Simon Peyton-Jones wrote:
I had a look at this. It's an old chestnut: lazy pattern matching. You
have
let ((commands, s), x) = run (read iters) 5
in do ...do something with commands...
print x
Trouble is, the 'x' hangs onto both components of the pair, even though
it only needs one.
It's not enough to force the pair earlier, which you probably tried,
because the state monad you are using is itself lazy, and uses masses of
lazy pattern matching.
You could return the state paired with each item, rather than just the
state at the end, perhaps, or use a stricter state monad.
This lazy-pattern-matching leak is a well-known problem, to which I do
not know a good solution. There was a paper from Chalmers about 8 years
ago about building more cleverness into the compiler, but it amounted to
extending Core with a lazy tuple binding. Fair enough, but quite a big
addition to Core and one I've never done.
Simon, I suggest that you take another look at Jan Sparud's paper.
No extension of Core is needed to implement it. All that is needed
is a strange (impure) primitive function. To maintain type correctness
you also need a family of functions that are really the identity, but
with different types. Read the paper, I think you could add it to ghc
without too much trouble.
-- Lennart
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