Patrick Bahr does something very similar in Modular Tree Automata [1],
also noting the relation to attribute grammars. It's implemented in the
compdata package [2].
[1] Patrick Bahr, Modular Tree Automata (MPC 2012),
http://dx.doi.org/10.1007/978-3-642-31113-0_14
[2] http://hackage.haskell.org/package/compdata
/ Emil
2013-01-26 23:03, Petr P skrev:
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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