On Thursday 20 January 2011 5:23:56 pm Henning Thielemann wrote:
Translating for me: liftA2 is essentially pair with subsequent fmap.
Yes. And I didn't mention it, but there's a similar restricted functor underlying the restricted applicative, which is a functor from a subcategory of Hask to Hask. So: rfmap :: (Suitable a, Suitable b) => (a -> b) -> f a -> f b In the normal applicative case: pure x = fmap (const x) unit f <*> x = fmap eval (pair (f, x)) where eval = uncurry ($) But the latter doesn't make sense if exponentials don't exist.
I already though about this and came to the conclusion, that this does not help in my case. A zipWith3 written as pure f <*> storableVectorA <*> storableVectorB <*> storableVectorC would require a Storable instance for pairs. I provided that in storable-tuple, but then it is not efficient to store intermediate pairs in StorableVector.
I can believe it isn't efficient. But it's possible. An implementation more like vector (for instance) would obviously work out better, because: unit = UnitVector pair (l, r) = PairVector l r instead of building a new vector and copying all the elements. Anyhow, my mails were more concerned with whether or not these things are 'unprincipled hacks,' not whether they're practically useful. -- Dan