I think this is covered by the assumed associativity of mappend.
I.e., if `f' is associative, then `foldr = foldl' modulo performance.
2008/10/1 Stephan Friedrichs
Hello,
I've got a suggestion for the documentation of the Data.Foldable module. The documentation [1] does not say anything about the order in which the elements are folded. But as far as I understand, the order in which the elements are traversed (i. e. the result of Data.Foldable.toList) has to bee deterministic. The documentation of the Foldable class IMHO should provide a clear statement about that.
Example: I implemented a heap that internally uses a tree of elements. It uses a tree to store the elements, but two heaps might be equal (contain the same elements) and still be represented by different trees. Then
instance Foldable (Heap p) where foldMap _ Empty = mempty foldMap f (Tree _ x l r) = foldMap f l `mappend` f x `mappend` foldMap f r
is a bug which is not indicated by the documentation.
Thanks in advance Stephan
[1] using ghc-6.8.3
--
Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so.
- Dieter Nuhr _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries