Proposal: New Eq and Ord instances for Double and Float

Simon Marlow marlowsd at
Thu Nov 10 17:48:33 CET 2011

On 10/11/2011 16:28, Roman Leshchinskiy wrote:
> Simon Marlow wrote:
>> On 27/09/2011 10:02, Roman Leshchinskiy wrote:
>>> Colin Paul Adams wrote:
>>>> A quick google for signalling NaNs seems to suggest that on the x86
>>>> architecture, you have to set a flag to raise an exception when
>>>> encountering signalling NaNs. Otherwise they are just treated as quiet
>>>> NaNs.
>>>> But you have to create the signalling NaNs manually. They are not
>>>> created as a result of arithmetic operations.
>>> IIRC, with the appropriate flags set, arithmetic operations throw
>>> exceptions instead of creating NaNs. You can't really create a
>>> signalling
>>> NaN in a register because as soon as you do, you get a signal.
>> But they don't throw a Haskell exception, they throw a processor
>> exception which kills your whole program.  If we want a Haskell
>> exception to result from 0/0, we have to insert extra checking code,
>> which I'm sure you won't like :-)
> I would, of course, expect the RTS to convert the processor exception to a
> Haskell exception!

You have high expectations :-)  I don't think it's possible to do that 
without some very low-level platform-specific and processor-specific 
hacking, which is why for example we have the current software test for 
divide-by-zero.  You basically get a signal and have to grovel around in 
the thread's registers and stack to recover the situation, and the 
exception could be thrown from *anywhere*.


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