[m-dev.] 0.13 release: op/3 syntax

[Peter Schachte]

On Sat, Mar 04, 2006 at 05:15:30PM +1100, Peter Moulder wrote:

Another Peter heard from!

> I believe the particular example Peter gave (where the argument of '***'
> was an arbitrary goal) wouldn't work in Mercury.

Not until the mode system is improved to allow output arguments in
closures.

> > I'd like to be able to use 'in' as an infix operator for list and set
> > membership.
> You can already use `member`.

X `member` L doesn't look any better to me than member(X,L).  Taste, I
guess.  Too bad we're stuck with ascii.

> > 	forall Template in Sequence :: Goal
> 
> I'm not sure what the above does.  Mercury already allows
> 
>   ( all[X] X `member` Sequence => Goal(X) )

My version means exactly the same as the Mercury one, except that it
can produce bindings.  Actually, I got it a little wrong; I forgot I
had changed things.  Here are a few examples:

	forall X in list(L) do X = 3

unifies every element of L with 3.

	forall X in list(L) as I in 1..N do X is I*I

unifies L with a list of the first N squares.

	forall I in 1..N as (0->S0->S->Sum) do S is S0 + I*I

unifies Sum with the sum of the first N squares.  I'm not entirely
happy with the syntax of this one.  It's meant to suggest that the
first S0 is 0, at each iteration we compute S from S0, each
iteration's S0 is the previous iteration's S, and Sum is the final S.
Basically, this does everything all the map/filter/fold predicates
(and all the weird combinations and arities) do, and is extensible to
allow iteration over sets, maps, etc.

This is all documented in

@InProceedings{sch-padl-03,
  author =       {Peter Schachte},
  title =        {Sequence Quantification},
  pages =        {128--144},
  year =         2003,
  booktitle =    {Practical Aspects of Declarative Languages},
  editor =       {Veronica Dahl and Philip Wadler},
  publisher =    springer,
  series =       lncs,
  Volume =       2562
}

It could all be done without operators, of course, but it wouldn't be
as easy to read.  Eg, the last example would have been something like

	forall((I,S0,S), (range(1,N),thread(0,Sum)), (S is S0 + I*I)))

-- 
Peter Schachte              Do not meddle in the affairs of wizards, for
schachte at cs.mu.OZ.AU        they are subtle and quick to anger.
www.cs.mu.oz.au/~schachte/      -- J. R. R. Tolkien 
Phone: +61 3 8344 1338      
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