A \future" is a typed synchronization mechanism used to represent a value that will come into existence at some point in time after the creation of the future. In this paper, a polymorphic future type is introduced that di ers from previous mechanisms in that it provides a rst-class, extensible facility for expressing synchronized access to the typed result value of a concurrent computation. The proposed mechanism requires write-once/read-many synchronization, which is implemented in terms of common thread synchronization primitives, thereby ensuring a high-degree of e ciency. The type de nition and type specialization features of C++ are used to illustrate how one would implement a polymorphic future type in a strongly typed object-oriented language supporting polymorphic type de nitions.
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