Hello Somewhat on the topic of optimistic evaluation, I've just thought of another way to evaluate thunks. When the program is about to block on some IO, what if we start a thread to evaluate (any) unevaluated thunks. We'll have additional system thread, but the blocked one will not actually consume any processor time. This would take place only when the program is compiled as threaded and run with -N k, k>1. The RTS knows at least about some operations that will block, those which IO operations are implemented with. for example. It could merely start a process of evaluating any (or something more clever) outstanding thunks right before going into one of those operations and stop it when it's back. Of course, it's not like optimistic evaluation because we don't avoid creating thunks. But in a sense it's similar. It could also be compared with incremental garbage collection :) Has something like that been done, discussed?