You could try and enable the {-# ScopedTypeVariables #-} language extension [1]. Then you can write an explicit forall so that the type variable a scopes over the where clause: myAPIServer :: forall a. Job a => Beamline -> JobQueue a -> Server (MyApi a) myAPIServer beam jobQueue = handleJobSpec :<|> handleHomepage :<|> handleXdsMe :<|> handleLogs :<|> handleResumXds :<|> handleSessionId where handleJobSpec :: JobSpecSub a -> Handler () handleJobSpec jobSpec = liftIO $ enqueue jobSpec jobQueue You could also do this without the language extension by explicitly passing the type variable via some proxy, but I recommend using the ScopedTypeVariables language extension. 1 - https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts... Op wo 16 okt. 2019 om 21:40 schreef PICCA Frederic-Emmanuel < frederic-emmanuel.picca@synchrotron-soleil.fr>:

Hello, I have a type for my APi like this

type SubscribeAPI a = "upload" :> ReqBody '[JSON] (JobSpecSub a) :> Post '[JSON] () type HomepageAPI = "homepage" :> QueryParam "id" Int :> Get '[HTML] Homepage type XdsMeAPI a = "xdsme" :> ReqBody '[FormUrlEncoded] XdsMeRequest :> Post '[JSON] (JobSpecSub a) type LogsAPI = "logs" :> ReqBody '[FormUrlEncoded] LogsRequest :> Post '[HTML] Html type ResumXdsAPI = "resumxds" :> ReqBody '[FormUrlEncoded] ResumXdsRequest :> Post '[HTML] Html type SessionIdAPI = "sessionid" :> ReqBody '[FormUrlEncoded] SessionIdRequest :> Post '[HTML] Html

type MyApi a = SubscribeAPI a :<|> HomepageAPI :<|> XdsMeAPI a :<|> LogsAPI :<|> ResumXdsAPI :<|> SessionIdAPI

myApi :: Job a => Proxy (MyApi a) myApi = Proxy

When I try to write the handler for this API, I have this error message for the next code

myAPIServer :: Job a => Beamline -> JobQueue a -> Server (MyApi a) myAPIServer beam jobQueue = handleJobSpec :<|> handleHomepage :<|> handleXdsMe :<|> handleLogs :<|> handleResumXds :<|> handleSessionId where handleJobSpec :: Job b => JobSpecSub b -> Handler () handleJobSpec jobSpec = liftIO $ enqueue jobSpec jobQueue

• Couldn't match type ‘a’ with ‘b’ ‘a’ is a rigid type variable bound by the type signature for: myAPIServer :: forall a. Job a => Beamline -> JobQueue a -> Server (MyApi a) at src/Web.hs:235:1-66 ‘b’ is a rigid type variable bound by the type signature for: handleJobSpec :: forall b. Job b => JobSpecSub b -> Handler () at src/Web.hs:238:5-56 Expected type: JobQueue b Actual type: JobQueue a • In the second argument of ‘enqueue’, namely ‘jobQueue’ In the second argument of ‘($)’, namely ‘enqueue jobSpec jobQueue’ In the expression: liftIO $ enqueue jobSpec jobQueue • Relevant bindings include jobSpec :: JobSpecSub b (bound at src/Web.hs:239:19) handleJobSpec :: JobSpecSub b -> Handler () (bound at src/Web.hs:239:5) jobQueue :: JobQueue a (bound at src/Web.hs:236:18) myAPIServer :: Beamline -> JobQueue a -> Server (MyApi a) (bound at src/Web.hs:236:1) | 239 | handleJobSpec jobSpec = liftIO $ enqueue jobSpec jobQueue | ^^^^^^^^

I understand that I need to explain haskell that a ~ b.

So my question is how can I do this :)

thanks for your help

Frederic _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.