I've been playing with par and pseq, and I wonder: Is there any reason to use

  a `par` b `pseq` (a + b)

instead of

  a `par` b `par` (a + b)

except that the second version sparks twice instead of just once (which probably degrades performance a bit)? It seems to me that the second variant would work as well: The main thread would block on one of the sparked computations, but the other one would still be evaluated in parallel.

The second variant seems to have one additional advantage: If the function that combines 'a' and 'b' isn't strict (and perhaps we don't know that in advance), the main thread won't be blocked by evaluating the computation it doesn't need. Like

    a `par` b `pseq` (const a b)

will block until both 'a' and 'b' are evaluated, but

will finish as soon as 'a' is evaluated.

I found this link on SO:

Why do we need 'seq' or 'pseq' with 'par' in Haskell? <http://stackoverflow.com/q/4576734/1333025>

but it doesn't really solve this objection.

  Thanks for help,

  Petr Pudlak