Setting a default list of items in project_attribute in a GNAT GPS Plugin - plugins

Question

I'm working on a custom GNAT GPS plugin (for GPS 6.1.2).
My plugin XML creates a project attribute "example_list_of_files".
This is a list of strings, that correspond the names of Ada files in the project.
I want to default the entries in that list to "a.adb","b.adb","c.adb". However I've been unable to find the correct syntax for this. Instead i end up with a single string of all the values.
What i want to see is what happens when you manually add three elements, as shown below:
Here is the code for this example:
GPS.parse_xml('<?xml version="1.0" ?>' + """
<my_plugin>
<project_attribute
name="example_list_of_files"
label="example_list_of_files"
description="A description...."
package="MyPackage"
editor_page="MyPage"
editor_section="Build"
hide_in="wizard library_wizard"
omit_if_default="false"
list="true"
base_name_only="true">
<string type="file" filter="project" default="'a.adb','b.adb','c.adb' " />
</project_attribute>
</my_plugin>""");
Notice the string element with the project attribute default. Instead of a list of entries in the project manager it gives me a single entry, containing the string "'a.adb', 'b.adb', 'c.adb'".
Anyone got any ideas? I've also tried multiple string elements, adding brackets, braces, square-brackets, space separators, prefixing with 'array(' with no luck.
thanks
Matt

Answer 1

It seems indeed this is not supported. The standard plug-in projects.py has several list attributes, but all of them have a single value as the default. I'll check what can be done to improve here.
However, your approach might be wrong in the first place. The default you are setting only concerns the project editor. That means that if a user uses a default project (like project default is end default) and never goes through the project editor, your attribute example_list_of_files will in fact not exist (and have a default empty value). So it seems that this should in fact be handled in your plug-in, when you query the value of the attribute (like via GPS.Project.get_attribute_as_list). If that function returns an empty list, then use ("a.adb", "b.adb", "c.adb") instead. That way, things work fine even with a default, unedited project.

Answer 2

From the GPS User's Guide:
The tag accepts the following attributes:
[...]
list (boolean, default false)
If true, the project attribute contains a list of values, as opposed
to a single value. An example is the list of source directories in
standard projects.
In your example:
<string type="file" filter="project" default="'a.adb','b.adb','c.adb' " />
This is a single string value. Instead, you should specify a list of string values, like this:
<string type="file" filter="project" default="a.adb" />
<string type="file" filter="project" default="b.adb" />
<string type="file" filter="project" default="c.adb" />