Adrian Hey wrote:
On Tuesday 17 Aug 2004 5:11 pm, Malcolm Wallace wrote:
It is also possible to use Wadler's garbage-collector fix for this space leak, as implemented in nhc98. P Wadler, "Fixing a Space Leak with a Garbage Collector", SP&E Sept 1987.
When the GC discovers a selector function applied to an evaluated argument, it "evaluates" the selector on-the-fly by just swizzling pointers. It needs some co-operation from the compiler to make selector functions look obvious, but that isn't too difficult.
So ghc doesn't do this (or something better)? I'm surprised because it seems like a really basic and simple feature to me. I implemented a toy FPL a few years ago and even my gc incorporated this optimisation. It's a bit strange that this should have been overlooked considering in all other respects ghc is far more sophisticated than my efforts were :-)
As Simon pointed out, it's not so easy to do this with an optimizing compiler that may inline selector functions. The "right" way to do this is to have specialized code that runs during GC for each thunk. This was investigated in Christina von Dorrien's licentiate thesis "Stingy evaluation". -- Lennart