never used in multi-threaded situation.
Erm, nearly all my code in Haskell is multi-threaded. One of the main reasons why I am using haskell is the low-cost light weight multi-threading. Surely this is a big win for Haskell on SMP/Numa machines - which are surely the future - as even Intel have realised they can't just keep ramping up clock speeds and have gone multi-core for their next pentium iteration. Just look at the benchmarks for the multi-cpu opteron machines!
I would hesitate to make any type unsafe for multi-threading by default - I think all the guards should be in as standard ... maybe a compile time (or run-time) flag to replace the guards with NO-OPS for single CPU machines might be a sensible option.
Since there seems to have been a bit of confusion in this thread, I thought I ought to clear it up. The issue is not about single vs. multiple CPU operation. GHC only runs code on a single CPU at the moment, due to the difficulties involved in sharing a heavily used resource (the heap) between multiple threads. The problem was whether DiffArrays should be thread-safe in the Concurrent Haskell sense, which means protecting access to the DiffArray with an MVar. Cheers, Simon