[Haskell-cafe] HANSEI in Haskell?

oleg at okmij.org oleg at okmij.org
Thu Feb 24 09:30:58 CET 2011

The topic of HANSEI in Haskell does come up from time to time. In
fact, there was a Haskell edition of the first version of Hansei:


It was written to see how the code would look in Haskell, and how
memoization (inherent in lazy evaluation of GHC) may help. The code
contains some of the tests from the HANSEI distribution. The
experience showed that lazy evaluation hurts -- a lot. The problem is
not in lack of seq -- adding seq would make things even worse. There
is a bit of explanation at the end of probM.hs. The gist of the
problem is that approximate inference (sampling, to be precise) trades
space for time. The search tree is way too large to fit into
memory. Therefore, it is better to recompute the values (the branches
of the tree) than to keep storing them. That is precisely the opposite
to the trade-off of lazy evaluation. The end result is attempting to
allocate gigabytes (really!), even for toy problems. We can get around
the problem by using thunks explicitly. But then we lose the remaining
benefits of Haskell syntax (the ones that are left after we have to
switch to the monadic notation). OCaml becomes quite compelling then.

It must be stressed that laziness in general is _crucial_ for solving
even moderately complex problems. However, the needed laziness is NOT
of the kind provided by GHC. We need memoization specific to a
possible world, rather than the one that affects all possible
worlds. GHC gives only the latter. The sort of laziness needed for
non-deterministic programming calls for first-class store.  It can be
emulated, albeit imperfectly, for example, as was described in the
ICFP09 paper. Hansei uses a quite similar idea.  Ideally first-class
memory is provided as built-in, since it requires interaction with
garbage collection.  Greg Morrisett has written extensively on this
topic and done a few implementations; see for example,

Sadly, the topic has been forgotten.

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