[Haskell] Image manipulation

[Simon Marlow]

Stefan O'Rear wrote:
> On Wed, Oct 31, 2007 at 10:58:40AM +0100, Ketil Malde wrote:
>> Bjorn Lisper <lisper at it.kth.se> writes:
>>
>>> Wait, Jerzy. Haskell implementations do not have to be lazy as long as they
>>> preserve the more abstract semantics. Optimistic evaluation has been
>>> considered, for instance by Robert Ennals.
>> I'm not sure if the GC hack proposed by Wadler¹ that lets the garbage
>> collector replace "fst (a,b)" with "a" (and similar for other unchecked
>> selectors) counts as optimistic evaluation, but I wonder what the
>> status of this is.  GHC doesn't seem to do it, and I wonder if there
>> is any particular reason?  Too specific?
> 
> GHC nominally does do it (look for 'selector thunks' in the RTS and
> commentary), but it doesn't work and we don't know why.
> 
> http://hackage.haskell.org/trac/ghc/ticket/1038

GHC HEAD has a fix for the example in that ticket, and 6.8.1 has a 
workaround (I bumped the recursion depth limit enough to make it run in 
constant space).  Do you have an example that still doesn't work, 
particularly with the HEAD?

Cheers,
	Simon