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
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