[Haskell-cafe] catamorphisms and attribute grammars
emax at chalmers.se
Mon Jan 28 12:05:42 CET 2013
Patrick Bahr does something very similar in Modular Tree Automata ,
also noting the relation to attribute grammars. It's implemented in the
compdata package .
 Patrick Bahr, Modular Tree Automata (MPC 2012),
2013-01-26 23:03, Petr P skrev:
> Dear Haskellers,
> I read some stuff about attribute grammars recently  and how UUAGC
>  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.
>  http://www.haskell.org/haskellwiki/Attribute_grammar
>  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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> Haskell-Cafe at haskell.org
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