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

a `par` b `par` a+b creates sparks for both a and b, but a+b is reached immediately so one of the sparks will fizzle (i.e., it is evaluated in the main thread). The problem with this is efficiency, as we created an unnecessary spark. If you're using this to implement parallel divide & conquer then the overhead will limit your speedup.

a `par` a+b seems better because it only creates a single spark. However, attempting to evaluate a before b will fizzle the spark for a, and as b does not have a spark this will result in sequential evaluation of a+b. Switching the order to b+a would solve this problem, but as code this doesn't enforce ordering and Haskell could still evaluate that as a+b.

So, we do a `par` b `pseq` a+b to force evaluation of b in the main thread before we attempt to evaluate a+b. This gives the a spark chance to materialise before we try evaluating a+b, and we haven't created any unnecessary sparks.