It does not seem like printing the result value from IO (), because with a more complicated example for lists. t :: [a] -> Bool t x = True then it randomly generates values of type [()]. *Quick> verboseCheck t 0: [] 1: [()] 2: [(),(),()] 3: [] 4: [()] 5: [(),(),(),()] I just wonder how the a got instantiated to (). Thanks, hugo On Tue, Jun 17, 2008 at 11:05 AM, Dougal Stanton wrote:

2008/6/17 Hugo Pacheco :

...

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?

I haven't got something here to check with, but from the formatting it looks like every odd line is a randomly-chosen boolean value, and the following line is the result? It might be just printing the result value of IO (). Maybe try a more complete test to see how that looks?

Cheers,

D

-- Dougal Stanton dougal@dougalstanton.net // http://www.dougalstanton.net

-- www.di.uminho.pt/~hpacheco

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 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