[Haskell-cafe] Example programs with ample use of deepseq?

Edward Z. Yang ezyang at MIT.EDU
Mon Jan 7 13:20:43 CET 2013

There are two senses in which deepseq can be overkill:

1. The structure was already strict, and deepseq just forces another
no-op traversal of the entire structure.  This hypothetically affects
seq too, although seq is quite cheap so it's not a problem.

2. deepseq evaluates too much, when it was actually sufficient only to
force parts of the structure, e.g. the spine of a list.  This is less
common for the common use-cases of deepseq; e.g. if I want to force pending
exceptions I am usually interested in all exceptions in a (finite) data
structure; a space leak may be due to an errant closure---if I don't
know which it is, deepseq will force all of them, ditto with work in
parallel programs.  Certainly there will be cases where you will want snip
evaluation at some point, but that is somewhat difficult to encode
as a typeclass, since the criterion varies from structure to structure.
(Though, perhaps, this structure would be useful:

data Indirection a = Indirection a
class DeepSeq Indirection
    rnf _ = ()


Excerpts from Joachim Breitner's message of Mon Jan 07 04:06:35 -0800 2013:
> Dear Haskellers,
> I’m wondering if the use of deepseq to avoid unwanted lazyness might be
> a too large hammer in some use cases. Therefore, I’m looking for real
> world programs with ample use of deepseq, and ideally easy ways to test
> performance (so preferably no GUI applications).
> I’ll try to find out, by runtime observerations, which of the calls ot
> deepseq could be replaced by id, seq, or „shallow seqs“ that, for
> example, calls seq on the elements of a tuple.
> Thanks,
> Joachim

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