How to use the output of a process to check for a condition?

[saberiato]

Hi,
I can't find a way to use the output of a process in a condition...
Consider we have this process:

process myTask {
    output:
    env FOO, emit: FOO

    script:
    '''
    FOO=0
    '''
}

And, I want to check the output FOO in a condition:

if(myTask.out.FOO == 0) {
...
}

But, It doesn't work... I tried toInteger(), toList()[0], first(), ..., but couldn't find a way to do that...

Also, I need to check if the output of a process is an empty file in a condition again... but no luck...

I appreciate any help!

[mribeirodantas]

The appropriate way to handle channels is to use channel operators. You can use the branch operator (docs here) for checking a condition. It will create N new channels, where N is the number of conditions that you described, distributing the channel elements according to your condition. If you want something more elaborate, you can also use a Nextflow process. If you want a process to only create tasks based on a condition, you can also use the when block (docs here).

The rule of thumb to remember: If you want to do something with a channel, you need to use a channel operator (or a process) 😉

[saberiato]

Thanks @mribeirodantas.
I used branch operator, and it worked.