1. A method performed by a computer system, the method including: defining a plurality of tasks according to a dependency graph; maintaining a plurality of parameter contexts, each associated with a different scope of the tasks; binding a value to a first parameter used in a first of the tasks, including identifying a first of the parameter contexts according to the dependency graph and retrieving the value for the parameter from the identified context; determining a value for assignment to the first parameter during execution of a second of the tasks; and storing the value for the first parameter in the first parameter context; wherein the first parameter context is associated with the second of the tasks.