Evaluation of Workflow Elements

As with tasks, declarations can appear in the body of a workflow in any order. Expressions in workflows can reference the outputs of calls, including in input declarations. For example:

Example: input_ref_call.wdl

version 1.2

task double {
  input {
    Int int_in
  }

  command <<< >>>

  output {
    Int out = int_in * 2
  }
}

workflow input_ref_call {
  input {
    Int x
    Int y = d1.out
  }

  call double as d1 { int_in = x }
  call double as d2 { int_in = y }

  output {
    Int result = d2.out
  }
}

{
  "input_ref_call.x": 5
}

{
  "input_ref_call.result": 20
}

The control flow of this workflow changes depending on whether the value of y is provided as an input or it's initializer expression is evaluated:

• If an input value is provided for y then it receives that value immediately and d2 may start running as soon as the workflow starts.
• In no input value is provided for y then it will need to wait for d1 to complete before it is assigned.