in reply to
Re^5: GHC shuffle more efficient than Perl5.
in thread Why is the execution order of subexpressions undefined?
Thanks for your kind words. In light of this thread's topic, I'd like to add that, because functions in pure FP never have side-effects, their reduction order can be entirely undefined. Actions defined by those pure functions, however, are defined as always sequential when main is executed.
This fact makes concurrency and automatic parallelization much, much easier to reason about; as an example, you can trivially write deadlock-free, composable, highly efficient concurrent code, using shared variables and inter-thread channels, without worrying about side effects going astray.
See http://homepages.inf.ed.ac.uk/wadler/linksetaps/slides/peyton-jones.ppt for more information... I think it will be enlightening :-)