Less exciting features also need to be done, return types

We have a feature request to support return types and to give back variable names. We currently return an array (of array) of just strings, with no typing. This doesn’t work very well for knowing whether a cell is (for example) unbound. Empty string isn’t the same as unbound. So what can you do?

With direct-access the implementation is easy, we’ll just read it from the SPARQL engine. We have all this info already anyway. For filedescriptor passing with D-Bus we need to marshal it over the protocol.

Although we might come back to this decision short term, we wont yet do it for our “normal” (non-FD passing) D-Bus query method. SPARQL’s type system is different from D-Bus’s, so we shouldn’t try to match them somehow. Any custom format that we’d come up with, would be arbitrary.

Maybe someday we’ll add another “normal” D-Bus method that gives you a big string with SPARQL Query Results in JSON or SPARQL Query Results in XML back. Right now this has no priority for us, plus it would be a lot slower due to serialization. Post 0.9 everybody should be using libtracker-sparql and that’ll select either FD passing or direct-access.

Anyway, this will likely be the API for Sparql.Cursor. The methods get_value_type and get_variable_name got added.

public enum Tracker.Sparql.ValueType {
	UNBOUND, URI, STRING, INTEGER,
	DOUBLE, DATETIME, BLANK_NODE
}

public abstract class Tracker.Sparql.Cursor : Object {
	public Connection connection { get; }
	public abstract int n_columns { get; }
+	public abstract ValueType get_value_type (int column);
+	public abstract unowned string? get_variable_name (int column);
	public abstract unowned string? get_string (int column, out long length = null);
	public abstract bool next (Cancellable? cancellable = null) throws GLib.Error;
	public async abstract bool next_async (Cancellable? cancellable = null) throws GLib.Error;
	public abstract void rewind ();
}

I usually post about work in progress, not about something that is done. Same this time, of course. You can find the branch where we’re working on this here.

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