Optional Types

A type may have a ? postfix quantifier, which means that its value is allowed to be undefined without causing an error. A declaration with an optional type can only be used in calls or functions that accept optional values.

Multi-level optionals are not allowed. A value cannot have multiple levels of optionality, for example, Int?? is not a valid type. However, nested optionals within compound types are allowed, such as Array[String?]?, where each ? applies to a different structural level of the type.

WDL has a special value None whose meaning is "an undefined value". The None value has the (hidden) type Union, meaning None can be assigned to an optional declaration of any type.

An optional declaration has a default initialization of None, which indicates that it is undefined. An optional declaration may be initialized to any literal or expression of the correct type, including the special None value.

Example: optionals.wdl

version 1.3

workflow optionals {
  input {
    Int certainly_five = 5      # an non-optional declaration
    Int? maybe_five_and_is = 5  # a defined optional declaration

    # the following are equivalent undefined optional declarations
    String? maybe_five_but_is_not
    String? also_maybe_five_but_is_not = None
  }

  output {
    Boolean test_defined = defined(maybe_five_but_is_not) # Evaluates to false
    Boolean test_defined2 = defined(maybe_five_and_is)    # Evaluates to true
    Boolean test_is_none = maybe_five_but_is_not == None  # Evaluates to true
    Boolean test_not_none = maybe_five_but_is_not != None # Evaluates to false
    Boolean test_non_equal = maybe_five_but_is_not == also_maybe_five_but_is_not
  }
}

{}

{
  "optionals.test_defined": false,
  "optionals.test_defined2": true,
  "optionals.test_is_none": true,
  "optionals.test_not_none": false,
  "optionals.test_non_equal": true
}

For more details, see the sections on Input Type Constraints and Optional Inputs with Defaults.