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
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