Types
The WDL type system and how types are declared
A declaration is a name that the user reserves in a given scope to hold a value of a certain type. In WDL all declarations (including inputs and outputs) must be typed. This means that the information about the type of data that may be held by each declarations must be specified explicitly.
In WDL all types represent immutable values. For example, a File represents a logical "snapshot" of the file at the time when the value was created. It is impossible for a task to change an upstream value that has been provided as an input - even if it modifies its local copy, the original value is unaffected.