October 2008 - Libraries

Data.Foldable documentation
by Stephan Friedrichs 04 Oct '08

04 Oct '08

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

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HackageDB bug in exposed modules
by Alfonso Acosta 03 Oct '08

03 Oct '08

Hi, I just uploaded a library to hackagedb: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ForSyDe Everything went smoothly, but the webpage shows certain hidden modules (other-modules section in the cabal description) as exposed. Am I missing something or is this a bug in HackageDB? Best regards, Alfonso Acosta

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Map / Set folding API changes
by Scott Dillard 02 Oct '08

02 Oct '08

Regarding this thread : http://www.haskell.org/pipermail/libraries/2008-September/010653.html and this ticket : http://hackage.haskell.org/trac/ghc/ticket/2580 I've attached a patch (to trac) which expands on Evan's changes and propagates them to IntSet and IntMap. Some (possibly controversial) changes of note : * Added foldr and foldl functions for the map types. This is redundant, but so is much of the interface. The argument to foldlWithKey has a strange type, (a -> key -> value -> a), that does not place nice with flip and const. So I think its convenient to have foldl as well, and since we've got that why not throw foldr in too. * Specialized default implementations of foldl and foldr for Foldable class. This is perhaps better than exporting then from the map libraries. Data.Sequence does it this way, but the map libraries do not. If the API clutter is found to be unbearable then perhaps a major cleanup is in order, hiding all folds in the map/set modules and exporting them via Foldable. Questions remaining : Strict versions? I'm happy to add them. The only question is if it (further) clutters the API. As mentioned, the type of foldlWithKey is strange. It didn't dawn on me until I went to define foldl in terms of foldlWithKey. Perhaps change it to (a -> (k,v) -> a) ? This goes against the established (and IMO unfortunate) convention that all the "WithKey" functions pass the key as an additional argument, instead of tupling it with the value. Scott

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