🚀 go-pugleaf

In article <BvBEny.4M8@gabriel.keele.ac.uk> csa09@seq1.keele.ac.uk (Paul Singleton) writes:

   Prolog is a sufficiently wide-spectrum language that we don't need to
   borrow bolt-on programming-in-the-large languages.  I think if we write

	   retract_all_unit_clause( X) :-
		   (	dialect( sepia)
		   ->	retract_all( X)
		   ;	dialect( sicstus)
		   ->	retractall( X)
		   ).

   then add either

	   dialect( sepia).
   or
	   dialect( sicstus).

   as a fact, and partially evaluate the program, we get the effect of macro
   substitution.  Isn't this a neater way of doing things?  

I have some problems in using partial evaluation as the means of
achieving this. In particular the partial evaluator will modify not
only those parts of the program that have to do with the dialects but
it will also modify other parts of the program that have nothing to do
with it. If for example, the code represents a library, you have two
alternatives:

1) To partially evaluate the code when the libraries are installed, in
this case the version of the libraries the the users have access to,
have been partially evaluated and therefore its logical structure has
being modified. It will not be easy to read since many predicates have
beeing optimized (partially evaluated).

2) To partially evaluate the code when the library is loaded. In this
case, the system must doit automatically. You can not put the burden
of nowing what must be partially evaluated and what not, to the user
of a shipped peace of software.

The advantage of using a macro processor (not necessarily CPP) is that
the instalation language is separated from the object language. It is
very clear what is a statement about how to install the software and
what is a statement of the code of the language.
With partial evaluation we mix the two as if they were talking about
the same thing.

	Jose Alberto.

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