You are right! If I try to compile it, then it complains about a Show instance for a, what in fact forces a monomorphic type signature for t. I have found under Test.Quickcheck.Poly a way to generate Int values for a polymorphic type, but because it requires an abstraction Poly a for some type a I do not see how it can improve. What it concretely states the module is - This is the basic pseudo-polymorphic object. - The idea is you can't cheat, and use the integer - directly, but need to use the abstraction. Is it possible to cheat? I would like to... Thanks, hugo On Tue, Jun 17, 2008 at 12:17 PM, Sebastiaan Visser <sfvisser@cs.uu.nl> wrote:
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
-- www.di.uminho.pt/~hpacheco