Hidden Types

Types that can only be instantiated by the execution engine

A hidden type is one that may only be instantiated by the execution engine, and cannot be used in a declaration within a WDL file. There is currently only one hidden type, Union; however, in WDL 2.0, Object will also become a hidden type.

ยงUnion

The Union type is used for a value that may have any one of several concrete types. A Union value must always be coerced to a concrete type. The Union type is used in the following contexts:

  • It is the type of the special None value.
  • It is the return type of some standard library functions, such as read_json.
  • It is the type of some reserved runtime attributes.