[Haskell-cafe] overloaded list literals?
illissius at gmail.com
Mon Sep 6 12:36:21 EDT 2010
On Mon, Sep 6, 2010 at 12:47 PM, Neil Brown <nccb2 at kent.ac.uk> wrote:
> On 06/09/10 11:23, Johannes Waldmann wrote:
>> We have overloaded numerical literals (Num.fromInteger)
>> and we can overload string literals (IsString.fromString),
>> so how about using list syntax ( , : )
>> for anything list-like (e.g., Data.Sequence)?
> I would have thought you have two obvious choices for the type-class (things
> like folding are irrelevant to overloading list literals):
> class IsList f where
> fromList :: [a] -> f a
> class IsList f where
> cons :: a -> f a -> f a
> empty :: f a
> I'd go for the first, as I'd imagine you are only overloading the [a,b,c]
> form, not the a:b:c: form, and the first reflects this better. Both of
> these could be used to convert a list literal into a list-like type (e.g.
> Sequence). But neither of them would be useful for sets or maps, because
> the classes lack an Ord constraint on the type a -- maybe this makes
> overloaded list literals fairly limited in utility.
I endorse the idea of a class along the lines of the first example.
That takes care of convenient syntax for literals; view patterns can
give you the other end, pattern matching*.
The fact that this doesn't work for Sets and the like is indeed
troublesome, but I think you can solve it:
class IsListLikeThingamabob f where
type ElemOf f
fromList :: [ElemOf f] -> f
then you can do:
instance Ord a => IsListLikeThingamabob (S.Set a) where
type ElemOf (S.Set a) = a
fromList = S.fromList
and that way you can also use it for *-kinded types like ByteString,
if for whatever reason you might want to.
I think the aim here should be just to gain access to the convenient
list syntax for use with other types -- a fully generalized interface
for collections of all shapes and sizes is out of scope, and _hard_.
(But Ivan Miljenovic seems to be working on it.)
* Especially if, as discussed on the wiki, view patterns also get
upgraded to hook into a type class:
data View a where
type ViewOf a
view :: a -> ViewOf a
where defining an instance would allow you to omit the name of the
viewing function, defaulting to 'view' instead. So if you define
ViewOf (MyContainer a) as [a], you could match using:
foo (-> ) = something
foo (-> x:xs) = something else
(I believe the main holdup wrt this is indecision over whether to use
a plain MPTC, or a fundep / associated type in one direction, or one
in the other. The version above looks clearly superior to me, but I
don't want to derail the thread further. If someone else does, fork
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