Prolog online compiler

What runs your code

Good to know

• Runs with SWI-Prolog 10. A program runs from an :- initialization(main). directive, as on GNU Prolog, and the run ends when it finishes or calls halt.
• read_line_to_string(user_input, Line) reads the next line of the Input tab; read(X) reads a term, which must end with a full stop.
• Double-quoted text is a string, as SWI-Prolog has it, not a list of codes as on GNU Prolog. Warnings, such as singleton variables, go to stderr; an error, or a main that fails, makes the exit code 1.

Run Prolog from your own code

Send the code and its input to the sync endpoint with an API key, and the output comes back in the response. The API docs cover the async endpoint with callbacks and every response field.

curl -X POST https://api.onlinecompiler.io/api/run-code-sync/ \
  -H "Authorization: YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "compiler": "swipl-10",
    "code": ":- initialization(main).\n\nmain :-\n    read_line_to_string(user_input, Name),\n    format(\"Hello, ~w!~n\", [Name]),\n    halt.",
    "input": "Ada"
  }'

The response has output ("Hello, Ada!\n" here), error, exit_code, time and memory.

Put a runnable Prolog example on your site

The editor above is the embeddable widget. Add the script and a div with your widget key, and your readers can edit and run the code without an account:

<script src="https://onlinecompiler.io/widget.js"></script>

<div class="runcode" data-key="YOUR_WIDGET_KEY" data-lang="swipl-10">
  <script type="text/plain">
    :- initialization(main).

    main :-
        read_line_to_string(user_input, Name),
        format("Hello, ~w!~n", [Name]),
        halt.
  </script>
</div>

On a platform that takes an iframe but not a script, such as many course sites, use the iframe embed. For Docusaurus, MkDocs, Sphinx and Mintlify, see runnable code examples in docs.

Other languages

Python, C, C++, Java, C#, F#, Go, Rust, PHP, Ruby, Haskell, TypeScript, JavaScript, SQL, Bash, Fortran, Visual Basic, Kotlin, R, Swift, COBOL, Perl, Julia, Pascal, Octave, Assembly, Lua, OCaml, Common Lisp, Erlang, Elixir, D, FreeBASIC, Dart, Clojure, Groovy, Scala, Zig, Nim, Crystal, Ada, PowerShell, Racket, Objective-C.

Frequently asked questions

Is this Prolog online compiler free?

Yes. Running Prolog on this page needs no account and shows no ads. To run Prolog from your own app or site, the API and the widget are free up to 100,000 runs a month, commercial use included.

Can my Prolog program read input?

Yes. Type it in the Input tab before you run, and the program reads it from standard input. The whole input is sent when you press Run, so a program can't ask for more while it runs.

Can I use Prolog libraries?

SWI-Prolog's bundled libraries, such as lists, apply, readutil and clpfd. Packs can't be installed, and programs have no network access.

How long can a program run?

Up to 30 seconds, with 512 MB of memory and 2 CPUs. A program still running at 30 seconds is stopped with exit code 124, and output over 64 KB per stream is cut.

Where does my code run?

On our servers, in a fresh gVisor sandbox with no network access that is deleted when the run ends. Each run is kept in execution history with its output; the security page says what is stored.