Example for using expressions with exclusive parameters

[pvanheus]

If you use exclusive parameters (as documented in the Advanced Parameters) section together with expressions you might end up with a situation where an expression is undefined. An example of such non-working CWL is here.

The solution is to use logical OR to provide a default value to the expression (thanks to @tetron) for explaining this. I.e. expressions like $(inputs.output.output_file || []). This works because expressions are ECMAScript expressions, something I didn't realise until last night.

I am preparing a pull request to add this explanation to the User Guide but I cannot right now come up with a simple example illustrating this, so I'm opening this issue to ask if anyone can come up with a straightforward example and also to remind myself to complete this addition to the guide.

[kinow]

Link to new Advanced Inputs section, which has the same contents as the old Advanced Parameters linked above: https://www.commonwl.org/user_guide/topics/inputs.html#advanced-inputs