fmap is a much more general concept.. I just want to be able to use map (or another term, like project) over "enhanced" lists. Such "enhanced" lists aren't Monads, btw, because I can't write a meaningful return/unit function: to put the list in context, I always need a context to be provided. Giacomo On Fri, May 31, 2013 at 11:34 AM, Nadir Sampaoli wrote:

Hi, I'm very new to Haskell and I don't know about head, take, etc. but I think the `Functor` class (and its `fmap` function) provides the abstraction for `map` (which, in my understanding, can be considered the list-specific `fmap`).

2013/5/31 Giacomo Tesio

...

I'm looking for a class abstracting Data.List. I mean something that enable the use of say head, map, take and so on into different kind of types.

Is there already something like that?

Giacomo

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-- Nadir

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http://hackage.haskell.org/packages/archive/ListLike/3.1.4/doc/html/Data-Lis... Peter On 31 May 2013 11:16, mukesh tiwari wrote:

Hi Giacomo, Data.List can be abstracted using Foldable and Traversable but unfortunately I could not find the functions corresponding to head, take.

Prelude Data.Traversable Data.Foldable> :t Data.Foldable. Data.Foldable.Foldable Data.Foldable.concatMap Data.Foldable.foldl' Data.Foldable.foldrM Data.Foldable.minimum Data.Foldable.sequenceA_ Data.Foldable.all Data.Foldable.elem Data.Foldable.foldl1 Data.Foldable.forM_ Data.Foldable.minimumBy Data.Foldable.sequence_ Data.Foldable.and Data.Foldable.find Data.Foldable.foldlM Data.Foldable.for_ Data.Foldable.msum Data.Foldable.sum Data.Foldable.any Data.Foldable.fold Data.Foldable.foldr Data.Foldable.mapM_ Data.Foldable.notElem Data.Foldable.toList Data.Foldable.asum Data.Foldable.foldMap Data.Foldable.foldr' Data.Foldable.maximum Data.Foldable.or Data.Foldable.traverse_ Data.Foldable.concat Data.Foldable.foldl Data.Foldable.foldr1 Data.Foldable.maximumBy Data.Foldable.product Prelude Data.Traversable Data.Foldable> :t Data.Traversable. Data.Traversable.Traversable Data.Traversable.for Data.Traversable.mapAccumR Data.Traversable.sequenceA Data.Traversable.fmapDefault Data.Traversable.forM Data.Traversable.mapM Data.Traversable.traverse Data.Traversable.foldMapDefault Data.Traversable.mapAccumL Data.Traversable.sequence Prelude Data.Traversable Data.Foldable> :i Foldable class Foldable t where fold :: Data.Monoid.Monoid m => t m -> m foldMap :: Data.Monoid.Monoid m => (a -> m) -> t a -> m Data.Foldable.foldr :: (a -> b -> b) -> b -> t a -> b foldr' :: (a -> b -> b) -> b -> t a -> b Data.Foldable.foldl :: (a -> b -> a) -> a -> t b -> a foldl' :: (a -> b -> a) -> a -> t b -> a Data.Foldable.foldr1 :: (a -> a -> a) -> t a -> a Data.Foldable.foldl1 :: (a -> a -> a) -> t a -> a -- Defined in `Data.Foldable' instance Foldable [] -- Defined in `Data.Foldable' instance Foldable Maybe -- Defined in `Data.Foldable' Prelude Data.Traversable Data.Foldable> :i Tr Traversable True Prelude Data.Traversable Data.Foldable> :i Traversable class (Functor t, Foldable t) => Traversable t where traverse :: Control.Applicative.Applicative f => (a -> f b) -> t a -> f (t b) sequenceA :: Control.Applicative.Applicative f => t (f a) -> f (t a) Data.Traversable.mapM :: Monad m => (a -> m b) -> t a -> m (t b) Data.Traversable.sequence :: Monad m => t (m a) -> m (t a) -- Defined in `Data.Traversable' instance Traversable [] -- Defined in `Data.Traversable' instance Traversable Maybe -- Defined in `Data.Traversable' Prelude Data.Traversable Data.Foldable> :t foldMap foldMap :: (Foldable t, Data.Monoid.Monoid m) => (a -> m) -> t a -> m

-Mukesh

On Fri, May 31, 2013 at 2:26 PM, Giacomo Tesio wrote:

...

I'm looking for a class abstracting Data.List. I mean something that enable the use of say head, map, take and so on into different kind of types.

Is there already something like that?

Giacomo

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Thanks, that's a good starting point. What would be the proper name for such an abstraction in Haskell? In C# it would be Enumerable (and indeed the static class Enumerable<T> contains such kind of functions applicable to anything implementing IEnumerable<T>). What would be the idiomatic name of such a *class* in Haskell? - List ? - Listable ? - Enumerable ? - ListWithContext ? Giacomo On Fri, May 31, 2013 at 12:16 PM, mukesh tiwari < mukeshtiwari.iiitm@gmail.com> wrote:

Hi Giacomo, Data.List can be abstracted using Foldable and Traversable but unfortunately I could not find the functions corresponding to head, take.

Prelude Data.Traversable Data.Foldable> :t Data.Foldable. Data.Foldable.Foldable Data.Foldable.concatMap Data.Foldable.foldl' Data.Foldable.foldrM Data.Foldable.minimum Data.Foldable.sequenceA_ Data.Foldable.all Data.Foldable.elem Data.Foldable.foldl1 Data.Foldable.forM_ Data.Foldable.minimumBy Data.Foldable.sequence_ Data.Foldable.and Data.Foldable.find Data.Foldable.foldlM Data.Foldable.for_ Data.Foldable.msum Data.Foldable.sum Data.Foldable.any Data.Foldable.fold Data.Foldable.foldr Data.Foldable.mapM_ Data.Foldable.notElem Data.Foldable.toList Data.Foldable.asum Data.Foldable.foldMap Data.Foldable.foldr' Data.Foldable.maximum Data.Foldable.or Data.Foldable.traverse_ Data.Foldable.concat Data.Foldable.foldl Data.Foldable.foldr1 Data.Foldable.maximumBy Data.Foldable.product Prelude Data.Traversable Data.Foldable> :t Data.Traversable. Data.Traversable.Traversable Data.Traversable.for Data.Traversable.mapAccumR Data.Traversable.sequenceA Data.Traversable.fmapDefault Data.Traversable.forM Data.Traversable.mapM Data.Traversable.traverse Data.Traversable.foldMapDefault Data.Traversable.mapAccumL Data.Traversable.sequence Prelude Data.Traversable Data.Foldable> :i Foldable class Foldable t where fold :: Data.Monoid.Monoid m => t m -> m foldMap :: Data.Monoid.Monoid m => (a -> m) -> t a -> m Data.Foldable.foldr :: (a -> b -> b) -> b -> t a -> b foldr' :: (a -> b -> b) -> b -> t a -> b Data.Foldable.foldl :: (a -> b -> a) -> a -> t b -> a foldl' :: (a -> b -> a) -> a -> t b -> a Data.Foldable.foldr1 :: (a -> a -> a) -> t a -> a Data.Foldable.foldl1 :: (a -> a -> a) -> t a -> a -- Defined in `Data.Foldable' instance Foldable [] -- Defined in `Data.Foldable' instance Foldable Maybe -- Defined in `Data.Foldable' Prelude Data.Traversable Data.Foldable> :i Tr Traversable True Prelude Data.Traversable Data.Foldable> :i Traversable class (Functor t, Foldable t) => Traversable t where traverse :: Control.Applicative.Applicative f => (a -> f b) -> t a -> f (t b) sequenceA :: Control.Applicative.Applicative f => t (f a) -> f (t a) Data.Traversable.mapM :: Monad m => (a -> m b) -> t a -> m (t b) Data.Traversable.sequence :: Monad m => t (m a) -> m (t a) -- Defined in `Data.Traversable' instance Traversable [] -- Defined in `Data.Traversable' instance Traversable Maybe -- Defined in `Data.Traversable' Prelude Data.Traversable Data.Foldable> :t foldMap foldMap :: (Foldable t, Data.Monoid.Monoid m) => (a -> m) -> t a -> m

-Mukesh

On Fri, May 31, 2013 at 2:26 PM, Giacomo Tesio wrote:

...

I'm looking for a class abstracting Data.List. I mean something that enable the use of say head, map, take and so on into different kind of types.

Is there already something like that?

Giacomo

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Do you see any issue with using ListLike? In my particular domain, the concept of position, is actually relevant. Giacomo On Fri, May 31, 2013 at 4:31 PM, Brandon Allbery wrote:

On Fri, May 31, 2013 at 6:16 AM, mukesh tiwari < mukeshtiwari.iiitm@gmail.com> wrote:

...

Data.List can be abstracted using Foldable and Traversable but unfortunately I could not find the functions corresponding to head, take.

Those are, again, more general than you want. What is the `head` of a HashMap? (Consider that an implementation may choose to randomize the hash function to avoid hash collision attacks.) Foldable and Traversable express the concept of a collection which has no meaningful concept of an element's relative position within the collection. ListLike adds the concept of position, thereby admitting an indexing operation (and, by extension, `head` which is index 0).

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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I need to add that it's perfectly valid to define all operation in terms of Folds and so it's possible to define take, head, and such functions for all Foldable instances as they have a path to fold over them: the key idea is to store a state while folding over a structure: headF :: (Foldable x) => x a -> a headF = snd . F.foldl (\(s,v) n -> if s then (s,v) else (True,n)) (False, error "empty") takeF :: (Foldable x) => Int -> x a -> [a] takeF c = reverse . snd . F.foldl (\(i,v) n -> if i < c then (i+1,n:v) else (i,v)) (0,[]) In domain of a lazy languages we will even stop evaluation as soon as we will get result without additional steps. On 31 May 2013 18:31, Brandon Allbery wrote:

On Fri, May 31, 2013 at 6:16 AM, mukesh tiwari < mukeshtiwari.iiitm@gmail.com> wrote:

...

Data.List can be abstracted using Foldable and Traversable but unfortunately I could not find the functions corresponding to head, take.

Those are, again, more general than you want. What is the `head` of a HashMap? (Consider that an implementation may choose to randomize the hash function to avoid hash collision attacks.) Foldable and Traversable express the concept of a collection which has no meaningful concept of an element's relative position within the collection. ListLike adds the concept of position, thereby admitting an indexing operation (and, by extension, `head` which is index 0).

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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-- Alexander