Conditional Tasks

[effigies]

In discussions at the Brainhack, the question of conditional tasks came up again, so it seems worth reviving.

I think the most comprehensive previous discussion was in nipy/nipype#2539, but I think we should probably start with desiderata and the state of the current API and see where we can get.

Desiderata

• Run-time conditions should be possible
• Build-time conditions should be detected and avoid generating unnecessary tasks
• ...

Questions

• Should conditionals be on/off, if/else, arbitrary branches (e.g. switch statements)? All of the above?
• Should it be a wrapper around a task e.g., wf.add(Conditonal(mytask)), or a parameter e.g., wf.add(mytask, cond=...)?
• ...