Type error while trying to implement the (>>=) function in order to create a custom monad transforme
- by CharlieP
Hello,
I'm trying to create a monad transformer for a future project, but unfortunately, my implementation of the Monad typeclasse's (=) function doesn't work.
First of all, here is the underlying monad's implementation :
newtype Runtime a = R {
unR :: State EInfo a
} deriving (Monad)
Here, the implementation of the Monad typeclasse is done automatically by GHC (using the GeneralizedNewtypeDeriving language pragma).
The monad transformer is defined as so :
newtype RuntimeT m a = RuntimeT {
runRuntimeT :: m (Runtime a)
}
The problem comes from the way I instanciate the (=) function of the Monad typeclasse :
instance (Monad m) => Monad (RuntimeT m) where
return a = RuntimeT $ (return . return) a
x >>= f = runRuntimeT x >>= id >>= f
The way I see it, the first >>= runs in the underlying m monad. Thus, runRuntimeT x >>= returns a value of type Runtime a (right ?). Then, the following code, id >>=, should return a value of type a. This value is the passed on to the function f of type f :: (Monad m) => a -> RuntimeT m b.
And here comes the type problem : the f function's type doesn't match the type required by the (=) function. Jow can I make this coherent ? I can see why this doesn't work, but I can't manage to turn it into something functionnal.
Thank you for you help, and do not hesitate to correct any flaws in my message,
Charlie P.
