Why do we need 'seq' or 'pseq' with 'par' in Haskell?

Question

I\'m trying to understand why we need all parts of the standard sample code:

a `par` b `pseq` a+b

Why won\'t the following be sufficient?

Answer 1

Ok. I think the following paper answers my question: http://community.haskell.org/~simonmar/papers/threadscope.pdf

In summary, the problem with

a `par` b `par` a+b 

and

a `par` a+b

is the lack of ordering of evaluation. In both versions, the main thread gets to work on a (or sometimes b) immediately, causing the sparks to "fizzle" away immediately since there is no more need to start a thread to evaluate what the main thread has already started evaluating.

The original version

a `par` b `pseq` a+b

ensures the main thread works on b before a+b (or else would have started evaluating a instead), thus giving a chance for the spark a to materialize into a thread for parallel evaluation.