[Haskell-beginners] Return a Foldable instance

[Antoine Latter]

On Wed, May 4, 2011 at 1:08 PM, Federico Mastellone <fmaste at gmail.com> wrote:
> On Wed, May 4, 2011 at 1:22 PM, Chaddaï Fouché <chaddai.fouche at gmail.com> wrote:
>> On Wed, May 4, 2011 at 2:21 PM, Federico Mastellone <fmaste at gmail.com> wrote:
>>> So, is it even possible to write a function with this type: getValues
>>> :: Foldable f => k -> MultiMap k v -> f v ???
>>>
>>
>> It isn't since there's nothing in the Foldable class that allows you
>> to build one (only to consume one, do a fold over it, no "unfold").
>>
>
> As it's impossible to build a function like this one below:
> getValues :: Foldable f => k -> MultiMap k v -> f v
> I know that it may be too difficult, but wouldn't be great to make
> this type illegal in Haskell ?
> You can't make a function that returns a class that has no method that
> builds an instance of that class unless another parameter of the
> function is of that class or allows you to build one instance of that
> class.
>
>> But since this signature is _not_ the one you really want (it says you
>> will deliver any Foldable your caller asks for), this isn't a problem,
>> what you really want is :
>>
>>> data ExFoldable a = forall f . (Foldable f) => ExFoldable (f a)
>
> This will do the trick and it is better than maintaining an
> intermediate structure as I previously said.
> Thank you and Alexander for this tip.
>

I really think that this is just a more complicated version of making
your own wrapper type as discussed above.

Both types:

> data ExFoldable a = ...

and

> newtype ResultType a = Result [a]
>  deriving (Foldable)

offer up the exact same API to the caller. The second one is just
easier to understand and requires fewer advanced language extensions.

And if your goal is optimization, the 'ExFoldable' version is likely
to be slower for the caller to fold over than if you had converted the
result to a list and returned it, because with the 'ExFoldable' return
type the compiler has less information to work with at the call site.
Keep in mind I haven't benchmarked any of this :-)

Antoine