[Haskell-cafe] Exception for NaN

[Daniel Fischer]

On Friday 13 May 2011 01:19:52, Ganesh Sittampalam wrote:
> On 12/05/2011 19:41, Nick Bowler wrote:
> > On 2011-05-12 21:14 +0400, Grigory Sarnitskiy wrote:
> >> I don't want NaN to propagate, it is merely stupid, it should be
> >> terminated.
> > 
> > NaN propagation is not stupid.  Frequently, components of a
> > computation that end up being NaN turn out to be irrelevant at a
> > later point, in which case the NaNs can be discarded.
> 
> Unfortunately, if a NaN reaches a comparison operation, it can lead to
> an end result that doesn't contain NaNs, but was still influenced by
> one.
> 
> Ganesh

For example:

Prelude Data.List> maximum [0,-1,0/0,-5,-6,-3,0/0,-2]
0.0
Prelude Data.List> minimum [0,-1,0/0,-5,-6,-3,0/0,-2]
-2.0
Prelude Data.List> sort [0,-1,0/0,-5,-6,-3,0/0,-2]
[-6.0,-5.0,-2.0,NaN,-3.0,NaN,-1.0,0.0]

As usual, when dealing with floating point numbers, you have to know what 
you're doing. There are situations where the appearence of NaNs is 
harmless, in others, they're poisonous.