Specific levels of monad transformers https://github.com/ivan-m/monad-levels
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Why not mtl?
The oft-spouted problem with the standard monad transformer library mtl and similar libraries is that instances are quadratic: you need a separate instance for each valid combination of transformer + typeclass.
For end users, this isn't really a problem: after all, all the required instances have already been written for you!
But what happens if you have a custom transformer, or a custom typeclass?
What about if you want to have something like
MonadIO but for a
different base monad?
Then you need to write all those extra instances.
What makes it more frustrating is that many of the instance
definitions are identical: typically for every transformer (using
StateT s m as an example) it becomes a matter of:
Possibly unwrap the transformer from a monadic value to get the lower monad (e.g.
StateT s m a -> m (a,s));
Possibly add internal values (e.g.
m a -> m (a,s));
Wrap the lower monad from the result of the computation back up into the transformer (e.g.
m (a,s) -> StateT s m a).
Ideally, instead we'd have something along the lines of (simplified):
class (Monad m) => MonadBase m where type BaseMonad m :: * -> * liftBase :: BaseMonad m a -> m a class (MonadBase m) => MonadLevel m where type LowerMonad m :: * -> * type InnerValue m a :: * -- A continuation-based approach for how to lift/lower a monadic value. wrap :: ( (m a -> LowerMonad m (InnerValue m a) -- unwrap -> (LowerMonad m a -> LowerMonad m (InnerValue m a)) -- addInternal -> LowerMonad m (InnerValue m a) ) -> m a
With these two classes, we could then use Advanced Type Hackery (TM) to let us instead just specify instances for the transformers/monads that do have direct implementations for a typeclass, and then have the rest defined for us!
It turns out that this approach is even powerful enough to make
liftBase redundant, and it isn't limited to just lifting a monad but
can instead be used for arbitrary functions.
Minimal specification required for adding new typeclasses: just specify the instances for monads that satisfy it, and then use the provided machinery to lift/lower methods to other transformers in the monadic stack.
Works even for polyvariadic functions.
Still allows specifying whether certain transformers do not allow some constraints to pass through (e.g.
ContTdoes not allow access to a
Requires a lot of GHC extensions.
Requires usage of proxies when lifting/lowering typeclass methods.
Large usage of associated types means type errors can be difficult to decipher.
Due to usage of closed type-families, it is not possible to add extra instances to typeclasses (i.e. it is not possible to use a custom
Currently un-benchmarked; as such, it's not known how much of a performance penalty this approach takes.
Lowering polyvariadic functions requires specifying the type of the function using a specific grammar (though the common
m a -> m acase is pre-defined).
Add compatibility for transformers-0.3
- Initial release