function - 编译器中止测试

Question

我试过在函数中指定变量，但没用。

功能:

prop_unzip_check :: forall a b. Ord a => [(a,b)] -> Bool
prop_unzip_check xs = length (unzip (xs::[(a,b)])) >= 0

prop_merge_check :: forall a. Ord a => [a] -> [a] -> Bool
prop_merge_check xs ys = length (merge (xs::[a]) (ys::[a]))
                           == length (sort ((xs::[a]) ++ (ys::[a])))

编译后我收到这个令人困惑的错误:

> Test property "prop_merge_check" ... aborted.
Reason: Ambiguous type variable `a0' arising from a use of `quickCheckBool'
prevents the constraint `(Arbitrary a0)' from being solved.
Probable fix: use a type annotation to specify what `a0' should be.
These potential instances exist:
  instance Arbitrary QCGen -- Defined in `Test.QuickCheck.Arbitrary'
  instance Arbitrary (a b c) => Arbitrary (WrappedArrow a b c)
    -- Defined in `Test.QuickCheck.Arbitrary'
  instance Arbitrary (m a) => Arbitrary (WrappedMonad m a)
    -- Defined in `Test.QuickCheck.Arbitrary'
  ...plus 80 others
  (use -fprint-potential-instances to see them all)
Ambiguous type variable `a0' arising from a use of `prop_merge_check'
prevents the constraint `(Ord a0)' from being solved.
Probable fix: use a type annotation to specify what `a0' should be.
These potential instances exist:
  instance Ord (Encoding' a)
    -- Defined in `Data.Aeson.Encoding.Internal'
  instance Ord DotNetTime -- Defined in `Data.Aeson.Types.Internal'
  instance Ord JSONPathElement
    -- Defined in `Data.Aeson.Types.Internal'
  ...plus 310 others
  (use -fprint-potential-instances to see them all)

Answer 1

通过将某些函数定义为 Int 而不是任何类型的函数 'a' 来解决问题。

> prop_unzip_check :: [(Int,Int)] -> Bool
> prop_unzip_check xs = length (unzip (xs::[(Int,Int)])) >= 0