MultiLisp was a functional programming language and dialect of Scheme, extended with constructs for parallel execution and shared memory. These extensions involve side effects, rendering MultiLisp non-deterministic. In addition to its parallel-programming extensions, MultiLisp also had some unusual garbage collection and task scheduling algorithms. Like Scheme, MultiLisp was oriented toward symbolic computation. Unlike some parallel programming languages, MultiLisp incorporated constructs for causing side effects and for explicitly introducing parallelism.
It was designed by Robert H. Halstead in the early 1980s for use on the 32-processor Concert multiprocessor being developed at MIT and implemented in Interlisp. It has influenced the development of the Scheme dialect Gambit [1], and Interlisp-VAX.
PCALL and FUTURE
MultiLisp achieves parallelism with the PCALL macro, where
(PCALL Fun A B C ...)
is equivalent to
(Fun A B C ...)
except that the arguments A, B, C, etc. are explicitly allowed to be evaluated in parallel; this circumvents the usual order of evaluation, which is sequential and left to right. It also makes use of a parallel programming construct called futures, which resembles forking, combined with lazy evaluation. Using this construct, an expression such as
(cons (FUTURE A) (FUTURE B))
can be written, which will overlap the evaluation of the expressions A and B, not only with each other, but with computations that use the result of the cons call, until an operation is performed that requires actual information about the value of A or B.