Editing Prolog (section)

== Programming in Prolog ==
In Prolog, loading code is referred to as ''consulting''. Prolog can be used interactively by entering queries at the Prolog prompt <code>?-</code>. If there is no solution, Prolog writes <code>no</code>. If a solution exists then it is printed. If there are multiple solutions to the query, then these can be requested by entering a semi-colon <code>;</code>. There are guidelines on good programming practice to improve code efficiency, readability and maintainability.<ref>{{Cite journal |last1=Covington |first1=Michael A. |last2=Bagnara |first2=Roberto |last3=O'Keefe |first3=Richard A. |author-link3=Richard O'Keefe |last4=Wielemaker |first4=Jan |last5=Price |first5=Simon |title=Coding guidelines for Prolog |doi=10.1017/S1471068411000391 |journal=[[Theory and Practice of Logic Programming]] |volume=12 |issue=6 |pages=889–927 |year=2011 |arxiv=0911.2899|s2cid=438363}}</ref>

Here follow some example programs written in Prolog.

=== Hello World ===
Example of a basic query in a couple of popular Prolog dialects:
{|
|-
! [[SWI-Prolog]]
! [[GNU Prolog]]
|-
|<syntaxhighlight lang="prolog">
?- write('Hello World!'), nl.
Hello World!
true.

?-
</syntaxhighlight>
|<syntaxhighlight lang="prolog">
| ?- write('Hello World!'), nl.
Hello World!

yes
| ?-
</syntaxhighlight>
|}

This comparison shows the prompt ("?-" vs "| ?-") and resolution status ("true". vs "yes", "false". vs "no") can differ from one Prolog implementation to another.

=== Compiler optimization ===
Any computation can be expressed declaratively as a sequence of state transitions. As an example, an [[optimizing compiler]] with three optimization passes could be implemented as a relation between an initial program and its optimized form:
<syntaxhighlight lang="prolog">
program_optimized(Prog0, Prog) :-
    optimization_pass_1(Prog0, Prog1),
    optimization_pass_2(Prog1, Prog2),
    optimization_pass_3(Prog2, Prog).
</syntaxhighlight>
or equivalently using [[definite clause grammar|DCG]] notation:
<syntaxhighlight lang="prolog">
program_optimized --> optimization_pass_1, optimization_pass_2, optimization_pass_3.
</syntaxhighlight>

=== Quicksort ===
The [[quicksort]] sorting algorithm, relating a list to its sorted version:
<syntaxhighlight lang="prolog">
partition([], _, [], []).
partition([X|Xs], Pivot, Smalls, Bigs) :-
    (   X @< Pivot ->
        Smalls = [X|Rest],
        partition(Xs, Pivot, Rest, Bigs)
    ;   Bigs = [X|Rest],
        partition(Xs, Pivot, Smalls, Rest)
    ).

quicksort([])     --> [].
quicksort([X|Xs]) -->
    { partition(Xs, X, Smaller, Bigger) },
    quicksort(Smaller), [X], quicksort(Bigger).
</syntaxhighlight>