Re: ‘match’ and ‘!’ features

[Per Bothner <per@bothner.com>]

On 09/06/2018 09:02 AM, Mathieu Lirzin wrote:
> Hello,
> 
> I am getting interested in using Kawa lately.  I am happy to discover
> that it implements pattern matching using the ‘match’ and ‘!’ macros.
> However I am a bit puzzled by the fact those features are added in the
> main module while the documentation on patterns [1] describe those
> features as work in progress.
> 
> In order to lower the retro-compatibility expectations for those
> experimentations, What do you think of moving the ‘match’ and ‘!’
> definitions in a dedicated library for example (kawa match) ?

Moving these core features to library may be a bit of work, and
I don't think it would be desirable - as a core feature.
I'm reasonably comfortable with the syntax and design, and am not
planning on changing it.  Of course there are almost certainly bugs as
well as under-optimized code.

The main feature where design and implementation are missing
is dealing with structural patterns.

Given a constructor function T, where T may be a type/class name
(see https://www.gnu.org/software/kawa/Allocating-objects.html),
what should be the pattern syntax for "destructuring"?  The syntax

   (T pat1 pat2 ... patN)

could be used to match a value created by (T e1 e2 ... eN)
by matching e1 against pat1 etc.  There is a symmetry blemish in that T
in (T pat1 pat2 ... patN) is "evaluated" while the "arguments" are patterns,
but it still seems a reasonable syntax.  Still, there are a number of
complications in doing this.  Patterns with keyword arguments are slightly
easier.  To match an obj against:

    (T x: pat1 ... z: pat1)

could be implemented by matching obj:x against pat1 etc.  In adddition
we should require (equal? obj (T obj:x ... obj:z)).

Then we could go further: Regular (tree) expressions.  Strings.
XML/HTML nodes.

So that's what I mean when I say this feature is a work-in-progress.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/