Swift: The Easy Scripting Language for Parallel Computing (Linux.com)

The crux is:

(from http://www.ci.uchicago.edu/swift/guides/release-0.94/user...)

>Non-array variables are single-assignment, which means that they must be assigned to exactly one value during execution. A procedure or expression will be executed when all of its input parameters have been assigned values. As a result of such execution, more variables may become assigned, possibly allowing further parts of the script to execute.

[...]

>Arrays in Swift are more monotonic - a generalisation of being assignment. Knowledge about the content of an array increases during execution, but cannot otherwise change. Each element of the array is itself single assignment or monotonic (depending on its type). During a run all values for an array are eventually known, and that array is regarded as closed.