RE: Optimizations for mutable structures?

On 07 December 2005 13:38, Malcolm Wallace wrote:

Jan-Willem Maessen writes:

...

- Fetch elimination for imperative reads: writeIORef r e >> acts >> readIORef r === writeIORef r e >> acts >> return e

This transformation is valid only on single-threaded systems. If there is any possibility of an IORef being shared across threads, you are out of luck.

(assuming 'acts' doesn't modify 'r'). No, Jan's transformation is correct even in a multithreaded setting. It might eliminate some possible outcomes from a non-deterministic program, but that's ok. There's no requirement that all interleavings according to the semantics have to be implemented. This is a hard property to state precisely, indeed we gave up trying to in the concurrency/FFI paper: http://www.haskell.org/~simonmar/papers/conc-ffi.pdf, see Section 6.1. Cheers, Simon