# Functor-Applicative-Monad Proposal

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=== For === |
=== For === |
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− | * <hask>fmap</hask>/<hask>join</hask> is more orthogonal than <hask>fmap</hask>/<hask>>>=</hask>, and the former is closer to the categorical definition. |
+ | * <hask>fmap</hask>/<hask>join</hask> is more orthogonal, and is closer to the categorical definition. |

* <hask>join</hask> is often easier to implement. See [http://article.gmane.org/gmane.comp.lang.haskell.libraries/14926]. |
* <hask>join</hask> is often easier to implement. See [http://article.gmane.org/gmane.comp.lang.haskell.libraries/14926]. |
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* <hask>>>=</hask> is used much more frequently in real-world code than <hask>join</hask>. |
* <hask>>>=</hask> is used much more frequently in real-world code than <hask>join</hask>. |
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− | * Performance: The default implementation of <hask>>>=</hask> requires two traversals. Any container-like type which only implements <hask>fmap</hask> and <hask>join</hask> would be slower. |
+ | * Performance: The default implementation of <hask>>>=</hask> requires two traversals. A container-like type which only implements <hask>join</hask> would most likely be slower. |

== Remove <hask>liftM</hask>, <hask>ap</hask>, etc. in favor of their Applicative counterparts == |
== Remove <hask>liftM</hask>, <hask>ap</hask>, etc. in favor of their Applicative counterparts == |
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=== Against === |
=== Against === |
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− | * A lot of code will be broken by this change. There is no compelling reason to remove these functions outright, rather than gradually deprecating them as with <hask>Prelude.catch</hask>. |
+ | * A lot of code will be broken by this change. Of course, we can gradually deprecate them as with <hask>Prelude.catch</hask>. |

− | * A common pattern is to write a full instance of Monad, then set <hask>fmap = liftM</hask> and <hask>(<*>) = ap</hask>. |
+ | * A common pattern is to write a full instance of Monad, then set <hask>fmap = liftM</hask> and <hask>(<*>) = ap</hask>. The functions are still useful for this purpose. |

== Split <hask>fail</hask> into its own class == |
== Split <hask>fail</hask> into its own class == |
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== Rename <hask>fmap</hask> to <hask>map</hask> == |
== Rename <hask>fmap</hask> to <hask>map</hask> == |
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+ | |||

+ | <haskell> |
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+ | class Functor f where |
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+ | map :: (a -> b) -> f a -> f b |
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+ | </haskell> |
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== Export <hask>Applicative</hask> in the Prelude == |
== Export <hask>Applicative</hask> in the Prelude == |

## Revision as of 02:09, 3 June 2013

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.

## 1 Make Applicative a superclass of Monad

Applicative

Monad

class Applicative m => Monad m where ...

### 1.1 For

- Code that is polymorphic over the Monad can use Applicative operators rather than the ugly andliftM.ap

- Most types that implement Monad also implement Applicative already. This change will only make explicit a current best practice.

### 1.2 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.

## 2 Add join as a method of Monad

join

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

### 2.1 For

- /fmapis more orthogonal, and is closer to the categorical definition.join

- is often easier to implement. See [1].join

- The analogous comonad package is written this way.

### 2.2 Against

- is used much more frequently in real-world code than>>=.join

- Performance: The default implementation of requires two traversals. A container-like type which only implements>>=would most likely be slower.join

## 3 Remove liftM, ap, etc. in favor of their Applicative counterparts

liftM

ap

### 3.1 For

- We will end up with a simpler base library.

### 3.2 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 andfmap = liftM. The functions are still useful for this purpose.(<*>) = ap

## 4 Split fail into its own class

fail

class Monad m => MonadFail m where fail :: String -> m a

## 5 Rename fmap to map

fmap

map

class Functor f where map :: (a -> b) -> f a -> f b

## 6 Export Applicative in the Prelude

Applicative

## 7 Redefine >> in terms of *> rather than >>=

>>

*>

>>=

## 8 Add a Pointed class

Pointed

class Pointed p where point :: a -> p a

This is already implemented in the pointed package.

### 8.1 For

### 8.2 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.

## 9 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.