On Thu, May 27, 2010 at 9:11 PM, Stefan Monnier <monn...@iro.umontreal.ca> wrote: > I.e. to make such an encoding really usable, you need "deep > polymorphism" (which GHC supports just fine, but which is not part of > the Haskell standard).
Ah, yes, and thank you for pointing that out. My message involved a great deal of hand-waving that I neglected to clearly identify as such. The same caveat of course applies to most Church encodings. For instance, the proper type of a (recursive) encoded list would be: type ChurchList a = ∀t. t -> (a -> t -> t) -> t ...where "a" is fixed as the type of the list elements. Thus, "cons" ought to look something like this charming type: cons :: ∀a. a -> (∀t. t -> (a -> t -> t) -> t) -> (∀t. t -> (a -> t -> t) -> t) For extra fun, try writing an instance such as: instance (Show a) => Show (ChurchList a) where [...etc.] ...at which point, perhaps, we remind ourselves that the language is named "Haskell", not "Alonzo", and drop the whole matter. - C. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
