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Prolog (Programming in Logic) is a declarative logic programming language created in 1972 by Alain Colmerauer and Robert Kowalski. Instead of describing step-by-step instructions, you state facts and rules, then pose queries that the system answers by searching for solutions through unification and backtracking. This makes Prolog especially well suited to symbolic reasoning, knowledge representation, parsing, and constraint problems.
On CompileBytes, Prolog programs run on a SWI-Prolog-style engine reported as version 8.2.4. SWI-Prolog is a popular, feature-rich open-source implementation widely used in artificial intelligence, natural language processing, expert systems, and education. You define a knowledge base of clauses, and the engine resolves goals against it, exploring alternatives automatically. Its relational, backtracking model lets a single rule run in multiple directions, which is unusual compared with imperative languages.
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:- initialization(main).
main :- write('Hello, World!'), nl.parent(tom, bob).
parent(bob, ann).
grandparent(X, Z) :-
parent(X, Y),
parent(Y, Z).
:- initialization(((grandparent(tom, ann) -> write(yes) ; write(no)), nl)).factorial(0, 1).
factorial(N, F) :-
N > 0,
N1 is N - 1,
factorial(N1, F1),
F is N * F1.
:- initialization((factorial(6, F), write(F), nl)).len([], 0).
len([_|T], N) :-
len(T, N1),
N is N1 + 1.
:- initialization((len([a,b,c,d], N), write(N), nl)).color(red).
color(green).
color(blue).
:- initialization((findall(C, color(C), Cs), write(Cs), nl)).sum_list_my([], 0).
sum_list_my([H|T], S) :-
sum_list_my(T, S1),
S is H + S1.
:- initialization((sum_list_my([1,2,3,4,5], S), write(S), nl)).male(tom).
male(bob).
female(ann).
parent(tom, bob).
parent(bob, ann).
father(F, C) :- parent(F, C), male(F).
:- initialization((father(F, bob), write(F), nl)).:- initialization(main).
main :-
( member(3, [1,2,3,4]) ->
write('found')
; write('not found')
), nl.:- initialization(main).
main :-
reverse([1,2,3,4], R),
write(R), nl.The editor uses Monaco, the engine behind VS Code, with language-aware editing tools and familiar keyboard controls.
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Prolog is one of 46 single-file compiler scratchpads. Each run submits only main.pl; local sibling-file imports are not enabled.
Linked-project languages (23): Node.js, TypeScript, Deno, Python, C, Go, Dart, Swift, Rust, Ruby, PHP, Java, Kotlin, C++, C#, Nim, Crystal, Haxe, Zig, Scala, D, Bun, Objective-C.
Single-file scratchpads (46): Matplotlib, NumPy, pandas, Haskell, Clojure, Perl, Lua, Tcl, jq, Raku, Elixir, Bash, Zsh, Nushell, R, Assembly (NASM), Julia, Octave, Groovy, PowerShell, Visual Basic .NET, Odin, Pascal, Lisp, Erlang, CoffeeScript, COBOL, BASIC, V, Racket, OCaml, Prolog, AWK, Emacs Lisp, Emojicode, Forth, F#, Fortran, Assembly (NASM64), Solidity, Mojo, Ada, WASM (WAT), Gleam, Shell Script (sh), Scheme.
Download file exports the active Prolog source file.