Functor-Applicative-Monad Proposal: Difference between revisions
Lambda Fairy (talk | contribs) (Add more things!) |
(Updated for AMP 2014) |
||
Line 1: | Line 1: | ||
Haskell calls a couple of historical accidents its own. While some of them, such as the "number classes" hierarchy, can be justified by pragmatism or lack of a strictly better suggestion, there is one thing that stands out as, well, not that: Applicative not being a superclass of Monad. | |||
This article | The topic has been discussed multiple times in the past (cf. link section at the bottom). '''This article was updated to describe the current, and very likely to succeed, Haskell 2014 Applicative => Monad proposal'''. | ||
== | == Pragmatic and short version == | ||
The following is a list of things you may have to change in your code so the AMP doesn't break it. | |||
* Add Applicative/Functor instances for all your Monads. If you don't care about efficiency, you can simply derive these instances from the Monad by adding the following code: | |||
<haskell> | <haskell> | ||
-- Monad m | |||
import Control.Monad (liftM, ap) | |||
import Control.Applicative (Applicative(..)) | |||
= | instance Functor m where | ||
fmap = liftM | |||
instance Applicative m where | |||
pure = return | |||
(<*>) = ap | |||
</haskell> | </haskell> | ||
* Add an Alternative instance for all instances of MonadZero. This can again be done easily using | |||
<haskell> | <haskell> | ||
-- MonadZero m | |||
import Control.Monad (mzero, mplus) | |||
import Control.Applicative (Alternative(..)) | |||
instance Alternative m where | |||
(<|>) = mplus | |||
empty = mzero | |||
</haskell> | </haskell> | ||
* Change your API to not define functions named <hask><*></hask>, <hask>join</hask> or <hask>pure</hask> | |||
== | == Detailed description == | ||
[https://github.com/quchen/articles/blob/master/applicative_monad.md Main text of the Haskell 2014 AMP] | |||
[http://article.gmane.org/gmane.comp.lang.haskell.libraries/19482 Mailing list discussion of the proposal] | |||
== | == Previous proposals == | ||
* From early 2011: [http://hackage.haskell.org/trac/ghc/ticket/4834 GHC ticket] – Makes Applicative into a superclass of Monad, but does not deprecate any existing names | * From early 2011: [http://hackage.haskell.org/trac/ghc/ticket/4834 GHC ticket] – Makes Applicative into a superclass of Monad, but does not deprecate any existing names | ||
Line 98: | Line 48: | ||
* [[The Other Prelude]] | * [[The Other Prelude]] | ||
[[Category:Proposals]] | [[Category:Proposals]] |
Revision as of 12:58, 17 June 2013
Haskell calls a couple of historical accidents its own. While some of them, such as the "number classes" hierarchy, can be justified by pragmatism or lack of a strictly better suggestion, there is one thing that stands out as, well, not that: Applicative not being a superclass of Monad.
The topic has been discussed multiple times in the past (cf. link section at the bottom). This article was updated to describe the current, and very likely to succeed, Haskell 2014 Applicative => Monad proposal.
Pragmatic and short version
The following is a list of things you may have to change in your code so the AMP doesn't break it.
- Add Applicative/Functor instances for all your Monads. If you don't care about efficiency, you can simply derive these instances from the Monad by adding the following code:
-- Monad m
import Control.Monad (liftM, ap)
import Control.Applicative (Applicative(..))
instance Functor m where
fmap = liftM
instance Applicative m where
pure = return
(<*>) = ap
- Add an Alternative instance for all instances of MonadZero. This can again be done easily using
-- MonadZero m
import Control.Monad (mzero, mplus)
import Control.Applicative (Alternative(..))
instance Alternative m where
(<|>) = mplus
empty = mzero
- Change your API to not define functions named
<*>
,join
orpure
Detailed description
Main text of the Haskell 2014 AMP
Mailing list discussion of the proposal
Previous proposals
- From early 2011: GHC ticket – Makes Applicative into a superclass of Monad, but does not deprecate any existing names
- See [1] for the associated discussion.
- The Other Prelude