What happens when using this to create variables? Is this a valid workaround?

[jrm5100]

I would like to create and output a variable in a process (a list of strings, for example) and also pass that variable into a function from my lib.

This example is about as simplified as I could manage.

process Echo {
  
  input:
    val message

  output:
    val output_list, emit: output_list_ch

  script:
    output_list = []
    ["Foo", "Bar"].each { s -> output_list.add(s) }
    def cmd = Test.echo(output_list)
    """
    $cmd
    """
}

The lib contains

Class Test {
    public static String echo(List<String> input) {
        "echo " + input.join(' ')
    }
}

This results in Variable `output_list` already defined in the process scope @ line 12, column 25.

If def is added at the beginning:

script:
    def output_list = []
    ["Foo", "Bar"].each { s -> output_list.add(s) }
    def cmd = Test.echo(output_list)
    """
    $cmd
    """

The output doesn't work: Missing value declared as output parameter: output_list

Digging into nextflow code, the "already defined" error doesn't apply to variables if they start with this.:

script:
    this.output_list = []
    ["Foo", "Bar"].each { s -> this.output_list.add(s) }
    def cmd = Test.echo(this.output_list)
    """
    $cmd
    """

This appears to work as I would like, but am I missing anything? Is this referring to the specific instance of the process, or could the namespace get mixed up somehow?

[jrm5100]

I ended up digging into this and found that this refers to the Script class created for the process, which is global for all uses of that process in the current run. So it may or may not work depending on how you use it.