On Thu, Apr 22, 2010 at 11:34 AM, Barak A. Pearlmutter
Comparison of exceptional IEEE floating point numbers, like Nan, seems to have some bugs in ghci (version 6.12.1).
Arguably, the "bug" in question is the mere existence of Eq and Ord instances for IEEE floats. They don't, can't, and never will work correctly. A similar topic was discussed here not too long ago; IEEE floating point so-called "numbers" lack reflexive equality and associativity of addition and multiplication, among other properties one might take for granted in anything calling itself a number. If memory serves me, someone provided this informative link in the previous thread: http://docs.sun.com/source/806-3568/ncg_goldberg.html That said, given that Haskell seems to be following the well-established tradition of willfully disregarding the inconvenient aspects of floats as far as the type system is concerned, I would say that compare returning GT is particularly unintuitive. If something must stand in for a result of "arguments are non-comparable", EQ is marginally more appealing, as it is expected to be reflexive, as "non-comparable" is. An invalid comparison evaluating to _|_ is arguably more correct, but I personally find the idea of introducing more bottoms rather distasteful. On the other hand, crashing the program is usually better than incorrect results, so in this case it's probably justified. The only correct solution would be to strip floating point types of their instances for Ord, Eq, and--therefore, by extension--Num. For some reason, no one else seems to like that idea. I can't imagine why... - C.