[Haskell-beginners] Return a Foldable instance

[Federico Mastellone]

So, is it even possible to write a function with this type: getValues
:: Foldable f => k -> MultiMap k v -> f v ???

On Wed, May 4, 2011 at 7:46 AM, Stephen Tetley <stephen.tetley at gmail.com> wrote:
> I don't think anyone's pointed it out, but you can't build a Foldable
> structure with just Foldable.
>
> The Foldable class only allows you to "reduce" objects that already exist.
>
>
> For instance you cannot write this function where the Map is Data.Map
> rather than your MultiMap:
>
> getValues :: Foldable f => k -> Data.Map k v -> f v
>
>
> Really all you can do is produce an API like Data.Map, Data.Set have:
>
> toList :: MultiMap k v -> [(k,v)]
> toMap  :: Ord k => MultiMap k v -> Map k v
> toSet  :: (Ord k, Ord v) => MultiMap k v -> Set (k,v)
>
>
> In a nutshell, you seem to be wanting "container oblivious code". You
> can't really do this in Haskell I'm afraid. You can traverse and
> reduce containers generically with the Traverse and Foldable classes
> but you can't build them generically[*].
>
> Best wishes
>
> Stephen
>
>
> [*] This might not be a disadvantage once you get over the initial
> disappointment - usually when you build something, you really do need
> to know what it is.
>
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-- 
Federico Mastellone
Computer Science Engineer - ITBA
".. there are two ways of constructing a software design: One way is
to make it so simple that there are obviously no deficiencies, and the
other way is to make it so complicated that there are no obvious
deficiencies. The first method is far more difficult."

Tony Hoare, 1980 ACM Turing Award Lecture.