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[Caml-list] Weird behavior with nan's and min/max
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Date: -- (:)
From: Yaron Minsky <yminsky@c...>
Subject: [Caml-list] Weird behavior with nan's and min/max
min and max have pretty strange behavior when it comes to nan's.  Here's
an example:

# min 3. nan;;
- : float = 3.
# min  nan 3.;;
- : float = nan

When you think about it, the reason for this is clear.  comparisons
involving nan's always return false, so if you simply implement min as
follows:

   if x < y then x else y

the the result will depend on the order.

Now here's the weird bit.  I decided I wanted a polymorphic comparison
that wouldn't have this problem.  But this is a little harder than it
seems, since it turns out that specialized float version of equality is
different from the polymorphic version.  Here's the example:

# let raw_min = min
val raw_min : 'a -> 'a -> 'a = <fun>
# let min x y =
  if not (y = y) then y
  else if not (x = x) then x
  else raw_min x y
;;
        val min : 'a -> 'a -> 'a = <fun>
# let fmin (x:float) y =
  if not (y = y) then y
  else if not (x = x) then x
  else raw_min x y
;;
        val fmin : float -> float -> float = <fun>
# fmin 3. nan;;
- : float = nan
# fmin nan 3.;;
- : float = nan
# min nan 3.;;
- : float = nan
# min 3. nan;;
- : float = 3.

So, does this count as a bug?  Any ideas about how to fix this behavior in
a reasonably efficient way?

Yaron

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