[Haskell-cafe] The difficulty of designing a sequence class
zednenem at psualum.com
Mon Jul 31 18:35:04 EDT 2006
ajb at spamcop.net writes:
> G'day all.
> Quoting Robert Dockins <robdockins at fastmail.fm>:
> > I've considered reformulating the Sequence class to be more similar
> > to the Collection classes (which use MPTCs, fundeps and mention the
> > element type),
> The redesign of the Collection hierarchy was from my tree. The main
> reason why I changed it was that ternary tries couldn't really be
> typed properly. (Chris' implementation of Patricia trees used a
> phantom key type along with a stern warning to only define the Int
> instance. That didn't work for ternary tries, since the key type is
> I didn't get around to fixing Sequence because there wasn't a need for
> it yet, but yes, it should be done.
That's a tough call to make. Changing the kind of Sequence to * from *
-> * means losing the Functor, Monad, and MonadPlus superclasses and all
the various maps and zips.
I guess you could separate those into an auxiliary class,
class (Functor s, MonadPlus s) => SeqFunctor s where
mapWithIndex :: (Int -> a -> b) -> s a -> s b
zip :: s a -> s b -> s (a,b)
and require that any instance of SeqFunctor also be an instance of
A pity we can't do something like,
class (Functor s, MonadPlus s, forall a. Sequence (s a) a) =>
SeqFunctor s where
David Menendez <zednenem at psualum.com> | "In this house, we obey the laws
<http://www.eyrie.org/~zednenem> | of thermodynamics!"
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