From: Eric Mahurin <eric_mahurin@...>
Date: 2005-08-03T22:14:02+09:00
Subject: Re: [RCR] array or with non-array

--- "David A. Black" <dblack@wobblini.net> wrote:

> Hi --
> 
> On Wed, 3 Aug 2005, Eric Mahurin wrote:
> 
> > I think a primary example where I would really like real
> > duck-typing in a built-in would be Regexp#match(str).  This
> > requires the arg to be a String.  I would really like to
> have
> > this be able to operate on a file.  If I implement a class
> that
> > walks like, talks like, quacks like a String but really
> > accesses a file (practically did that in my rubyforge
> cursor
> > project), it wouldn't do any good because Regexp#match only
> > takes a String - period.
> 
> You can define #to_str on your object to get Regexp#match to
> accept
> it as an argument:
> 
>    irb(main):006:0> o = Object.new
>    => #<Object:0x401f3db4>
>    irb(main):007:0> def o.to_str; "hi"; end
>    => nil
>    irb(main):008:0> /i/.match(o)
>    => #<MatchData:0x401eec60>

This doesn't really help in the polymorphism department.  This
is no different than writing:

/i/.match(o.to_str)

> > The Regexp#match method could be implemented to take ANY
> object
> > that implemented a some subset of the String API.
> 
> I think there's a semantic or perhaps definitional issue
> here, though.
> What does it mean for a regular expression to "match" an
> arbitrary
> object?  I don't think it's just a matter of what methods the
> object
> has.  The object has to match the pattern, and the patterns
> are
> descriptions of strings.  I'm not sure how you would detect a
> pattern
> like /[A-Z]{3}(\d\d)?/ in something that wasn't a string.

By the same methods used in String.  It could get away with
just one method to accomplish the task: #[positive_int].  We
could put this in IO for example:

class IO
  def [](i)
    self.pos = i
    if eof? # pos can go beyond the eof
      self.pos = i-1
      return(nil) if eof?
      self.pos = i
    end
    getc
  end
end

If Regexp#match(obj) used just obj[pos], we could match a
Regexp across a file with the above.

Like I said earlier, for efficiency the code should special
case the String case and use the same implementation there is
now.



		
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