On Jun 17, 2008, at 11:53 AM, Hugo Pacheco wrote:
Hi all,
There is something about polymorphic tests in QuickCheck that I do not understand.
If you write the simplest dummy test function
tst :: a -> Bool tst _ = True
and evaluate it we get
verboseCheck tst 0: () 1: () ...
How come did the polymorphic value a get instanciated to ()? Is this done via the Testable type class? Could someone please explain this to me? I'm not saying it does not make sense, but I would like to understand how it works. Being possible to change this behavior, this is, assign another "default type" whenever polymorphic types occur, would be awesome.
I think it is GHCi that is instantiating the type with (). Try compiling the program, it will probably be rejected by the compiler when you do not specify a concrete type. You can, for example, try this: verboseCheck (tst :: [Int] -> Bool)
Cheers, hugo