On Wed, May 04, 2011 at 02:18:14AM -0300, Federico Mastellone wrote:
I want to create different implementations of a multimap, for example using lists and using Data.Set, and instead of providing functions getValuesList and getValuesSet that return a [] and a Set respectively I want to provide a more generic one, getValues that returns a Foldable and avoid unnecessary conversions to and from lists. This way the user folds directly the underlying structure, without having to worry about which function is the best to fold the values. But I don't know how to do this without an extra intermediate data type.
A couple thoughts: First, I am not sure why you are so worried about avoiding an intermediate list. It smells like premature optimization to me. I doubt it will really make that big of a difference; and depending on how the result is used, the intermediate list might be optimized away anyway. Second, if you really want to *only* allow the caller of getValues to be able to fold the result, then you can just "inline" the fold, like so: getValues :: Monoid m => MultiMap k v -> (v -> m) -> m That is, instead of getting a Foldable thing and doing the fold themselves, they provide a mapping from values to some monoid and you do the fold for them. -Brent