I've been working on a little project, and one of the things I need to do is dynamically compile and import a Haskell Source file containing filtering definitions. I've written a small monad called Filter which is simply: type Filter a = Reader (Config, Email) a To encompass all the email filtering. The method I need to import, filterMain, has type: filterMain :: Deliverable a => Filter a where Deliverable is a type class which abstracts over delivery to a path in the file system. The notion is that I can write a type like: data DEmail = {email :: Email, path :: FilePath} newtype Maildir = MD DEmail instance Deliverable Maildir where {- ... omitted -} However, Filter a should not be restricted to Deliverable types- it also encompasses the results of regular expression matching, etc, which are not -- in general -- Deliverable instances. My question is this, when importing the file containing the definitions of filterMain, I have the following code to grab filterMain and return it as a function. getFilterMain :: Deliverable a => FilePath -> Interpreter (Filter a) getFilterMain fMainLoc = do loadModules [fMainLoc]; setTopLevelModules [(takeWhile (/='.') fMainLoc)] fMain <- (interpret "(filterMain)" (as :: Deliverable a => Filter a)) return (fMain) However, when I try to compile this, I get the type error: Hackmain.hs:70:43: Ambiguous type variable `a' in the constraint: `Deliverable a' arising from a use of `getFilterMainStuff' at Hackmain.hs:70:43-60 Probable fix: add a type signature that fixes these type variable(s) My understanding is that a type like "Foo a => Bar a" (where Foo is a class and Bar is a datatype) would simply restrict the values of a to only those implementing Foo. But evidently I'm wrong. Is there a good (read, easy... :) ) fix to this? Any help would be greatly appreciated. /Joe PS. All the actual code is on patch-tag, here http://patch-tag.com/repo/Hackmail/home -- if anyone prefers to look at that directly, the relevant files are in Src, namely, Hackmain.hs, Filter.hs, and Deliverable.hs