Re: term_expansion/2 with DCG clauses

On 08/14/2013 04:28 AM, Richard A. O'Keefe wrote:
>
> On 14/08/2013, at 3:44 AM, Jan Wielemaker wrote:
>> This seems a logical way to access DCG expansion. The only price is that
>> you cannot created rules for -->/2 (nor for []/0 and ./2). Do these
>> systems have an agreed way to creates DCGs from expansion rules? I guess
>> Richard will know the rationale for not doing DCG after user expansion.
>
> If I recall correctly, DCG expansion was done *before* user expansion.
> With there being no goal_expansion in those days, this was important if
> you wanted users to be able to plug their own goal_expansion code in and
> have it apply to the result of DCG expansion.
> Further, DCG expansion was available as library code, so if you wanted it,
> you could very easily call it yourself.

So not doing DCG expansion if user expansion is involved seems out of
date. The idea to chain `translation packages' is also implemented in
Ciao and ECLiPSe (and Logtalk?), although the way the packages are
specified there is much more fine-grained.

If you want DCG translation and do further term expansion on the result,
you can call dcg_translate_rule/2. Would be nice if this had the same
name in all systems supporting DCGs ... As I understand it, some people
assume you can call expand_term/2 to achieve DCG translation. There are
two issues with that: it might do a lot more and it is probably a bad
idea to call expand_term/2 from term_expansion/2.  It surely easily gets
you into a loop.

I see no reason to change anything right now.

	Cheers --- Jan