Re: [RCR] array or with non-array

Hi --

On Wed, 3 Aug 2005, Eric Mahurin wrote:

--- "David A. Black" <dblack@xxxxxxxxxxxx> 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) 

I think it's quite different, certainly in appearance and to some
extent in logic.  I'm not sure how much more polymorphic one could
get, unless one had every object present its .to_s representation for
matching, which would not be good.

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.


scanf.rb does something along those lines.  (It gets tricky with
scanf, because of whitespace and stuff, but it's basically position
and index manipulation.)  Then again, scanf has always been
stream-oriented.  I'm not sure I agree conceptually with the idea of
matching an IO object to a pattern.  It actually feels to me like
there *should* be an explicit, intervening string representation.

Nor do I think this is a sign of failure or rejection of the principle
of duck typing or anything like that.  Everything doesn't have to do
everything.  For instance, you can't do File.rename on an integer, or
divide a hash by a float.  And yes, I know that the reductio ad
absurdum is not proof of anything :-)  I just think there's some
nuance to some of the cases, including the specificity of the
pattern/string connection.  I don't see pattern matching as strictly a
matter of integer indexability.


David

--
David A. Black
dblack@xxxxxxxxxxxx


.

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