One of our projects uses the ECLiPSe constraint logic programming language (a more powerful flavor of Prolog) in conjunction with Java (which is a good way to integrate graphics, unit testing, etc). One of the most popular developer tools for Java is the Eclipse IDE (yes, same name, totally different application), and we’d like to find a similarly-powerful editor for our pure-Prolog (.pro) and ECLiPSe (.ecl) files — one that provides indicators of (im)balanced parens, coloring of built-in predicates and syntax, and maybe code completion and compiler warnings.

ECLiPSe had a project called Saros on sourceforge that aimed to provide all this functionality, plus a debugger. Unfortunately, I wasn’t able to get any of these features working in its 1.0 version (after installing it as a plugin in the IDE). I’ve also heard that development on Saros has stopped.

However, I would be happy just getting paren-balancing, and I found a way to do that in the Eclipse IDE:

1. Go to Window | Preferences
2. Select General | Content Types in the left pane, and then on the right, select Text in the Content Types list.
3. In the File Associations list below that on the right side, you should see “*.txt (locked)”. Click the Add button and add “.ecl” when prompted for the Content Type. Do this again for .pl files (or .pro files, as I prefer, to distinguish from Perl files).
4. Back in the left pane, select General | Editors | File Associations.
5. On the right side, add “.ecl” and “.pl” (or “.pro”) if they aren’t already present.
6. Select “*.ecl” in the upper list on the right. Then in the lower listbox, if “Standalone Structured Source Editor (default)” isn’t shown, use the Add button to select “Standalone Structured Source Editor” and then click the Default button.
7. Select “*.pl” (or “*.pro”) in the upper list, and make sure it has the same default editor.
8. Click OK.

Now, if you open an .ecl or .pro file using either the Java or Debug perspective, and you place the text cursor after a ) or ] that is balanced, you will see a blue rectangle around the corresponding ( or [. There is no indicator if the ) or ] is not balanced.

Note that if you quit the IDE and launch it again, you will get a warning dialog about “Unsupported content type in editor.” I believe that’s because we selected “Text” in the Content Types preferences – there being no way to add a content type for ecl and pro files in particular – and that this resulted in our files being “locked” to the generic text editor in the File Associations preference. However, by marking the structured text editor as the default, we override that locking, and you’ll notice that if you dismiss the warning, paren-balancing still works. So, I selected the checkbox for “Do not show this message again” in the warning. Relaunching the IDE again gives me paren-balancing with no warning dialog.

UPDATE: Christian Wirth said on the eclipse-clp-users mailing list: “You have to install the Web Tools Project also.”