User-defined class coercion?

ruby-talk
1

I would like to define my own class that works as a number when
treated as a number but also has other properties. Can I do this?
If so, how?

-Mark

2

Hi,

I would like to define my own class that works as a number when
treated as a number but also has other properties. Can I do this?
If so, how?

By defining “coerce” method:

class Foo
def coerce(other)
…
return converted_other, self
end
end

						matz.
···

In message “User-defined class coercion?” on 03/08/06, “Mark J. Reed” markjreed@mail.com writes:

3

Mark J. Reed wrote:

I would like to define my own class that works as a number when
treated as a number but also has other properties. Can I do this?
If so, how?

Here’s an extract from the new reference material for the coerce method:

  Coerce is both an instance method of Numeric and part of a
  type conversion protocol. When a number is asked to perform an
  operation, and is passed a parameter of class different to its
  own, it must first coerce both itself and that parameter into a
  common class so that the operation makes sense. For example, in
  the expression $1 + 2.5$, the Fixnum ``1'' must be converted
  to a Float to make it compatible with ``2.5''. This
  conversion is performed by coerce.  For all numeric
  object, coerce is straightforward: if aNumeric is the same
  type as self, returns an array containing aNumeric and
  self. Otherwise, returns an array with both aNumeric and
  self represented as Float objects.
 
  If a numeric object is asked to operate on a non-numeric, it
  tries to invoke coerce on that other object. For
  example, if you write:

      1 + "2"

 Ruby will effectively execute the code as:

      n1, n2 = "2".coerce(1)
      n2 + n1

  In the more general case, this won't work, as most non-numerics
  don't define a coerce method. However, you can use this
  (if you feel so inclined) to implement part of Perl's automatic
  conversion of strings to numbers in expressions.

    class String
      def coerce(other)
        case other
        when Integer
          begin
            return Integer(self), other
          rescue
            return Float(self), Float(other)
          end
        when Float
          return Float(self), other
        else super
        end
      end
    end

    1 + "2"
    1 + "2.3"
    1.2 + "2.3"
    1.5 + "2"
4

MR = me
YM = Matz

I would like to define my own class that works as a number when
treated as a number but also has other properties. Can I do this?
If so, how?

By defining “coerce” method:

class Foo
def coerce(other)
…
return converted_other, self
end
end

That works wonderfully. Thank you, for everything! Didn’t expect my
dumb newbie question to be answered by the creator of the language. :slight_smile:

-Mark

5 
  In the more general case, this won't work, as most non-numerics
  don't define a coerce method. However, you can use this
  (if you feel so inclined) to implement part of Perl's automatic
  conversion of strings to numbers in expressions.

    class String
      def coerce(other)
        case other
        when Integer
          begin
            return Integer(self), other
          rescue
            return Float(self), Float(other)
          end
        when Float
          return Float(self), other
        else super
        end
      end
    end

    1 + "2"
    1 + "2.3"
    1.2 + "2.3"
    1.5 + "2"

But…

1 - “4” => 3
1 - “2.4” => 1.4

The return values were the wrong way around, this should work better:

class String
def coerce(other)
case other
when Integer
begin
return other, Integer(self)
rescue
return Float(other), Float(self)
end
when Float
return other, Float(self)
else super
end
end
end

But even so…

8 | “16”
TypeError: cannot convert String into Integer
from (irb):47:in `|’
from (irb):47
from :heart::0

I feel it is an anomaly for the bitwise operators to not call coerce.
Yes, it makes no sense to coerce to or from floats for bitwise, but
there are other cases as the above shows.

Stephen
± S.D.Sykes - www.stephensykes.com

···

“You are a woman, aren’t you?” – The Duke of Edinburgh checks his
facts after receiving a gift from a woman, Kenya 1984.

6

And that’s only one of the many charms of Ruby. :slight_smile:

I started out very enthusiastic about Ruby and it just keeps getting
better. In some ways, encountering Ruby is like having a veil that was
obscuring your vision lifted from your eyes.

Regards,

Mark

···

On Tuesday, August 5, 2003, at 11:41 PM, Mark J. Reed wrote:

[snip]
That works wonderfully. Thank you, for everything! Didn’t expect my
dumb newbie question to be answered by the creator of the language.
:slight_smile:

-Mark

7

haldane wrote:

But…

1 - “4” => 3
1 - “2.4” => 1.4

The return values were the wrong way around, this should work better:

Oops - and I even documented that… sigh

Thanks

Dave