Space usage

[Lennart Augustsson]

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