[Haskell-cafe] Computing sums
andrewcoppin at btinternet.com
Sat Feb 20 06:00:05 EST 2010
Luke Palmer wrote:
> On Sat, Feb 20, 2010 at 3:30 AM, Andrew Coppin
> <andrewcoppin at btinternet.com> wrote:
>> Have I just invented arrows?
> No... you have a data type which is *an* Arrow (probably/almost).
Well, OK, that's kind of what I meant. ;-)
> pure implementation bugs me because of its use of undefined. Might
> still be okay though. I would be more comfortable if it could not
> output until it has *some* input;
Yes, this irritates me also.
> data Property' a b = forall s. Property' s (a -> s -> (b,s))
Hmm. I hadn't thought of that. I will work though the consequences and
see what happens...
> Anyway, for yours: try to implement (.) :: Property b c -> Property a
> b -> Property a c, and first :: Property a b -> Property (a,c) (b,c).
> Then you will have an arrow.
(.) = flip (>==>)
first p = p >==< pure id
> This is a Causal Commutative Arrow, even, similar to the kinds of
> things that are done in Yampa. It might be more recognizable as the
> non-recursive version of:
> newtype Property a b = Property b (a -> Property a b)
...OK, I'm lost...
More information about the Haskell-Cafe