free-auth.hs

data UnAuthErr = MkErr deriving (Eq, Show)
data Req = MkReq deriving (Eq, Show)
data Credentials = MkCredentials deriving (Eq, Show)

data Auth a = Error a
            | Request Req a
            | Auth Credentials a
            deriving (Eq, Show)

instance Functor Auth where
  fmap f (Error a) = Error (f a)
  fmap f (Request Req a) = Req Req (f a)
  fmap f (Auth creds a) = Auth Creds (f a)

onlyIfAuth :: Free Auth (Either UnAuthErr a) ->
onlyIfAuth = iterM $ \auth ->
  -- interpret here

tryMe = onlyIfAuth someFreeTerm