Editing Logic programming (section)

===Metalogic programming===
[[Metaprogramming]], in which programs are treated as data, was already a feature of early Prolog implementations.<ref>Colmerauer, A. and Roussel, P., 1996. The birth of Prolog. In History of programming languages---II (pp. 331-367).</ref><ref name="Warren">Warren, D.H., Pereira, L.M. and Pereira, F., 1977. Prolog-the language and its implementation compared with Lisp. ACM SIGPLAN Notices, 12(8), pp.109-115.</ref> For example, the Edinburgh DEC10 implementation of Prolog included "an interpreter and a compiler, both written in Prolog itself".<ref name="Warren"/> The simplest metaprogram is the so-called "[[Vanilla (computing)|vanilla]]" meta-interpreter:
<syntaxhighlight lang="prolog">
    solve(true).
    solve((B,C)):- solve(B),solve(C).
    solve(A):- clause(A,B),solve(B).
    </syntaxhighlight>
where true represents an empty conjunction, and (B,C) is a composite term representing the conjunction of B and C. The predicate clause(A,B) means that there is a clause of the form	A :- B.

Metaprogramming is an application of the more general use of a ''[[metalogic]]'' or ''[[metalanguage]]''  to describe and reason about another language, called the ''object language''.

Metalogic programming allows object-level and metalevel representations to be combined, as in natural language. For example, in the following program, the atomic formula <code>attends(Person, Meeting)</code> occurs both as an object-level formula, and as an argument of the metapredicates <code>prohibited</code> and <code>approved.</code>

<syntaxhighlight lang="prolog">
prohibited(attends(Person, Meeting)) :- 
    not(approved(attends(Person, Meeting))).

should_receive_sanction(Person, scolding) :- attends(Person, Meeting), 
    lofty(Person), prohibited(attends(Person, Meeting)).
should_receive_sanction(Person, banishment) :- attends(Person, Meeting), 
    lowly(Person), prohibited(attends(Person, Meeting)).

approved(attends(alice, tea_party)).
attends(mad_hatter, tea_party).
attends(dormouse, tea_party).

lofty(mad_hatter).
lowly(dormouse).

?- should_receive_sanction(X,Y).
Person = mad_hatter,
Sanction = scolding.
Person = dormouse,
Sanction = banishment.
</syntaxhighlight>