| Suppose you want to define an instance of MonadState for another monad like
| MaybeT. You would need to write code like this:
|
| instance (MonadState s m) => MonadState s (MaybeT m) where
| get = lift get
| put = lift . put
|
| This code fails the coverage condition, because class argument
| (MaybeT m) does not contain the type variable s.
The issue doesn't even arise with type families:
class MonadState m where
type State m :: *
instance MonadState m => MonadState (MaybeT m) where
type State (MaybeT m) = State m
So examples that fail the coverage condition in fundeps, but (as you argue) are
ok because the context expresses the dependency, are sometimes just fine with
type families.
| Now if, in addition to lifting the coverage condition, you add
| OverlappingInstances, you can do something even better--you can write
| one single recursive definition of MonadState that works for all
| MonadTrans types (along with a base case for StateT). This is far
| preferable to the N^2 boilerplate functions currently required by N
| monad transformers:
|
| instance (Monad m) => MonadState s (StateT s m) where
| get = StateT $ \s -> return (s, s)
|
| instance (Monad (t m), MonadTrans t, MonadState s m) =>
| MonadState s (t m) where
| get = lift get
| put = lift . put
Why do you need the first instance? Isn't the second sufficient for (StateT s
m) as well?
Simon
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