Is there an easy way to make the processes/workflows in a dsl2 fork, change based on a condition?

[dileep-kishore]

Consider this example:

workflow example_workflow {
    take:
        input_channel
    main:
        input_channel \
        | foo \
        | (bar1 & bar2 & bar3) \
        | mix
}

What I want to do is to change the processes in the fork based on a condition or variable:

variable = ["bar1", "bar2"]
workflow example_workflow {
    take:
        input_channel
    main:
        input_channel \
        | foo \
        // Some condition or function here
        | (bar1 & bar2) \
        | mix
}

Is there any way other than writing if and switch statements for every possible combination?

[pditommaso]

The branch operator may help https://www.nextflow.io/docs/latest/operator.html#branch

[dileep-kishore]

I think that I was a little unclear while framing my question. I don’t want to split the channels but instead would like to create dynamic workflows based on user input or a parameter value:

variable = ["bar1", "bar2"]  // This is the user input
workflow example_workflow {
    take:
        input_channel
    main:
        input_channel \
        | foo \
        | (bar1 & bar2) \  // <- what we want to change based on the value of variables
        | mix
}

If the variable was changed the workflow needs to change to:

variable = ["bar1", "bar2", "bar3"]  // Now we want to execute 3 processes
workflow example_workflow {
    take:
        input_channel
    main:
        input_channel \
        | foo \
        | (bar1 & bar2 & bar3) \  // <- changed from the previous set of 2 processes to 3
        | mix
}

Here bar1, bar2, etc are processes and I want to take in a list of processes as input from the configuration and dynamically do the forking.

[dileep-kishore]

One idea I had was to write multiple workflows using a loop structure and select the appropriate one based on the input. Is there a better way?

[dileep-kishore]

I realized that I can use the when directive on processes. Eg:

process bar1 {
input:
    ...
when:
    "bar1" in variable
output:
    ...
script:
    """
    ...
    """
}