Mongo Example

mongo.hs

{-# LANGUAGE OverloadedStrings, ExtendedDefaultRules #-}

{-
    db <- mdb 
    db $ delete $ select [] "tags"
-}

module Mongo (mdb) where

import Database.MongoDB
import Data.Bson
import Data.CompactString
import Control.Monad.IO.Class

main = do
    -- Here's the normal way you access mongo stuff
    -- I want to put it all in one function, and return a function called "db"
    -- so I can think of it more like normal mongo: db.mycollection.remove({})
    pipe <- runIOE $ connect $ host "127.0.0.1"
    let run action = access pipe master "testdb" action
    run $ delete $ select [] "mycollection"
        
    -- here's what the shortcut looks like
    db <- mdb "127.0.0.1" "testdb" 
    db $ delete $ select [] "mycollection"
    -- but without the above line, I get the error pasted at the bottom. Why?

    return () 


mdb :: (MonadIO m) => String -> String -> IO (Action m a -> m (Either Failure a))
mdb hostname dbname = do
    pipe <- runIOE $ connect $ host hostname
    return (access pipe master (pack dbname))



{-
/Users/seanhess/itv/tvtagdemos/Mongo.hs:24:11:
    Ambiguous type variable `m0' in the constraint:
      (MonadIO m0) arising from a use of `mdb'
    Probable fix: add a type signature that fixes these type variable(s)
    In a stmt of a 'do' expression: db <- mdb "127.0.0.1" "testdb"
    In the expression:
      do { pipe <- runIOE $ connect $ host "127.0.0.1";
           let run action = access pipe master "testdb" action;
             run $ delete $ select [] "mycollection";
           db <- mdb "127.0.0.1" "testdb";
           .... }
    In an equation for `main':
        main
          = do { pipe <- runIOE $ connect $ host "127.0.0.1";
                 let run action = ...;
                   run $ delete $ select [] "mycollection";
                 .... }
-}