[Haskell-cafe] Re: Haskell-Cafe Digest, Vol 39, Issue 6

[Lemmih]

On 11/2/06, Chad Scherrer <chad.scherrer at gmail.com> wrote:
> Lemmih wrote:
> > Using 'seq' is generally a bad idea. It can worsen the performance if
> > not used carefully and GHCs strictness analyser is usually good
> > enough.
>
> Is GHC.Conc.pseq any better? Usually the whole point of making things
> more strict is to optimize performance for pieces you know will be
> evaluated anyway. It's frustrating if there's not a consistent way to
> do this that works well.

pseq is just as bad. The problem is excessive use of strictness
annotations in the hope of a magical performance improvement.
Strictness annotations should be used with care and only placed where
they're needed.

> Lately, I've been using lots of strictness annotations and bang
> patterns - are there non-obvious places this could slow things down?

In the case of the MersenneTwister, forcing 'y4' from 'next32' would
evaluate it almost 10,000,000 times more than needed.

> Would it be possible for the type system to distinguish at compile
> time whether something would need to be evaluated, and optimize away
> redundant `seq`s? Maybe this is what the strictness analyzer does
> already.

Evaluating the seq's isn't costly, afaik.

-- 
Cheers,
  Lemmih