[previously sent by mistake to Simon only - new para at end] "Simon Marlow" writes:

Now, take the original program, but change the creation of m2 to "newMVar ()", i.e. m2 starts off full. Is the transformation valid now? Well maybe, because in some interleavings acts does not block, and we can prove that at compilation time.

I don't think it is valid for a compiler to say that "one possible execution path permits me to remove some code, therefore I will remove that code on all possible execution paths". The example I had in mind was a GUI where the action writeIORef r e >> acts >> readIORef r is intended to capture a situation where first we record some global configuration data for the application, then permit some arbitrary GUI actions to occur, and then we retrieve the configuration data again. My expectation is that the config data /will/ have been changed by some other GUI thread. Surely it cannot be OK for the compiler to silently deliver the original unchanged data here - it goes against the programmer's intention. Surely, if a Haskell programmer is going to write code that explicitly reads from a reference after writing to it, that sequence must 9/10 be intentional: otherwise wouldn't she have just used a let-binding? Regards, Malcolm