CODEWITHSUNDEEP
haskellfunctional-programmingfunctorquickcheck
Alimagadov K.
Alimagadov K.

Reputation: 325

Functor laws checking with QuickCheck-classes library

I want to check functor laws for instance:

instance Functor Stream where
    fmap f (x :> xs) = (f x) :> fmap f xs

where Stream is

data Stream a = a :> Stream a

I use QuickCheck.Classes (http://hackage.haskell.org/package/quickcheck-classes) to check the laws but I have some problem. When I try to run

lawsCheck (functorLaws (Proxy :: Proxy (Stream Int)))

I get the following:

* Couldn't match kind `*' with `* -> *'
      When matching types
        proxy0 :: (* -> *) -> *
        Proxy :: * -> *
      Expected: proxy0 f0
        Actual: Proxy (Stream Int)
    * In the first argument of `functorLaws', namely
        `(Proxy :: Proxy (Stream Int))'
      In the first argument of `lawsCheck', namely
        `(functorLaws (Proxy :: Proxy (Stream Int)))'
      In the expression:
        lawsCheck (functorLaws (Proxy :: Proxy (Stream Int)))

What is "proxy0 f0"? Is there any way to fix this error? I can't understand why this code is not being executed.

Upvotes: 0

Views: 245

Answers (1)

Ari Fordsham
Ari Fordsham

Reputation: 2515

As @leftaroundabout commented, functorLaws takes the unapplied functor, with kind * -> *. The correct version of your code would be:

lawsCheck (functorLaws (Proxy :: Proxy Stream))

Upvotes: 0

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