fundep

FunctionalDependencies example:

> {-# LANGUAGE GeneralizedNewtypeDeriving
>     , FunctionalDependencies
>     , MultiParamTypeClasses #-}

> module Test where

> import Control.Monad.State
> import Control.Monad.Identity

> class (Monad m) => MonadFoo a m | m -> a where
> -- class (Monad m) => MonadFoo a m  where
>     -- save :: m [a]
>     save :: m ()

save has type (MonadFoo a m) => m () . And, although Foo (see below) is an instance of MonadFoo Char, GHC complains that there is No instance for (MonadFoo a Foo) arising from a use of 'save', if functional dependencies isn't used (see commented out code above).

One way to get around is to make save :: m [a] and explicitly annotate each usage of save with :: Foo String (see commented out code of action).

But, using functional dependencies, one can say that m uniquely identifies a. So, GHC will instantiate a with Char on Foo's save, since Foo is an instance of MonadFoo Char.

>     restore :: m [a]
>     push :: a -> m ()

> newtype FooT m a = FooT {
>     runFooT :: (StateT (String, Int) m) a
>     } deriving (Monad, Functor, MonadState (String, Int))

> instance (Monad m) => MonadFoo Char (FooT m) where
>     save = do
>         (current, _) <- get
>         put (current, 0)
>         -- return current
>     restore = do
>         (current, n) <- get
>         put (drop n current, 0)
>         return $ take n current
>     push x = do
>         (current, n) <- get
>         put (x : current, n + 1)

> newtype Foo a = Foo {
>     runFoo :: FooT Identity a
>     } deriving (Monad, Functor
>         , MonadState (String, Int), MonadFoo Char)

> action :: Foo String
> action = do
>     push 'a'
>     save
>     -- save :: Foo String
>     push 'b'
>     push 'c'
>     restore
> test = runIdentity $ runStateT (runFooT (runFoo action)) ("", 0)

FunctionalDependencies example:

> {-# LANGUAGE GeneralizedNewtypeDeriving
>     , FunctionalDependencies
>     , MultiParamTypeClasses #-}

> module Test where

> import Control.Monad.State
> import Control.Monad.Identity

> class (Monad m) => MonadFoo a m | m -> a where
> -- class (Monad m) => MonadFoo a m  where
>     -- save :: m [a]
>     save :: m ()

save has type (MonadFoo a m) => m () .
And, although Foo (see below) is an instance of MonadFoo Char,
GHC complains that there is No instance for (MonadFoo a Foo)
arising from a use of 'save', if functional dependencies isn't used
(see commented out code above).

One way to get around is to make save :: m [a] and explicitly annotate
each usage of save with :: Foo String (see commented out code of action).

But, using functional dependencies, one can say that m uniquely identifies a.
So, GHC will instantiate a with Char on Foo's save,
since Foo is an instance of MonadFoo Char.

>     restore :: m [a]
>     push :: a -> m ()

> newtype FooT m a = FooT {
>     runFooT :: (StateT (String, Int) m) a
>     } deriving (Monad, Functor, MonadState (String, Int))

> instance (Monad m) => MonadFoo Char (FooT m) where
>     save = do
>         (current, _) <- get
>         put (current, 0)
>         -- return current
>     restore = do
>         (current, n) <- get
>         put (drop n current, 0)
>         return $ take n current
>     push x = do
>         (current, n) <- get
>         put (x : current, n + 1)

> newtype Foo a = Foo {
>     runFoo :: FooT Identity a
>     } deriving (Monad, Functor
>         , MonadState (String, Int), MonadFoo Char)

> action :: Foo String
> action = do
>     push 'a'
>     save
>     -- save :: Foo String
>     push 'b'
>     push 'c'
>     restore
> test = runIdentity $ runStateT (runFooT (runFoo action)) ("", 0)