Is there already a list class?
robdockins at fastmail.fm
Wed Jan 10 09:30:07 EST 2007
On Jan 9, 2007, at 9:12 PM, Spencer Janssen wrote:
> On Jan 9, 2007, at 7:52 PM, Marc Weber wrote:
>> Is here the right place to request a list class?
>> class List l e where
>> (:) :: e -> l e -> l e
>> head :: ..
> Note that this approach isn't quite flexible enough. Your example
> forces the container type to have kind * -> *, and therefore can't
> support certain specialized containers like ByteString.
> There is a class like this in the Edison library (http://
> www.eecs.tufts.edu/~rdocki01/edison.html), it is called Seq
> (Haddocks: http://www.eecs.tufts.edu/~rdocki01/docs/edison/Data-
> Edison-Seq.html). However, it suffers the same kind flexibility
> issues as your List class. Other classes in Edison take a MPTC
> +fundep approach and I'm not sure why Seq doesn't. Can you comment
> on this, Rob?
I consider this mostly a historical artifact. The Edison design
dates back to about 1998, before fundeps were available in a Haskell
implementation. The Set/Bag and Finite Map classes were designed
with MPTC, but no fundeps. The sequence class was nicer because it
was more elegant, and played nicer with type inference. In the
course of time, Set/Bag and Finite Map classes got fundeps, and
became a bit nicer. When I took over maintenance, the typeclass
hierarchy was much as it is now.
I am personally in favor of the idea of changing the sequence class
to the MPTC+fundep approach, for largely the reasons you've
mentioned. The downsides are twofold: 1) Functor, Monad, and
MonadPlus could no longer be superclasses of Sequence and 2) its a
pretty major API change. Despite the downsides, I've become
convinced this is the right direction to go, and this change will
almost certainly take place sometime in the not too distant future.
I'm now finished with some work which was occupying most my attention
for the last six months or so.
I am also considering a more sweeping API reorganization, where the
typeclasses become less monolithic (especially the sequence class),
and the non-observable classes go away, but I'm still trying to
figure out the most optimal way to restructure things, so this change
won't happen for awhile.
>> This might be used in Data.Set, Data.Map
>> class StorableAsList l e t where
>> fromList :: l e -> t
>> toList :: t -> l e
> This is subsumed by other Edison functionality.
>> I'd like to help implementing/ writing it.
>> Do you consider this beeing a useful enhancement?
> Oh yes, but let's avoid reinventing the wheel if at all possible.
> Spencer Janssen
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
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