[Haskell-beginners] Return a Foldable instance

[Federico Mastellone]

On Wed, May 4, 2011 at 2:46 PM, Antoine Latter <aslatter at gmail.com> wrote:
>
> On May 4, 2011 12:32 PM, "Federico Mastellone" <fmaste at gmail.com> wrote:
>>
>> On Wed, May 4, 2011 at 12:25 PM, Stephen Tetley
>> <stephen.tetley at gmail.com> wrote:
>> > On 4 May 2011 13:21, 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 ???
>> >>
>> >
>> > I hadn't thought about it, but it's not possible to write that either.
>> > (!)
>> >
>> > You would need something like an Insertable class instead, plus Monoid
>> > so you can get an initial empty Set or List:
>> >
>> > getValues :: (Monoid (f v), Insertable f) => k -> MultiMap k v -> f v
>>
>> But it won't be Foldable either, or is there a way to traverse a
>> Monoid that I don't know?
>
> With this signature the caller gets to pick the representation type; if they
> want something foldable they can choose it themselves.

Now I understand, I just build the answer using mzero (from Monoid)
and insert (From Insertable) and the calling context decides which
representation is constructed.

Thanks!

>
>>
>> >
>> > class Insertable f where
>> >  insert :: a -> f a -> f a
>> >
>> > instance Insertable [] where
>> >  insert a xs = (a:xs)
>> >
>> >
>> > -- Won't actually compile due to Ord constraint on Set
>> > -- (though there are ways with GHC to around that).
>> > --
>> > instance Insert Set.Set where
>> >  insert = Set.insert
>> >
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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.
>>
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>> Beginners mailing list
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>> http://www.haskell.org/mailman/listinfo/beginners
>



-- 
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.