Ease optional access with scatter-like block

[cjllanwarne]

Equivalent to scatter blocks for arrays, I propose a new ifDefined block for optionals:

workflow foo {
  String? bar
  ifDefined (String concreteBar in bar) {
    call baz { input: x = concreteBar }
  }
  output {
    # Just like outputs from inside a scatter become array outputs, isDefined creates optional outputs:
    String? baz_out = baz.out
  }
}

task baz {
  String x
  command { ... }
  output { String out = ... }
}

[cjllanwarne]

NB this could be pure syntactic sugar for:

workflow foo {
  String? bar
  if (length(select_all([bar])) == 1) {
    String concreteBar = select_first([bar])
    call baz { input: x = concreteBar }
  }
  output {
    String? baz_out = baz.out
  }
}

[katevoss]

@cjllanwarne can you explain when a user would encounter this? Was it something you heard requested, or was it a "we should improve this because we know it's not as good as it could be"?

[cjllanwarne]

I'm moving this WDL spec change issue to openwdl/wdl#187 and closing this one.