From: Robert Klemme <shortcutter@...>
Date: 2008-09-26T20:41:19+09:00
Subject: Re: Getting the right class with inheritance and super()

2008/9/25 Kyle Schmitt <kyleaschmitt@gmail.com>:
> On Thu, Sep 25, 2008 at 1:31 PM, Robert Klemme
> <shortcutter@googlemail.com> wrote:
>> 2008/9/25 Kyle Schmitt <kyleaschmitt@gmail.com>:
>>> Reading the example you gave, and even some on the net, it looks like
>>> I'd need the Array class to have a coerce method that can change an
>>> Array into a FixedSizeArray?
>>
>> No.  Your coerce method needs to take care.  Look at my example.  I
>> did not change Fixnum or Float.
> Right, you didn't change it, but in the example you're calling coerce
> on whatever it is that's passed in (perhaps this is the source of my
> confusion):

The critical thing to understand is that the _other_ instance's coerce
method is invoked!

13:38:43 ~$ ruby -r bigdecimal -e
'bd=BigDecimal.new("2");set_trace_func lambda {|*a|p a};1+bd'
["line", "-e", 1, nil, #<Binding:0x1002e3c8>, false]
["c-call", "-e", 1, :+, #<Binding:0x1002e38c>, Fixnum]
["c-call", "-e", 1, :coerce, #<Binding:0x1002e274>, BigDecimal]
["c-return", "-e", 1, :coerce, #<Binding:0x1002e080>, BigDecimal]
["c-call", "-e", 1, :+, #<Binding:0x1002e044>, BigDecimal]
["c-return", "-e", 1, :+, #<Binding:0x1002de8c>, BigDecimal]
["c-return", "-e", 1, :+, #<Binding:0x1002de50>, Fixnum]

Look at trace lines 2 and 3: Fixnum#+ is invoked and then BigDecimal#coerce.

> Class X
> ... most everything removed ...
>  def + other
>   if self.class === other
>     # fake only
>     self.class.new
>   else
>     #only going to get here if other isn't of class X
>     a, b = other.coerce(self)
>     #doesn't whatever class other is have to have a coerce method?
>     a + b
>   end
>  end
> end
>
>
> Now if we pull up irb.
> fixnum=10
> float=11.0
> hash={}
> array=[]
> string="Wax Rabbits"
>
> fixnum.respond_to?"coerce"
> => true
> float.respond_to?"coerce"
> => true
> hash.respond_to?"coerce"
> => false
> array.respond_to?"coerce"
> => false
> string.respond_to?"coerce"
> => false
>
> So since Arrays Hashes and Strings don't have a coerce, does that mean
> they can't be used in this fashion?

13:39:04 ~$ ruby -e '1+[]'
-e:1:in `+': Array can't be coerced into Fixnum (TypeError)
        from -e:1
13:41:04 ~$ ruby -e '[]+1'
-e:1:in `+': can't convert Fixnum into Array (TypeError)
        from -e:1
13:41:12 ~$ ruby -e '1+{}'
-e:1:in `+': Hash can't be coerced into Fixnum (TypeError)
        from -e:1
13:41:19 ~$ ruby -e '{}+1'
-e:1: undefined method `+' for {}:Hash (NoMethodError)

Cheers

robert


-- 
use.inject do |as, often| as.you_can - without end