Question

par is declared as:

par  :: a -> b -> b

Notice, that argument one is thrown away. In order to use par you need to play tricks like using the same expression multiple times.

If its purpose is to execute a and b in parallel, why wasn’t it defined like this?:

par  :: (a, b) -> (a, b)

Taking a tuple of (unevaluated) expressions and returning the same expressions – while they are potentially being materialized on background threads.

It seems the latter model is simpler than the former. Why was the design chosen that way?

Answer 1

In the former, you can easily spark more than two computations,

c1 `par` c2 `par` c3 `par` c4 `pseq` something c1 c2 c3 c4

which would be rather cumbersome in the latter.