ContT inference question

ContTInferenceQuestion.purs

-- Some code excerpts to illustrate the following question:
-- Why is the type ascription
-- 
--   :: forall eff. ContT _ _ [F Number]
--
-- below necessary? Why isn't `[F a]` inferred to be `[F Number]` from
-- our use of `:: Query a` in the call to `runQueryCont`?

-- My assumption: choosing a `Query a` fixes `a` as well as the `[F a]` in the result type
runQueryCont :: forall eff a rxx. (IsForeign a) => Query a -> Client -> ContT rxx (DBEff eff) [F a]
runQueryCont query client = ContT $ runQuery query client


main = 
  runContT
    (withConnection $ \con -> do
      ns <- (
              -- Here, we instantiate `a` at `Number`, by our ascription `:: Query Number`,
              -- which I assumed would make the explicit `:: forall eff. ContT _ _ [F Number]` unnecessary.
              (runQueryCont ("insert into artist values ('Deep Purple', 1968) returning year as result" :: Query Number) con)
              :: forall eff. ContT _ _ [F Number]
              -- Omitting the type ascription [F Number] above causes a compile error.
              -- Why is this, since `Query a` above fixes `a` and therefore `[F a]`?
            )
     lift $ printNumbers ns
     runQueryCont ("select * from artist" :: Query Artist) c
    )
    printArtists
    

printNumbers :: forall eff. [F Number] -> Eff (trace :: Trace | eff) Unit
printNumbers xs = trace $ "numbers:\n" <> foldMap stringify xs
  where stringify = show >>> flip (<>) "\n"

printArtists :: forall eff. [F Artist] -> Eff (trace :: Trace | eff) Unit
printArtists artists = trace $ "numbers:\n" <> foldMap stringify artists
  where stringify = show >>> flip (<>) "\n"

For future reference: jutaro suggested on irc that this "might be a bug in the typechecker, see 810, 812, 814, ..." purescript/purescript#810, purescript/purescript#812, purescript/purescript#814

Problem turned out to be that Query was a type synonym instead of a proper type.