Functional Programming to the rescue? No side effects makes parallelism easy right? It is always safe to speculate on pure code Execute each sub-expression in its own thread? Alas, the 80s dream does not work Far too many parallel tasks, many of which are too small to be worth the overhead of forking them Difficult/impossible for compiler to guess which are worth forking Idea: Give the user control over which expressions might run in parallel No side effects makes parallelism easy, right? - It is always safe to speculate on pure code. - Execute each sub-expression in its own thread?  Alas, the 80s dream does not work. - Far too many parallel tasks, many of which are too small to be worth the overhead of forking them. - Difficult/impossible for compiler to guess which are worth forking. Idea: Give the user control over which expressions might run in parallel