There's a special function to handle these `ok`-statuses: `Database.MongoDB.Internal.Util.true1 :: Label -> Document -> Bool`

I would suggest s.th. like that:

```haskell
aggregate aColl agg = do
  response <- runCommand ["aggregate" =: aColl, "pipeline" =: agg]
  if true1 "ok" response
    then lookup "result" response
    else fail $ "aggregate: Server response was\n" ++ show response
```