Hey, I knew about the forall (I use that to represent OO style collections, very handy), but not about the exists. Thanks. But GHC 6.8.2 (with -fglasgow-exts) does not seem to accept this "exists" keyword?
Does a book or document already exist (except the website) that tells more about not standarized yet very cool Haskell thingies that make writing real world applications possible? I would LOVE such a book. Cheers, Peter On Mon, 2008-01-21 at 16:10 -0500, Stefan Monnier wrote: > > How does caller choose which particular instance of Num they want? > > By passing the type they want. That's what the "Num a =>" thingy does. > > > In object-oriented language If function return type is an interface it means > > that it can return any implementation of this interface, but caller can't > > choose which particular inplementation they want. > > The full type of "f" you've given is: > > forall a . (Num a) => Integer -> a > > where the "forall a ." is normally not written. What you describe (a > function that returns something where the type can be chosen by the > function itself) would have type: > > Integer -> (exists a . (Num a) => a) > > I.e. the "a" is not passed as a (type) argument, but instead it's > returned by the function. > > > What the difference between haskell class and interface in object-oriented > > languge such Java or C#? > > >From a low-level point of view, the difference is that the vtable is > manipulated separately from the objects. The "Num a" basically stands > for the type of the vtable (which is called "dictionary" in Haskell). > > To bundle an object with its vtable as is traditionally done in OO > languages, you need to create an existential package, e.g. something of > type (exists a . (Num a) => a). > > > Stefan > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
