compare on Double

Robert Dockins robdockins at
Thu Mar 22 10:25:56 EDT 2007

On Mar 22, 2007, at 8:21 AM, Ketil Malde wrote:

> Neil Mitchell wrote:
>> I'm still not happy with adding more incompleteness into something
>> that intuitively feels safe - compare is pretty benign. Having a  
>> crash
>> on division is something that does appeal to me more.
> Are you suggesting that 0.0/0.0 should crash also?  I'm no expert,  
> but think returning a NaN is according to the floating point  
> standard (IEEE 754), and I'm not convinced Haskell should break that.

IIRC, signaling NaNs (ie, throwing an error), are also allowed under  
IEEE 754, albeit less common.  In the context of Haskell, I think it  
makes a lot more sense.  Remove NaN as a value and Float/Double  
regain a total orders, and real equivalence relations (not PERs).   
Also, if I get a NaN in a calculation, I'd personally rather get an  
exception than  have NaNs infect the entire computation and only find  
out later.

Another potentially attractive option is to recast Eq as being only a  
PER (remove the reflexivity assumption) and allow Ord to be a partial  
order.   Or maybe:

class PEq a where { (==) (/=) :: a -> a -> Bool }
class PEq a => Eq a

class POrd a where { compare :: ....; (<=) :: ...; etc }
class POrd a => Ord a

so we introduce new classes with the relaxed semantics and make Ord/ 
Eq subclasses with no new operations.  Although, this is obviously  
Haskell' territory.

>   If you use floating point, you need to know the pitfalls anyway.   
> Maybe comparing NaNs should give random results? :-)
> The bad thing is that comparison can hide the NaNs (for arithmetic,  
> NaNs will be contagious, so you'll likely get a NaN riddled  
> output).  One thing I remember seeing (from Fortran compilers?) is  
> that when a program execution involves a NaN, it is reported on  
> termination, even if the NaN was in an intermediate calculation and  
> not part of the final result.  Something like that could perhaps be  
> useful if the current behavior is kept?
> -k

Rob Dockins

Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
           -- TMBG

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