[Haskell-cafe] Type problem with simple takusen code (was: Trivial database access in Haskell)

[Seth Gordon]

Paul Moore wrote:
> db.hs:10:64:
>    Couldn't match expected type `forall mark. DBM mark Session a'
>           against inferred type `a1 b'
>    In the second argument of `($)', namely
>        `do r <- doQuery
>                   (sql "select username from all_users") query1Iteratee []
>              liftIO $ (putStrLn $ (show r))'
>    In the expression:
>          (withSession (connect "USER" "PASSWORD" "DB"))
>        $ (do r <- doQuery
>                     (sql "select username from all_users")
> query1Iteratee []
>                liftIO $ (putStrLn $ (show r)))
>    In the expression:
>        do (withSession (connect "USER" "PASSWORD" "DB"))
>         $ (do r <- doQuery
>                      (sql "select username from all_users")
> query1Iteratee []
>                 liftIO $ (putStrLn $ (show r)))

I had the same problem...

($) doesn't have the expected semantics with existential types (like
"forall mark. DBM mark Session a").  Some day I will be able to explain
why this is so, but in the meantime, if you replace "liftIO $ putStrLn $
show r" with "liftIO (putStrLn (show r))", it should work.