[Haskell-cafe] Re: map (-2) [1..5]
wnoise at ofb.net
Mon Sep 11 12:28:01 EDT 2006
On 2006-09-11, Henning Thielemann <lemming at henning-thielemann.de> wrote:
> On Sun, 10 Sep 2006, Aaron Denney wrote:
>> >> Of course, there's always a typeclass, where we could add all sorts of
>> >> other encodings of the Peano axioms, such as binary trees,, but I don't
>> >> see that that buys us much if we don't also get access to operations
>> >> beyond them, such as (an _efficient_) `div` for fastexp.
>> > I don't see why Natural can't have an instance of whatever
>> > class ends up owning "div". It's perfectly well behaved on
>> > Naturals.
>> True. It seems odd to have a multiplicative (pseudo) inverse, but
>> not an additive, though. Breaking up the numeric hierarchy too finely
>> seems like it would be a pain -- take it to the limit of a
>> separate class per function. What else would you drag in with "div"?
>> "mod", (*), ...?
> from http://darcs.haskell.org/numericprelude/src/Algebra/Core.lhs :
>> class (Num a) => Integral a where
>> div, mod :: a -> a -> a
>> divMod :: a -> a -> (a,a)
>> -- Minimal definition: divMod or (div and mod)
>> div a b = fst (divMod a b)
>> mod a b = snd (divMod a b)
>> divMod a b = (div a b, mod a b)
That particular division means that Naturals can't support div because
they're not a ring, which the (Num a) in that snippet above means for
this numeric hierarcy, right? So this does drag in div, mod and (*),
which all seem mostly reasonable, but it also drags in (-) and "negate",
which you don't want for Naturals.
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