In my tool-config I try to use conditional number of output files. Is this possible? For input files the <CONDITIONAL> and <WHEN> statements are able to do that. What about output files?
If I have four listed I will always get 4 history items in my work flow. If I only generate 3 I still get 4 items of which one contains no data. I want to prevent this cluttering by providing the user a selective set of output files.

Hi Alex,

Yes, it is possible to have different numbers of tool outputs using the <filter> tag for outputs. E.g. in the cuffcompare wrapper this code is used:

--
        <data format="tabular" name="input2_tmap" label="${tool.name} on ${on_string}: data ${second_gtf.input2.hid} tmap file">
            <filter>second_gtf['use_second_gtf'] == "Yes"</filter>
        </data> 
--

In practice, you should be able to use any boolean condition in <filter>. 

This doesn't seem to be documented on the tool config wiki page right now ( http://bitbucket.org/galaxy/galaxy-central/wiki/ToolConfigSyntax ), but we'll add it soon.

Best,
J