[Haskell-cafe] Non Empty List?
Conor McBride
conor at strictlypositive.org
Fri Jun 5 10:22:18 EDT 2009
Hi folks
data NE x = x :> Maybe (NE x)
?
It's Applicative in at least four different
ways. Can anyone find more?
Conor
On 5 Jun 2009, at 01:34, Edward Kmett wrote:
> Günther,
>
> Miguel had the easiest suggestion to get right:
>
> Your goal is to avoid the redundant encoding of a list of one
> element, so why do you need to get rid of the Many a [] case when
> you can get rid of your Single a case!
>
> > module NE where
>
> > import Prelude hiding (foldr, foldl, foldl1, head, tail)
> > import Data.Foldable (Foldable, foldr, toList, foldl, foldl1)
> > import Data.Traversable (Traversable, traverse)
> > import Control.Applicative
>
> > data NE a = NE a [a] deriving (Eq,Ord,Show,Read)
>
> Now we can fmap over non-empty lists
>
> > instance Functor NE where
> > fmap f (NE a as) = NE (f a) (map f as)
>
> It is clear how to append to a non-empty list.
>
> > cons :: a -> NE a -> NE a
> > a `cons` NE b bs = NE a (b:bs)
>
> head is total.
>
> > head :: NE a -> a
> > head (NE a _) = a
>
> tail can return an empty list, so lets model that
>
> > tail :: NE a -> [a]
> > tail (NE _ as) = as
>
> We may not be able to construct a non-empty list from a list, if its
> empty so model that.
>
> > fromList :: [a] -> Maybe (NE a)
> > fromList (x:xs) = Just (NE x xs)
> > fromList [] = Nothing
>
> We can make our non-empty lists an instance of Foldable so you can
> use Data.Foldable's versions of foldl, foldr, etc. and nicely foldl1
> has a very pretty total definition, so lets use it.
>
> > instance Foldable NE where
> > foldr f z (NE a as) = a `f` foldr f z as
> > foldl f z (NE a as) = foldl f (z `f` a) as
> > foldl1 f (NE a as) = foldl f a as
>
> We can traverse non-empty lists too.
>
> > instance Traversable NE where
> > traverse f (NE a as) = NE <$> f a <*> traverse f as
>
> And they clearly offer a monadic structure:
>
> > instance Monad NE where
> > return a = NE a []
> > NE a as >>= f = NE b (bs ++ concatMap (toList . f) as) where
> > NE b bs = f a
>
> and you can proceed to add suitable instance declarations for it to
> be a Comonad if you are me, etc.
>
> Now a singleton list has one representation
>
> NE a []
>
> A list with two elements can only be represented by NE a [b]
>
> And so on for NE a [b,c], NE 1 [2..], etc.
>
> You could also make the
>
> > data Container a = Single a | Many a (Container a)
>
> definition work that Jake McArthur provided. For the category theory
> inspired reader Jake's definition is equivalent to the Cofree
> comonad of the Maybe functor, which can encode a non-empty list.
>
> I leave that one as an exercise for the reader, but observe
>
> Single 1
> Many 1 (Single 2)
> Many 1 (Many 2 (Single 3))
>
> And the return for this particular monad is easy:
>
> instance Monad Container where
> return = Single
>
> In general Jake's non-empty list is a little nicer because it avoids
> a useless [] constructor at the end of the list.
>
> -Edward Kmett
>
> On Thu, Jun 4, 2009 at 5:53 PM, GüŸnther Schmidt
> <gue.schmidt at web.de> wrote:
> Hi,
>
> I need to design a container data structure that by design cannot be
> empty and can hold n elements. Something like a non-empty list.
>
>
> I started with:
>
> data Container a = Single a | Many a [a]
>
> but the problem above is that the data structure would allow to
> construct a Many 5 [] :: Container Int.
>
> I can't figure out how to get this right. :(
>
> Please help.
>
> Günther
>
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