Daniel McAllansmith wrote:
I'm trying to control the scope within which functions can be used by putting them in a type class. Unfortunately I can't seem to figure out how to get it done. Any advice would be much appreciated.
Hopefully the following is close to what you wanted. The idea is to describe each appropriate function (like getInfo) with a label and to identify each monadic scope with a label, and then define the relationship which tells which labeled functions can be used within which scopes. If we attempt to obtain InfoC within the scope B, we get an error ``no instance (LabelOK ScopeC ScopeB)'' -- which seems clear. The more elaborate version of similar example can be found here: http://pobox.com/~oleg/ftp/Haskell/types.html#monadic-regions Using labels to enforce well-formedness term constraints (content model constraints for HTML/XML terms) can be found here: http://www.haskell.org/pipermail/haskell/2006-March/017656.html http://www.haskell.org/pipermail/haskell/2006-March/017684.html {-# OPTIONS -fglasgow-exts #-} module Scopes where import Control.Monad.Trans -- Our scopes newtype ScopeA m a = ScopeA{runScopeA :: m a} deriving (Monad, MonadIO) newtype ScopeB m a = ScopeB{runScopeB :: m a} deriving (Monad, MonadIO) newtype ScopeC m a = ScopeC{runScopeC :: m a} deriving (Monad, MonadIO) runScopeBA (ScopeB x) = ScopeA x runScopeCB (ScopeC x) = ScopeB x -- The deriving Show part is just to make the example better instance Show (ScopeA m a) where show _ = "ScopeA" instance Show (ScopeB m a) where show _ = "ScopeB" instance Show (ScopeC m a) where show _ = "ScopeC" -- Here we define the relationship that tells which labeled functions -- (identified by label') can appear within which scope (identified by label) class LabelOK (label' :: ( * -> * ) -> * -> * ) (label :: ( * -> * ) -> * -> * ) -- If we use overlapping isntances extension, the number of instances -- below can be significantly reduced. instance LabelOK ScopeA ScopeA instance LabelOK ScopeA ScopeB instance LabelOK ScopeA ScopeC instance LabelOK ScopeB ScopeB instance LabelOK ScopeB ScopeC instance LabelOK ScopeC ScopeC -- The function getInfo is labeled. The constraint LabelOK tells -- the the function can be used within any (monadic) scope where -- it is OK... getInfo :: (Monad m, Monad (label m), Show (label' [] ()), LabelOK label' label) => label' [] () -> label m String getInfo l = return (show l) -- The latter functions are to be defined analogously -- putInfo :: LabelOK label' label => label' -> String -> m () -- updateInfo :: UpdateOK label' label => label' -> Int -> m () main = runScopeA aScoped >>= print aScoped = do bResult <- runScopeBA bScoped -- updateAInfo bResult return "done" -- Inferred type: bScoped :: ScopeB IO Int bScoped = do i1 <- b1 i2 <- b2 return (i1 + i2) b1 :: (Monad m, MonadIO m) => ScopeB m Int b1 = do -- Within B scope, we can request AInfo and BInfo getInfo (ScopeA [()]) >>= liftIO . print getInfo (ScopeB [()]) >>= liftIO . print -- But we can't request CInfo -- If we uncomment the following, we get -- no instance (LabelOK ScopeC ScopeB) -- I think the error message is quite clear -- getInfo (ScopeC [()]) >>= liftIO . print return 2 b2 :: (Monad m, MonadIO m) => ScopeB m Int b2 = runScopeCB cScoped >>= return . fromEnum cScoped :: (Monad m, MonadIO m) => ScopeC m Char cScoped = do -- Within C scope, we can request AInfo and BInfo and CInfo getInfo (ScopeA [()]) >>= liftIO . print getInfo (ScopeB [()]) >>= liftIO . print getInfo (ScopeC [()]) >>= liftIO . print return '('