[Haskell-beginners] Nested Iterations with Takusen

18 Apr 2011

      Hello everyone,

I am new to Haskell and I am trying to use the Takusen library to query an SQL database. My desire is two perform a two level query query where the first query will iterate through some values  that will be used in the second level to get the data of interest. 

I am having issues with the types and I haven't been able to figure out what am I doing wrong. Below is some sample code that shows the problem I am dealing with:

-- sample.hs
import Database.PostgreSQL.Enumerator
import Database.Enumerator

db_host        = CAhost "host"
db_user        = CAuser "user"
db_password    = CApassword "pass"
db_name        = CAdbname "name"
db_auth_tokens = [db_user, db_password, db_host, db_name]
connection     = connect db_auth_tokens

secondLevelQuery :: IO Integer
secondLevelQuery = do
  let query = sql "SELECT count(*) from alarms"
      iter :: Monad m => Integer -> IterAct m Integer
      iter count _ = result' count
  withSession connection (doQuery query iter 0)

firstLevelQuery :: IO (Integer, Integer)
firstLevelQuery = do
  let query = sql "SELECT count(*) from alarms"
      iter :: Monad m => Integer -> IterAct m (Integer, Integer)
      iter count _ = do
                    count' <- secondLevelQuery
                    result' (count', count)
  withSession connection (doQuery query iter (0,0))

main = firstLevelQuery
-- end sample.hs

And the output from ghc is:

sample.hs:23:31:
    Could not deduce (m ~ IO)
    from the context (Monad m)
      bound by the type signature for
                 iter :: Monad m => Integer -> IterAct m (Integer, Integer)
      at sample.hs:(22,7)-(24,43)
      `m' is a rigid type variable bound by
          the type signature for
            iter :: Monad m => Integer -> IterAct m (Integer, Integer)
          at sample.hs:22:7
    Expected type: m Integer
      Actual type: IO Integer
    In a stmt of a 'do' expression: count' <- secondLevelQuery
    In the expression:
      do { count' <- secondLevelQuery;
           result' (count', count) }
    In an equation for `iter':
        iter count _
          = do { count' <- secondLevelQuery;
                 result' (count', count) }

I would appreciate any help in figuring out what am I doing wrong.

Thanks,
Pepe

[Haskell-beginners] Nested Iterations with Takusen

Pepe Barbe