Functor-Applicative-Monad Proposal
The standard class hierarchy is a consequence of Haskell's historical development, rather than logic.
This article attempts to document various suggestions that have been brought up over the years, along with arguments for and against.
Make Applicative
a superclass of Monad
Applicative
a superclass of Monad
class Applicative m => Monad m where
...
For
- Code that is polymorphic over the Monad can use Applicative operators rather than the ugly
liftM
andap
.
- Most types that implement Monad also implement Applicative already. This change will only make explicit a current best practice.
Against
- Monad is part of standard Haskell, but Applicative is not. If Monad is made a subclass of Applicative, then we will need to add Applicative to the language standard.
- Some libraries, such as blaze-markup, only implement Monad for its do-notation. For these types, an Applicative instance would have no meaning.
Add join
as a method of Monad
join
as a method of Monad
class Applicative m => Monad m where
(>>=) :: (a -> m b) -> m a -> m b
join :: m (m a) -> m a
...
m >>= k = join (fmap k m)
join m = m >>= id
For
fmap
/join
is more orthogonal, and is closer to the categorical definition.
join
is often easier to implement. See [1].
- The analogous comonad package is written this way.
Against
>>=
is used much more frequently in real-world code thanjoin
.
- Performance: The default implementation of
>>=
requires two traversals. A container-like type which only implementsjoin
would most likely be slower.
Remove liftM
, ap
, etc. in favor of their Applicative counterparts
liftM
, ap
, etc. in favor of their Applicative counterpartsFor
- We will end up with a simpler base library.
Against
- A lot of code will be broken by this change. Of course, we can gradually deprecate them as with
Prelude.catch
.
- A common pattern is to write a full instance of Monad, then set
fmap = liftM
and(<*>) = ap
. The functions are still useful for this purpose.
Split fail
into its own class
fail
into its own classclass Monad m => MonadFail m where
fail :: String -> m a
Rename fmap
to map
fmap
to map
class Functor f where
map :: (a -> b) -> f a -> f b
Export Applicative
in the Prelude
Applicative
in the PreludeRedefine >>
in terms of *>
rather than >>=
>>
in terms of *>
rather than >>=
Add a Pointed
class
Pointed
classclass Pointed p where
point :: a -> p a
This is already implemented in the pointed package.
For
Against
- This class has seen little real-world use. On Hackage, there are only 9 reverse dependencies for
pointed
, most of which are by the same author.
Related proposals
- From early 2011: GHC ticket – Makes Applicative into a superclass of Monad, but does not deprecate any existing names
- See [2] for the associated discussion.
- The Other Prelude
Context alias would also be a great help with backwards compatibility. The class system extension proposal may also help.