On Sun, Dec 14, 2008 at 6:54 AM, David Menendez <d...@zednenem.com> wrote: > 2008/12/13 Nathan Bloomfield <nblo...@gmail.com>: >> I want to be able to parse a string of digits to a type level numeral as >> described in the Number parameterized types paper. After fiddling with the >> problem for a while, I'm not convinced it's possible- it seems as though one >> would need to know the type of the result before parsing, but then we >> wouldn't need to parse in the first place. :) > > This can be done with existential types. I think Oleg Kiselyov has an > example somewhere of a parser that determines the type of its output > from its input, but here's the basic idea: > > data SomeCard = forall a. (Card a) => SomeCard a > > Now you can define parseP :: Parser SomeCard > > Unfortunately, all you know about the value inside the SomeCard is > that it's a member of the class Card, which may not be very helpful. > Depending on how general you want to be, you can bundle more > operations with SomeCard, or you can return a GADT that reflects the > type-level natural at the value level. > > -- > Dave Menendez <d...@zednenem.com> > <http://www.eyrie.org/~zednenem/> > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >
Depending on what you actually want to do, you may be looking for Oleg Kiselyov's reifyIntegral function, which he defines in the paper "Functional Pearl: Implicit Configurations -- or Type Classes Reflect the Value of Types" (at http://www.cs.rutgers.edu/~ccshan/prepose/prepose.pdf ). It has type something like reifyIntegral :: forall a. Int -> (forall n. Numeral n => n -> a) -> a encoding n's existential type with continuation passing style. Cheers, Reiner _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
