[Haskell-cafe] Traversals of monomorphic containers

[Mario Blažević]

On 09/02/13 06:53, Nicolas Trangez wrote:
> # Redirected to haskell-cafe
>
> On Sun, 2013-09-01 at 14:58 +0400, Artyom Kazak wrote:
>> Would this be an appropriate place to propose adding mapM_ (and then
>> possibly mapM) to bytestring library?
>>
>> Was it suggested before? If yes, why was it rejected?
>
> This got me wondering: there are several type-classes useful for
> polymorphic container types, e.g. Functor, Foldable & Traversable which
> all apply to some type of kind (* -> *).
>
> Are there related things for monomorphic containers, like ByteString,
> Text or some newtype'd Vector with fixed element type, e.g.
>
> class MFunctor f a where
>      mfmap :: (a -> a) -> f -> f
>
> instance MFunctor ByteString Word8 where
>      mfmap = ByteString.map


	I'm not aware of this particular class, but I have considered it. In 
the end I've chosen to generalize the class to FactorialMonoid instead:

class Monoid m => FactorialMonoid m where
    ...
    foldMap :: Monoid n => (m → n) → m → n

	ByteString and Text are instances of the class, and so are lists, maps, 
and other containers, and Sum and Product as well.


http://hackage.haskell.org/packages/archive/monoid-subclasses/0.3.2/doc/html/Data-Monoid-Factorial.html



>
> or (maybe even better)
>
> class MFunctor f where
>      type Elem
>      mfmap :: (Elem -> Elem) -> f -> f
>
> instance MFunctor ByteString where
>      type Elem = Word8
>      mfmap = ByteString.map
>
> and similar for other classes.
>
> Nicolas
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>