[Haskell-cafe] strange type mismatch when trying to use takusen

23 Nov 2006

      I have a simple test program for takusen and PostgreSQL:
...
import Database.Enumerator
import Database.PostgreSQL.Enumerator
import Control.Monad.Trans
gazdbSession dbname = connect [CAdbname dbname]
resultCollector :: (Monad m) => String -> IterAct m [String]
resultCollector str accum = result' (str:accum)
main = do withSession (connect [CAdbname "template1"]) $ main'
main' = do r <- doQuery (sql "select 'fred'") resultCollector []
           liftIO $ putStrLn $ show r
But it won't compile:

[1 of 1] Compiling Main             ( takusen-test.hs, takusen-test.o )

takusen-test.hs:11:57:
     Couldn't match expected type `forall mark. DBM mark Session a'
            against inferred type `DBM mark sess ()'
     In the second argument of `($)', namely `main''
     In the expression:
           (withSession (connect [CAdbname "template1"])) $ main'
     In the expression:
         do (withSession (connect [CAdbname "template1"])) $ main'

Adding an explicit "main' :: DBM mark Session ()" type signature doesn't 
change the error message.

The takusen source that I'm using was updated from darcs on November 14 
and compiled with ghc 6.6.

[Haskell-cafe] strange type mismatch when trying to use takusen

Seth Gordon