Thanks but I'm afraid that's still not quite what I'm looking for; guess I'll have to define my desire by my implementation - so once it's ready I'll show the result to cafe :)
2011/7/1 Alexey Khudyakov <alexey.sklad...@gmail.com>: > On Fri, Jul 1, 2011 at 12:54 PM, Eugene Kirpichov <ekirpic...@gmail.com> > wrote: >> Alexey, your definition of "mean" does not look like "liftS2 (/) sum >> length" - you have to manually "fuse" these computations. >> > Well it was fused for numerical stability > >> I'm asking for a formalism that does this fusion automatically (and >> guaranteedly). >> > Joining accumulators is quite straightforward. So is joining of initial > state. Just creating a >> joinAcc :: (acc1 -> x -> acc1) -> (acc2 -> x -> acc2) -> (acc1,acc2) -> x -> >> (acc1,acc2) >> joinAcc f1 f2 (s1,s2) x = (f1 s1 x, f2 s2 x) > > Still you have to handle them separately. >> sum' = foldl (+) 0 >> len = foldl (\n _ -> n+1) 0 >> sumLen = foldl (joinAcc (+) (\n _ -> n+1)) (0,0) > > There is more regular approach but it only works with statistics. > (function which do not depend on order of elements in the sample) > For every statistics monoid for its evaluation could be constructed. > For example sum: >> newtype Sum a = Sum a >> instance Num a => Monoid (Sum a) where >> mempty = Sum 0 >> mappend (Sum a) (Sum b) = Sum (a+b) > > Composition of these monoids becomes trivial. Just use > > > I pursued this approach in monoid-statistics[1] package. > It's reasonably well documented > > [1] http://hackage.haskell.org/package/monoid-statistics > -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/ _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
