[q-lang-users] New stuff in cvs: multichar ops, views

[Albert G. <Dr....@t-...>]

Hi all,

I had some time to work on the new Q release over the Whitsun holidays.
As a result, you can find some interesting new stuff in cvs today:

Multichar operator symbols: These have been asked for on the mailing
list a while ago. I've given in to the popular demand now and
implemented them. So you can write, e.g.:

  public (--) Xs Ys @(-);
  Xs:List--Ys:List = foldl (flip (filter . neq)) Xs Ys;

(Q's lexical syntax had to be revised to support this, but most old
scripts should be unaffected.)

Wadler/Okasaki-style views (good stuff!): These are now implemented,
too. Virtual constructors can be declared with the new 'virtual' keyword
and can then be used in pattern-matching definitions like real
constructors (if the appropriate views a.k.a. unparsings are defined).
Note that in order to go with the customary terminology, I renamed the
builtin 'unparse' routine to 'view' so you will have to change your
scripts accordingly.

For example, here is how the 'Rational' type and its view are now
defined in the standard library:

  // from rational.q
  public type Rational : Real = virtual (%) P Q @ (/)
                              | private const rat N D;

  // from prelude.q
  view Q:Rational = '(N % D) where (N:Int,D:Int) = num_den Q;

Having (%) as a virtual constructor of 'Rational' lets you use the
operator in your definitions just like a real constructor, e.g.:

  def X%Y =  3%14 + 7%6;

  foo (X%Y) = (X,Y);

This yields:

  ==> (X,Y); foo (4%6)
  (29,21)
  (2,3)

The same applies to the container ADTs in the standard library. E.g.:

  def set Xs = set [1..10]+set [5..12];

  mymembers (set Xs) = Xs;

Which yields:

  ==> Xs; mymembers (set ["a".."c"]+set ["A".."C"])
  [1,2,3,4,5,6,7,8,9,10,11,12]
  ["A","B","C","a","b","c"]

It goes without saying that this makes the handling of abstract data
types *much* more convenient and elegant.

(Note that 'def's in the interactive command loop of the interpreter
don't quite work like that yet, as they don't handle virtual
constructors right now; but I'm working on that.)

Enjoy! :)
Albert

-- 
Dr. Albert Gr"af
Dept. of Music-Informatics, University of Mainz, Germany
Email:  Dr....@t-..., ag...@mu...
WWW:    http://www.musikinformatik.uni-mainz.de/ag