40.0) + 2.0 ) # Float 42.0


     harp:~ > ruby a.rb
     42.0
     42
     42.0
     42.0
     42
     42.0
     42.0
     42.0


i assume one would do something nice with the coerce method using Time, Date,
and DateTime objects too...

regards.

-a
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