1 Running a Prolog program

Running a Prolog program involves

1. creating a file containing the program
2. saving the file
3. loading the file into Prolog (called compiling, consulting, or reconsulting, the file in Prolog jargon)
4. finally, calling some goal defined in the program

The version of Prolog we will be using is one which runs on a variety of different platforms and is called SWI-Prolog. At York, it is accessible on the university’s Windows network. (Information about how to get your own copy is provided at the end of this documentation.)

1.1 Getting started

1. Log in to your account with your username and password.
2. from the Start menu follow the path Accessories|Windows Explorer. Click with the mouse.
3. In the window that opens use the panel on the left to navigate your way down the path (by clicking on the + icon) My Computer|Teaching on cserv $\ldots$|Applications|Departments|Language|Prolog 5|files|bin and double-click on the last item to open it.
4. Find the file named plwin.exe and right-click on it
5. In the box that opens, select Create shortcut and agree with the suggestion to put the shortcut on your desktop
6. Double-clicking the icon for plwin.exe on your desktop will launch the program.

You should be rewarded by the appearance of a new window containing some welcome messages and the Prolog prompt |?- , which indicates that Prolog is ready for input.

You may find it convenient to create a shortcut to the Prolog application on your desktop.

1.2 Creating a Prolog program.

To create a Prolog program, you must use an editor to create a file. There are a number of editors available and you can use whatever you prefer. If you have no preference, I suggest using the emacs editor that comes with SWI-prolog. It is Prolog-aware and will format and colour-code Prolog programs for you, as well as providing a number of other Prolog-related conveniences. To use emacs,

1. Start Prolog (by double-clicking the shortcut to plwin.exe on your desktop).
2. From the Menu bar select  Settings|User init file
3. Scroll down the file until you see  %:- set_prolog_flag(editor, pce_emacs).
4. Delete the percentage sign at the beginning of the line.
5. Save the file (Either Save from the File menu, or use the key combination Ctrl-x Ctrl-s -- that is, hold down the Control key and type xs).
6. Quit Prolog. Next time you start Prolog, the emacs editor will be available.

Here is how you create a Prolog program.

1. Double-click the Prolog program icon.
2. Use the File menu of the resulting window to create a new file and give it a name, with the extension .pl, making sure you save it at the root level of your M drive. .
3. Enter the text of your file in the file window.
4. You save your file by using the Save option from the File menu on the menu bar at the top of the editor’s window.
5. Make sure that you give your Prolog files the extension .pl .
6. You can copy text in the usual manner by using Copy and Paste from the Edit menu, or by selecting the text with the cursor (this copies automatically) and using using the key combination Ctrl-y to paste.
7. The same procedures work in the SWI-Prolog window; select the text to copy and use Ctrl-y to paste.

1.3 The Web

All these course notes are available over the web by using a browser such as Internet Explorer. The URL is:

http://www-users.york.ac.uk/ sjh1/

1.4 An Example

Use the emacs editor to create a file called family.pl.

When you have created a new file window, type the following, being particularly careful to type exactly what appears here, including full stops and commas:

1.4.1 Saving a file

Before you can do anything with this, you must save your file. Use Save from the File menu (or Ctrl-x Ctrl-s).

1.4.2 Consulting a file

Now that you have created a file, you need to inform Prolog of its contents. You do this by consulting the file. There are two ways to do this.

First, use the mouse to select the SWI-Prolog window in which Prolog is running. Then, type either

or

Note: It is conventional to give Prolog source files the extension .pl. SWI-Prolog knows about this, so you do not need to give the full filename and extension when consulting it. In fact, if you do type consult(test.pl). or [family.pl], you will receive an error message for your pains!

This is the standard procedure in all Prolog systems.

Alternatively, put the cursor in your emacs file window and use the key combination Ctrl-c Ctrl-b (For ‘consult buffer’ -- windows are called ‘buffers’ in emacs-speak).

If your file contains no errors, you should receive a message in the SWI-prolog window telling you that the file has been consulted successfully. You are now in a position to get Prolog to do some work.

1.4.3 Posing a goal

To get Prolog to do some computation, you need to issue a goal. Type

Type this exactly, including the capital W and the final full-stop, and, hit <return>. Prolog will respond:¹

Next, type a semi-colon followed by <return>. Prolog will respond

Again, type a semi-colon followed by <return>. Prolog will respond:

This means that you have now received all the responses to the goal that Prolog can compute on the basis of the information supplied in your program file.

1.4.4 Tracing

You will frequently want to see what Prolog does while it is evaluating your programs. This is often necessary when you are trying to track down bugs in your program. Unfortunately, Prolog’s debugging environment is not very good. To trace the execution of a goal (such as grandmother(liz, Who).), type

and then a goal, such as

Prolog will then output to the screen the sequence of steps it carries out in executing this goal

At each point where it calls a goal, Prolog pauses and requires a carriage return from you before proceeding.

To turn off tracing, type

(Note the full stop!)

1.5 A note on filenames and directories in Prolog

You may be in the (good) habit of organising your work by putting material related to a particular topic into a (sub-)directory. If you decide to do this with your Prolog files, you need to be aware of difference in the way you specify the filename using consult.

If the file is in the root directory of your M drive, and the file has the form <filename>.pl, you simply type

If, however, you have created a directory called, say myprolog and your file <filename>.pl lies inside that directory, the above instruction will not work and you will receive an error message.

What you have to do is:

1. to supply the full filename, including the .pl extension
2. the path from your $HOME directory to the file
3. enclose the whole expression in single quotes, like this:

   | ?- consult('myprolog/<filename>.pl'). <return>

Alternatively, start Prolog and select Consult or Edit from the File menu and navigate to the relevant folder.