| > 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 :-) It does; see my reply below | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Simon Peyton-Jones | Sent: 17 August 2004 17:25 | To: Malcolm Wallace; glasgow-haskell-users@haskell.org | Subject: RE: Space usage | | | | You probably mean | | J. Sparud, "Fixing Some Space Leaks without a Garbage Collector", | FPCA'93. | | http://citeseer.ist.psu.edu/sparud93fixing.html | | Indeed. | | | as implemented in hbc. 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. | | GHC does this, but it does not always work. What starts as | let x = fst t in | let h = case x of { True -> 1; False -> 2 } in | ... | may become | let h = case t of { (x,_) -> case x of {True->1;False->2} } | ... | and now there is no selector thunk any more. So the selector stuff is | fragile to otherwise-valuable optimisations. And that's what's happening | in this case. A non-optimising compiler is fine. | | Simon | | Regards | -- | Adrian Hey | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users