[Haskell-cafe] deepSeq vs rnf

Cale Gibbard cgibbard at gmail.com
Sun Oct 22 23:19:14 EDT 2006

On 22/10/06, Chad Scherrer <chad.scherrer at gmail.com> wrote:
> Hi,
> I had posted this question a while back, but I think it was in the
> middle of another discussion, and I never did get a reply. Do we
> really need both Control.Parallel.Strategies.rnf and deepSeq? Should
> we not always have
> x `deepSeq` y == rnf x `seq` y
> ?
> Maybe there's a distinction I'm missing, but it seems to me they're
> basically the same.

I agree, they are the same. The Strategies library also gives much
more general operations for working with strictness and
parallelisation. That library seems to need more love, I think it's a
great idea, but it doesn't really get noticed all that much. The
Hierarchical libraries documentation for it is a little lacking -- it
doesn't even provide a reference or link to the paper, and many of the
combinators, as well as the general idea of how to use it are
undocumented from there. It also spuriously contains an Assoc
datatype, which if I recall correctly, was an example from the paper,
but doesn't really belong in the library as far as I can tell. It
would also be really nice to see the list of instances for the NFData
class expanded to include other datatypes in the libraries, possibly
also with compiler support for deriving, since it's mostly

Speaking of boilerplate and the scrapping thereof, Data.Generics could
theoretically also be used to write a relatively generic rnf/deepSeq,
but in my attempts, it seems to be much much slower than using a
specific normal form class. Here's my code from quite a while back. As
I recall, it's semantically correct, but ran about an order of
magnitude slower. There might be a much better way to do it, I don't
really know Data.Generics all that well.

rnf :: (Data a) => a -> ()
rnf x = everything (\x y -> x `seq` y) (\x -> x `seq` ()) x

deepSeq x y = rnf x `seq` y

f $!! x = rnf x `seq` f x

 - Cale

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