REPL & Consult

When starting scryer-prolog (oder another prolog interpreter), you can specify which files with facts and rules should be loaded.


scryer-prolog src/facts_about_prolog.pl

Using the the -g argument allows defining a goal (your entrypoint into the program).

When starting prolog without -g (or when it finished executing the goal), it will open a prompt, allowing to write queries interactively. This environment is called REPL read–eval–print loop.

In the default query mode, you can't directly enter new facts or rules, but only write queries. The following describes how to update your knowledge base:

Consulting facts and rules from the REPL

Loading Prolog files is called “consulting”.


?- 
consult('src/facts_about_prolog.pl').

The same can be achieved using this special notation:


?- 
['src/facts_about_prolog.pl'].

Once we have the knowlede base loaded, we can ask queries. Let's ask for facts about prolog:


?- 
fact("prolog", _, P, O).

Interactive

At REPL you can use the [user] special notation to read prolog text from standard input.

When finished, you press Ctrl-d. Than the entered prolog text will be consulted.


?-
[user].
fruit(orange).
fruit(banana).

<Ctrl-d>


?-
fruit(X).

Prolog will search for a solution and display it:


X = orange

If you are only interested whether any solution exists, we are done. Press Enter and you get back to the query prompt.

If you want to see all results, press Space for the next one…