[Haskell-cafe] Why not some subclass of Floating to model NaNs as some handleable bottom?

Olaf Klinke olf at aatal-apotheke.de
Fri Aug 6 22:16:39 UTC 2021

On Fri, 2021-08-06 at 22:21 +0800, YueCompl wrote:
> Thanks Olaf,
> Metaphors to other constructs are quite inspiring to me, though I don't have sufficient theoretical background to fully understand them atm.
Pen-and-paper or GHCi experiments suffice here, no fancy theoretical
background needed. Say Q is the type of rationals 0 < q and we express
type NonNegativeNumber = ([Q],[Q])
where the first (infinite) list is the lower approximants and the
second the upper approximants. Multiplication is then defined as
(l,r) * (l',r') = ([x*x'|x <- l, x' <- l'],[y*y'|y <- r, y' <- r'])
The extremes of this type are 
0     = ([],Q)
infty = (Q,[])
It is easily seen that 
0 * infty = ([],[]) 
a number with no lower and no upper approximants, in other words, NaN. 
Excercise: Define division for this type and find out what 1/0 and 0/0

> Am I understanding it right, that you mean `0/0` is hopeful to get ratified as "a special Float value corresponding to one non-proper Dedekind-cuts", but `NaN` as with IEEE semantics is so broken that it can only live outlaw, even Haskell the language shall decide to bless it?
Yes. I think it is vital that we provide a migration path for
programmers coming from other languages. Under the Dedekind
cut/interval interpretation, NaN would behave differently, as I pointed
out. So I'd leave Float as it is, but be more verbose about its
violation of type class laws. To this end, one could have (and now I
might be closer to your initial question) numerical type classes like
HonestEq, HonestOrd, HonestMonoid and HonestRing whose members are only
those types that obey the laws in all elements. Naturally, Float would
not be a member. Who would use these new classes? Probably no one,
because we all like to take the quick and dirty route. But at least it
says clearly: Careful, you can not rely on these laws when using Float.


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