How to write a function to fire quickCheck prop_xxx?

I am using QuickCheck v1. Here is a simple prop_xxx defined as below:

prop_foo :: (Num a) =>[a] -> Bool
prop_foo xs = (reverse.reverse) xs == id xs

This can be tested in GHCi correctly: quickCheck prop_foo开发者_如何转开发

However, when I tried to wrap the call in a function like:

f :: IO ()
f = quickCheck prop_foo

It reported the error:

Ambiguous type variable `a' in the constraints:
  `Num a' arising from a use of `prop_foo' at Foo.hs:147:15-22
  `Arbitrary a'
    arising from a use of `quickCheck' at Foo.hs:147:4-22
Probable fix: add a type signature that fixes these type variable(s)

Shall I provide something like

instance Arbitrary Xxx where
    arbitrary     = ...
    coarbitrary c = ...

Thanks a lot.

-- Larry

You have to give it a monomorphic type signature, like

prop_foo :: [Int] -> Bool

After all, the question is: in your original version, which type a should quickCheck choose to test the function with? a = Int? a = Double? Something else? The error message complains that a is ambiguous, i.e. there is no unique choice for it.