"No instance for" error - but the instance exists (kinds did not match)

Question

I get the following error from ghc for my servant library:

No instance for (GetEndpoint
                       (Verb 'GET 200 '[JSON] Position)
                       (Verb 'GET 200 '[JSON] Position)
                       'True)
arising from a use of `callServer3'

but I have an instance in scope, which looks like that:

instance GetEndpoint (Verb n s ct a) (Verb n s ct a) 'True where
  getEndpoint _ _ _ _ server = server

which looks exactly like the one ghc can not find. I am a bit confused right now.

Full code can be found here:

• Instance definition - right at the bottom of the file.
• File triggering the error - relevant code also at the bottom.

Any clues? Thanks a lot for any hints!

Answer 1

The given instance has default kind '*' for n s ct and a. Either use poly kinds as for n or the right concrete kinds:

(Verb (n :: k1) (s :: Nat) (ct :: [*]) a)

The correct instance would look like this:

instance GetEndpoint (Verb (n :: k1) (s :: Nat) (ct :: [*]) a) (Verb n s ct a) 'True where
  getEndpoint _ _ _ _ server = server

If you don't want to enable PolyKinds (it introduced a bunch of other errors), you can use the more restricted StdMethod for n:

instance GetEndpoint (Verb (n :: StdMethod) (s :: Nat) (ct :: [*]) a) (Verb n s ct a) 'True where
  getEndpoint _ _ _ _ server = server

Full code (compiling and even working as expected), can be found here.

Thanks again Carsten for this really quick help!