Jean-Philippe Bernardy wrote:
foldr on ElemsView is defined as such:
foldr f i (ElemsView c) = foldr (f . snd) i c
so, for example:
getElementList = toList . ElemViews
Ok, thanks, this helps. I had forgot to look at the instances.
When I designed this code (some years ago), I didn't like the "fold" of Map to have the type:
fold :: (a -> b -> b) -> b -> Map k a -> b
This just doesn't make sense if we see maps as a collection of (key, value) pairs. (Indeed, toList :: Map k a -> [(k, a)])
In order to be consistent, but to provide an easy way to migrate to the new collection classes I was designing, I provided the ElemViews/KeyViews to "switch" to the former behaviour on a case by case basis.
I understand the motivation. Could you say something about the functions withElems and withKeys? Their types contain a mysterious type constructor 'T' that is not documented in the haddock docs. The source file says 'type T a = a->a' so I guess this is for cases where I want to 'lift' a function mapping e.g. keys to keys to a fucntion on Maps?
This also allows for definining optimized versions of foldr, etc. for each types that supports "Views", but this was tedious, so I never did it. GHC "RULE" pragma is probably better suited to the purpose anyway.
As for the lack of documentation, everyone is very welcome to contribute ;)
I'd love to, but my understanding is still somewhat limited... Cheers (and thanks) Ben P.S. The collections framework is great, but the type errors one sometimes gets are horrible. When I started to see whether I had understood what you wrote above I got: Couldn't match expected type `c' (a rigid variable) against inferred type `Str' `c' is bound by the type signature for `macros' at MacroSubst.hs:13:24 Expected type: (Str, c) Inferred type: (Str, Str) When using functional dependencies to combine Foldable (Data.Map.Map k a) (k, a), arising from the instance declaration at Defined in Data.Collections Foldable (Data.Map.Map Str Str) (Str, c), arising from use of `unions' at Gadgets.hs:95:13-30 When trying to generalise the type inferred for `macros' Signature type: forall c1. (Collection c1 String, Set c1 String) => Attributes -> c1 Type to generalise: forall c1. (Collection c1 String, Set c1 String) => Attributes -> c1