Hi Petr, Congratulations -- you've just implemented a Moore machine! [1]
I posted something very much like this just last year [2]. It's a very common pattern in Haskell, forming the basis of coroutines and iteratees and many other things. Edward Kmett includes it in his machines package [3]. His variation, like mine, hides the state inside a closure, removing the need for existentials. pipes 2.0 contains one implemented as a free monad [4]. [1] http://en.wikipedia.org/wiki/Moore_machine [2] http://hackage.haskell.org/packages/archive/machines/0.2.3/doc/html/Data-Machine-Moore.html [3] http://www.haskell.org/pipermail/haskell-cafe/2012-May/101460.html [4] http://hackage.haskell.org/packages/archive/pipes/2.0.0/doc/html/Control-Pipe-Common.html Chris On Sun, Jan 27, 2013 at 11:03 AM, Petr P <[email protected]> wrote: > Dear Haskellers, > > I read some stuff about attribute grammars recently [1] and how UUAGC [2] > can be used for code generation. I felt like this should be possible inside > Haskell too so I did some experiments and I realized that indeed > catamorphisms can be represented in such a way that they can be combined > together and all run in a single pass over a data structure. In fact, they > form an applicative functor. > > [1] http://www.haskell.org/haskellwiki/Attribute_grammar > [2] Utrecht University Attribute Grammar Compiler > > To give an example, let's say we want to compute the average value of a > binary tree. If we compute a sum first and then count the elements, the > whole tree is retained in memory (and moreover, deforestation won't happen). > So it's desirable to compute both values at once during a single pass: > > -- Count nodes in a tree. > count' :: (Num i) => CataBase (BinTree a) i > count' = ... > > -- Sums all nodes in a tree. > sum' :: (Num n) => CataBase (BinTree n) n > sum' = ... > > -- Computes the average value of a tree. > avg' :: (Fractional b) => CataBase (BinTree b) b > avg' = (/) <$> sum' <*> count' > > Then we can compute the average in a single pass like > > runHylo avg' treeAnamorphism seed > > My experiments together with the example are available at > https://github.com/ppetr/recursion-attributes > > I wonder, is there an existing library that expresses this idea? > > Best regards, > Petr Pudlak > > > _______________________________________________ > Haskell-Cafe mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
