LISP and Symbolic Computation, 7(1)83-110
SubcontinuationsRobert Hieb, Indiana University Computer Science Department, Bloomington, IN 47405
R. Kent Dybvig, Indiana University Computer Science Department, Bloomington, IN 47405
Claude W. Anderson, III, Rose-Hulman Institute of Technology Computer Science Department, Terre Haute, Indiana 47803
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Abstract: Continuations have proven to be useful for
implementing a variety of control structures, including exception
handling facilities and breadth-first searching algorithms. However,
traditional continuations are not useful in the presence of
concurrency, because the notion of the rest of the computation
represented by a continuation does not in general make
sense. Traditional continuations can also be difficult to use in
nonconcurrent settings, since their global nature is sometimes
problematic. This article presents a new type of continuation, called
a subcontinuation. Just as a traditional continuation represents the
rest of a computation from a given point in the computation, a
subcontinuation represents the rest of a subcomputation from a given
point in the subcomputation. Subcontinuations may be used to control
tree-structured concurrency by allowing nonlocal exits to arbitrary
points in a process tree and allowing the capture of a subtree of a
computation as a composable continuation for later use. In the absence
of concurrency the localized control achievable with subcontinuations
makes them more useful than traditional continuations. Keywords: continuations, control structure, control delimiters, concurrency, engines, scheme |
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