Adrian Hey
But seriously, once you admit the possibility that even if x == y it still matters which of x or y is used in expressions than all hell breaks loose. I shudder to think just how much Haskell code there must be out there that is (at best) ambiguious or just plain "broken" if this is a serious possibility.
Just search for "copy" (on ByteStrings). One program of mine was extracting substrings from a large file. Since the file was pretty huge, I used lazy bytestrings for this purpose. Unfortunately, the short substrings I retained pulled with them rather large chunks from the file -- since a bytestring is essentially a pointer to a chunk, an offset, and a length. The solution is "copy", which creates a new string, indistinguishable from within Haskell, but with very different effects on the program. -k -- If I haven't seen further, it is by standing in the footprints of giants