I think this explanation is positively brilliant. Byorgey++
On Fri, Jan 23, 2009 at 9:59 AM, Brent Yorgey
AbstractInterface a = new ConcreteClass();
In Java, if a variable has type AbstractInterface, it is *existentially* quantified: it means, this variable references a value of *some* type, and all you know about it is that it is an instance of AbstractInterface. Whatever code sets the value of the variable gets to choose its concrete type; any code that uses the variable cannot choose what type it should be, but can only use AbstractInterface methods on it (since that's all that is known).
However, a Haskell variable with type
var :: AbstractInterface a => a
is *universally* quantified: it means, this variable has a polymorphic value, which can be used as a value of any type which is an instance of AbstractInterface. Here, it is the code which *uses* var that gets to choose its type (and indeed, different choices can be made for different uses); the definition of var cannot choose, and must work for any type which is an instance of AbstractInterface (hence it must only use methods of the AbstractInterface type class).
Writing
a :: Num n => n a = 3 :: Integer
is trying to define a universally quantified value as if it were existentially quantified.
Now, it *is* possible to have existentially quantification in Haskell; I can show you how if you like, but I think I'll stop here for now.
Does this help?
-Brent _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners