[Haskell-cafe] parallel Haskell

Claude Heiland-Allen claude at mathr.co.uk
Thu Jul 6 17:16:30 UTC 2017

Hi Dennis,

On 06/07/17 17:53, Dennis Raddle wrote:
> I have a program which does backtracking search in a recursive
> function.  I followed the chapter in "Real World Haskell" to parallelize it.
> There's no effect from the R.W.H. ideas. Can I get some suggestions as
> to why? 

You can get timing and other useful diagnostics by compiling with
-rtsopts and running with +RTS -s, no need to measure CPU time in your
own program.

> force :: [a] -> ()
> force xs = go xs `pseq` ()
>   where go (_:xs) = go xs
>         go [] = 1

This force doesn't do enough, it just walks the spine. Try this, which
forces the elements as well as the shape:

force :: [a] -> ()
force xs = go xs `pseq` ()
  where go (x:xs) = x `pseq` go xs
        go [] = 1



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