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-Mac...
[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-Pip...
Chris
On Sun, Jan 27, 2013 at 11:03 AM, Petr P
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
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