Is there a "null" value that can be used in WDL? Could "else" statements be added to conditionals?

Say I have a task that has different behavior depending on the setting of optional parameters. Is there a null or undefined value in WDL that will cause the defined function to evaluate to false that I can use to effectively hide some of these parameters in the workflow section rather than basically have nearly the exact same task defined twice in my WDL?

In the below example I have a preprocess step that only runs if unprocessed_file is defined and processed_file is undefined. Many correlations use categorical data, and may have a preprocess step to convert continuous data into categorical data via binning.

Additionally, there is result_archive, a zip file that is passed from task to task, with all outputs added to it, until it is finally attached as an attribute to the entity the method configuration was run on.

Because opt_preprocess is optional, the initialization of result_archive can occur either there, or in process, thus requiring a more flexible implementation of process. I do this by having two optional inputs (a File and a String), only one of which should ever be defined.

It would be really useful if there is a way to force the input result_archive to be undefined in the call process as process_alt block below, and the same for result_archive_name in the call process block. Then there would not be as much potential for accidentally breaking things in the Method Config, as my_workflow.process.result_archive_name and my_workflow.process_alt.result_archive would not be accessible to it.

I'm using conditionals in my workflow definition, and as you can see, the code would be a lot cleaner if else statements were available rather than having to test for the opposite state.

task opt_preprocess {
    File unprocessed_file
    String result_archive_name
    String result_archive="${result_archive_name}.zip"

    command {
        preprocess ${unprocessed_file} > preprocessed.dat
        zip -r ${result_archive_name} . -x \
            "fc-[a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9]/*" \
            lost+found/\* \
            "tmp.[a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9]/*" \
            exec.sh
    }

    output {
        File preprocessed_file="preprocessed.dat"
        File result_archive_pkg=result_archive
    }

    ...
}

task process {
    File input_file
    File? result_archive
    String? result_archive_name

    command {
        process  ${input_file} > processed.dat
        result_archive_name=${if defined(result_archive) then basename(result_archive) else result_archive_name}
        ${defined(result_archive)} && mv ${result_archive} .
        zip -r $result_archive_name . -x \
            "fc-[a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9]/*" \
            lost+found/\* \
            "tmp.[a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9]/*" \
            exec.sh
    }

    output {
        File processed_file="processed.dat"
        File result_archive_pkg="${if defined(result_archive) then basename(result_archive) else result_archive_name}"
    }

    ...
}

task postprocess {
    File in_file
    File result_archive
    String result_archive_name=basename(result_archive)

    command {
        postprocess ${in_file}
        mv ${result_archive} .
        zip -r $result_archive_name . -x \
            "fc-[a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9]-[a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9]/*" \
            lost+found/\* \
            "tmp.[a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9][a-zA-Z0-9]/*" \
            exec.sh
    }

    output {
        File result_archive_pkg="${result_archive_name}"
    }

    ...
}

workflow my_workflow {
    File? preprocessed_file
    File? unprocessed_file
    String result_archive_name="my_workflow"

    if (defined(preprocessed_file)) {
        call process as process_alt {
            input: input_file=preprocessed_file,
                   result_archive_name=result_archive_name
        }
    }

    if (!defined(preprocessed_file) && defined(unprocessed_file)) {
        call opt_preprocess {
            input: unprocessed_file=unprocessed_file
        }
        call process {
            input: input_file=opt_preprocess.preprocessed_file,
                   result_archive=opt_preprocess.result_archive_pkg
        }
    }

    post_file="${if defined(process.out_file) then process.out_file else process_alt.out_file}"
    result_archive="${if defined(process.result_archive_pkg) then process.result_archive_pkg else process_alt.result_archive_pkg}"

    call postprocess {
        input: in_file=post_file,
               result_archive=result_archive
    }

    output {
        postprocess.result_archive_pkg
    }
}