Re: Perl 6 How Do I? (Was: Perl 6 Summary for 2005-01-11 through 2005-01-18)

[Austin Hastings]

Luke Palmer wrote:
Austin Hasting writes:
 

How do I concisely code a loop that reads in lines of a file, then
calls mysub() on each letter in each line?
Or each xml tag on the line?
   

And I guess the answer is the same as in Perl 5.  I don't understand
what the problem is with Perl 5's approach:
   for <> {
       mysub($_) for .split: /<null>/;
   }
Or:
   use Rule::XML;
   for <> {
       mysub($_) for m:g/(ÂRule::XML::tagÂ)/;
   }
My problem with this is one of abstraction. P6 is, in a lot of places, a 
giant step forward in terms of abstracting away 'well-understood' 
operations. Grammar/Regex is one example, properties another.

I'd like to see a similar, simple notation for expressing composite 
operations. Perhaps this is a macro thing, but macros are still a little 
fuzzy to me (and I have these horrid memories from Lisp... :(

In general, though, this ties back to my long-ago wish for "separable 
verb" syntax support: I'd like to see a relatively concise, expressive 
notation for doing something like a double-loop or arbitrary traversal. 
The outer product was a delightful example of this kind of thinking -- 
it's obviously code, but it's totally data-driven.

Iterators may provide some of this, of course. But they provide it in 
scenarios where the data structure has been "comprehended" beforehand. 
Dynamic comprehension, if such a phrase can exist, is the obvious next 
step in DWIMmery.

Given a Tree, or a Trie, or a AvlTree, or a RBTree, it's easy to figure 
out what $CLASS->getIterator() is going to do. But what's the right way 
to traverse a C source file? In preprocessor mode it's one thing, in 
lexer mode it's another.

Is it possible to talk about an iteration template? Say I've got a list 
of numbers, but I want to iterate only the primes. Something like:

 class PrimeNumberIterator
   is Iterator
 {
   method _is_prime() {...}
   method .next()
   {
      my $cand;
      $cand = SUPER::next() while !_is_prime($cand);
      return $cand;
   }
 }
How do I impose that iteration scheme?
 for @list -> $x is PrimeNumberIterator { say $x ; }
Does that work?
Anyway, the point is that we as humans can say things like "look at all 
the prime numbers in the list ...", so I would like to see a similar 
level of expressivity in P6. I think P6l was heading that way some time 
ago, but we got sidetracked by an Apocalypse. :)

Maybe Larry's concept of 'type' has a place here? (Remember that 'type' 
was described as a restriction of 'class', such that 'odd number' is a 
type that restricts 'number'.)

Would
 for @list -> $x is Prime { say $x ; }
work? What about
 for [EMAIL PROTECTED] ~~ Prime ] -> $x { say $x; }
(That last one is really hard to read -- it would wind up demanding a 
layer of sugar...)

But if type is the only way to do filtering, then we'll wind up with a 
metatype mechanism for defining types on the fly, so ...

=Austin
   


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