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