问题
I'm trying to add
instance YesodAuthEmail App
to the Yesod-Postgres scaffolding (yesod version 1.6) and getting stuck on a compile error.
The relevant code is:
instance YesodAuth App where
type AuthId App = UserId
....
authPlugins :: App -> [AuthPlugin App]
authPlugins app = [authOpenId Claimed []] ++ extraAuthPlugins
where extraAuthPlugins = [ authEmail ]
instance YesodAuthEmail App where
type AuthEmailId App = UserId
afterPasswordRoute _ = HomeR
addUnverified email verkey =
runDB $ insert $ User email Nothing
The error I receive is:
/home/justin/code/yesodemail/src/Foundation.hs:273:11: error:
• Could not deduce: m ~ HandlerFor site0 from the context: MonadAuthHandler App m
bound by the type signature for:
addUnverified :: Yesod.Auth.Email.Email -> VerKey -> AuthHandler App (AuthEmailId App)
....
Expected type: m (AuthEmailId App)
Actual type: HandlerFor site0 (Key User)
Based on the types,
getEmail :: AuthEmailId site -> AuthHandler site (Maybe Email)
type MonadAuthHandler master m = (MonadHandler m, YesodAuth master, master ~ HandlerSite m, Auth ~ SubHandlerSite m, MonadUnliftIO m)
type AuthHandler master a = forall m. MonadAuthHandler master m => m a
I would have thought this would compile. What I am misunderstanding?
P.S. I've tried to include everything relevant, but the full Foundation.hs is at https://gist.github.com/hyperpape/39d4d2baf67d3bdbdba45a943e7e0425
回答1:
The type of runDB is:
runDB :: YesodDB site a -> HandlerFor site a
in order to call it in AuthHandler you need to lift it to HandlerFor.
If I am not mistaken this is what the liftHandler method from MonadHandler is for.
If you compose your runDB call with it, it should work:
addUnverified email verkey =
liftHandler . runDB $ insert $ User email Nothing
I found a detailed answer to your question here.
来源：https://stackoverflow.com/questions/50808477/yesodauthemail-could-not-deduce-m-handlerfor-site0