Ticket #1239 (closed proposal: wontfix)
compare on exceptional Doubles and Floats should raise an error
let n = 0/0 :: Double in (n `compare` n, n < n, n == n, n > n)
In GHC and YHC this gives
while in hugs it gives
Neither of these is very satisfactory, as I would expect
x `compare` y === EQ => (x == y) === True x `compare` y === GT => (x > y) === True
and it's even less pleasant that the implementations differ for no good reason.
The Haskell report isn't very helpful on how comparing exceptional Doubles should behave, as it doesn't even say you need to have NaN etc:
http://haskell.org/onlinereport/basic.html#sect6.4 The results of exceptional conditions (such as overflow or underflow) on the fixed-precision numeric types are undefined; an implementation may choose error (_|_, semantically), a truncated value, or a special value such as infinity, indefinite, etc.
I think that the right answer is that
n `compare` n
above (and more generally such a comparison for any incomparable Doubles or Flaots) should raise an error (i.e. be _|_).
The changes needed are simple, e.g. for GHC
(D# x) `compare` (D# y) | x <## y = LT | x ==## y = EQ | otherwise = GT
(D# x) `compare` (D# y) | x <## y = LT | x ==## y = EQ | x >## y = GT | otherwise = error "Incomparable values"
Deadline: 1 week after discussion ends.