Space usage

[Simon Peyton-Jones]

| 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