[Haskell] Optimistic Evaluation?

Hi, I remember once reading a paper by SPJ about Optimistic Evaluation that is somewhere between lazy and strict. If I remember correctly, they even implemented it in GHC. Now, it seems that current version of GHC doesn't support optimistic evaluation (not even as an optional feature), so my question is: what happened to OE? Was it that it seemed like a good idea on paper but not in reality? Or was it too costly to maintain? Or it didn't deliver the goods it promised? I'm just curious. Regards, Lajos Nagy