On Sunday 08 January 2006 06:43 am, Tim Walkenhorst wrote:
{- Disclaimer: I'm rather new to Haskell and completely new to this board. I may not use the correct terminology in all cases, but I hope my intention becomes clear when you have a look at the code-snippets. -}
Hey ho,
Is there any way two find out whether two variables refer to the same cell? That is I'm looking for a notion of identity (compareable to == in Java) as opposed to equality (== in Haskell).
Another poster has already replied with a link to a long answer. The short answer is "no, not really". An attempted rationale: The semantics of such an "identity" operator are unclear. The operator could be a test for leibenz equality (ie, structural equality, ie replaceability in all contexts), but such an operator cannot be decidable (proof due to Church), so that wouldn't help in your situation anyway. The general usefulness of such an operator is debatable. We could instead provide an implementation-dependent operation that tests for identical heap location, but such an operator would give different results with different Haskell implementations and would be sensitive to optimizations. That would either be a horribly broken operator or (to fix the brokeness) greatly constrain the avaliable optimizations and implementation strategies. For the problem at hand (involving the STLC), you will not be able to type omega because omega is a non-normalizing closed term and STLC has the strong normalization property. You will have to move to a more expressive calculus to type omega. Rob Dockins