[Haskell-cafe] parMap doesn't work fine

[Grigory Sarnitskiy]

Hello!

I've just started using parallel computations in Haskell. parMap works fine, it is so easy to use. However, parMap fails with functions returning lazy structures, e.g. tuples.

This code works as expected:

(parMap rpar) bm tvalues

bm :: Double -> Double is some heavy function. But if I want to return list of pairs (t, bm t) it doesn't use cpu cores fine (only one is in use):

(parMap rpar) (\t -> (t, bm t)) tvalues

The same is valid for functions returning lists. How do I use multiple cores with functions returning tuples?