The following example:

params.outdir = 'results'

workflow {
    workflow.onComplete {
        if( workflow.success )
            log.info "Success! View results: $params.outdir"
        else
            log.info "Failure!"
    }
}

Fails with an error that workflow is null in the onComplete handler, even though it should be in scope. The same error will happen for params if it is accessed first. These handlers should work regardless of whether they are defined at the script level or in the entry workflow.

There are two workarounds:

1. Assign the onComplete handler: workflow.onComplete = { ...
2. Declare local variables that reference workflow and params:

   def myParams = params
   def myWorkflow = workflow
   workflow.onComplete {
     // ...
   }