Related Constructs
An I-var (as in the language Id) is a future with blocking semantics as defined above. An I-structure is a data structure containing I-vars. A related synchronization construct that can be set multiple times with different values is called an M-var. M-vars support atomic operations to take or put the current value, where taking the value also sets the M-var back to its initial empty state.
A concurrent logic variable is similar to a future, but is updated by unification, in the same way as logic variables in logic programming. Thus it can be bound more than once to unifiable values (but cannot be set back to an empty or unresolved state). The dataflow variables of Oz act as concurrent logic variables, and also have blocking semantics as mentioned above.
A concurrent constraint variable is a generalization of concurrent logic variables to support constraint logic programming: the constraint may be narrowed multiple times, indicating smaller sets of possible values. Typically there is a way to specify a thunk that should be run whenever the constraint is narrowed further; this is necessary to support constraint propagation.
Read more about this topic: Futures And Promises
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