‘toSample’ is not a (visible) method of class ‘ToSample’

[juan....@klarna.com]

Hey,

I'm trying to use servant-docs with a fairly simple example. Something like:

```

newtype User = User { username :: String }

  deriving (Eq, Show, Arbitrary, Generic)

instance ToJSON User

type AuthAPI = "user" :> Get '[JSON] User

instance ToSample User where

  toSample = Just g

    where g = User "john-doe"

api :: Proxy AuthAPI

api = Proxy

md :: String

md = markdown $ docs api

```

And I get 

```

.../src/AuthAPI.hs:23:3: error:

    ‘toSample’ is not a (visible) method of class ‘ToSample’

```

If it helps I'm using servant 0.8.1 and haskell2010. Any idea?