# Difference between revisions of "The Other Prelude"

Uchchwhash (talk | contribs) (class system extension proposal updates, and various other issues raised) |
CaleGibbard (talk | contribs) |
||

Line 75: | Line 75: | ||

-- this leaves little left for the actual Monad class | -- this leaves little left for the actual Monad class | ||

class Applicative m => Monad m where | class Applicative m => Monad m where | ||

− | + | (>>=) :: m a -> (a -> m b) -> m b -- bind | |

− | (>>=) :: m a -> (a -> m b) -> m b | + | join :: m (m a) -> m a -- combining levels of structure |

− | -- | + | -- Minimal complete instance: one of join or (>>=). |

− | + | -- Default definitions: | |

− | |||

− | -- | ||

x >>= f = join (map f x) | x >>= f = join (map f x) | ||

join x = x >>= id | join x = x >>= id |

## Revision as of 01:21, 15 January 2007

## Contents

## Call For Contribution

This fun project, called *The Other Prelude*, is a creative reconstruction of the standard Prelude. By disregarding history and compatibility, we get a clean sheet.

## Committee

This project has no committee whatsoever. Haskell community discussed the issues here.

## Naming Conventions

- Function names should be easy for beginners to consume.
- Specifically,
*The Other Prelude*naming convention is to use- descriptive symbols for functions that are naturally infix (
*e.g.*,`mplus`

is replaced by`(++)`

) - whole English words and camelCase for functions (
*e.g.*,`orElse`

but not`fmap`

)

- descriptive symbols for functions that are naturally infix (

## The Hierarchy

Although, not Haskell 98, hierarchical modules will definitely be in Haskell'. We take it for granted.

`TheOtherPrelude`

- Minimalistic module.`TheOtherPrelude.Utilities`

- Convenient definitions. The reasoning behind its existence is that we want the Prelude to be very concise. It should not steal good names.

## Open Issues

- Should
`Functor`

imply`Monad`

or the other way around? - When the same function has an infix and a prefix implementation, should one of them be outside the class to enforce consistency?
- Should Prelude functions use
`Integer`

instead of`Int`

? - Should
`String`

be a class rather than a type synonym? - The current proposal lacks a well thought
`fail`

mechanism. Should it be integrated into`MonadZero`

, or have a class of his own, or remain in the`Monad`

class?

## Reality

What we have here right now is not ready to be adopted by existing projects. May be the class system extension proposal can make a difference.

## The Code

Currently, the code is in Wiki form. If people do agree that the collaborative decisions begot something pretty, we'll have a group of files in darcs.haskell.org some time.

The imaginery Prelude as it stands,

`TheOtherPrelude`

```
-- module: TheOtherPrelude
import Prelude () -- hide everything
-- the idea is to remove 'fmap'.
-- both map :: (a -> b) -> [a] -> [b] ('fmap' for the monad [])
-- and (.) :: (a -> b) -> (e -> a) -> (e -> b) ('fmap' for the (->) e monad)
-- are good names, and are intuitively prefix and infix respectively.
class Functor f where
-- 'fmap' is guilty of nothing but a bad name
map, (.) :: (a -> b) -> f a -> f b
-- implementing either is enough
map = (.)
(.) = map
-- the following has been shamelessly copied,
-- from the Functor hierarchy proposal[1] wiki page.
class Functor f => Applicative f where
-- lifting a value
return :: a -> f a
-- lifted application, in prefix and infix form
apply, (<*>) :: f (a -> b) -> f a -> f b
-- when the second is independent of the first
(>>) :: m a -> m b -> m b
-- implementing either is enough
apply = (<*>)
(<*>) = apply
-- is there a better definition?
f >> g = (map (const id) f) <*> g
-- this leaves little left for the actual Monad class
class Applicative m => Monad m where
(>>=) :: m a -> (a -> m b) -> m b -- bind
join :: m (m a) -> m a -- combining levels of structure
-- Minimal complete instance: one of join or (>>=).
-- Default definitions:
x >>= f = join (map f x)
join x = x >>= id
-- we shamelessly copy from the MonadPlus reform proposal[2] now.
-- zero will be used when pattern matching against refutable patterns in
-- do-notation as well as to provide support for monad comprehensions.
-- should satisfy 'left zero': zero >>= f = zero
class Monad m => MonadZero m where
zero :: m a
-- should satisfy 'monoid'
-- zero ++ b = b, b ++ zero = b, (a ++ b) ++ c = a ++ (b ++ c)
-- and 'left distribution'
-- (a ++ b) >>= f = (a >>= f) ++ (b >>= f)
class MonadZero m => MonadPlus m where
(++) :: m a -> m a -> m a
-- should satisfy 'monoid'
-- zero `orElse` b = b, b `orElse` zero = b
-- (a `orElse` b) `orElse` c = a `orElse` (b `orElse` c)
-- and 'left catch'
-- (return a) `orElse` b = a
class MonadZero m => MonadOr m where
orElse :: m a -> m a -> m a
```

[1]: Functor hierarchy proposal

[2]: MonadPlus reform proposal

[3]: Class system extension proposal

`TheOtherPrelude.Utilities`

```
-- module: TheOtherPrelude.Utilities
import Prelude () -- hide everything
-- this is the if-then-else proposal
-- the name has been chosen to reflect the magic of Church booleans!
boolean True x _ = x
boolean False _ y = y
```

## How To Use

```
-- ''The Other Prelude'' is an alternative, not a replacement.
-- So we need to hide everything from the Prelude
import Prelude ()
-- This is just an example assuming there is nothing to hide
import TheOtherPrelude
-- Hopefully, this module will contain lift,...
-- Standard convention is to use M.lift (instead of liftM)
import qualified TheOtherPrelude.Monad.Kleisli as M
```

## See also

- Class system extension proposal - Makes this proposal worth reading at last
- Functor hierarchy proposal - Making
`Monad m`

imply`Functor m`

(adopted by*The Other Prelude*). - If-then-else - Making
`if`

a function (partially adopted by*The Other Prelude*, we are silent on the bigger issue of sugar). - MissingH - Functions "missing" from the Haskell Prelude/libraries.
- MonadPlus reform proposal - Clarifies ambiguities around MonadPlus laws (adopted by
*The Other Prelude*) - Mathematical prelude discussion - A numeric Prelude in good shape already. Will a merger be ever possible?
- Prelude extensions and Prelude function suggestions - Unlike
*The Other Prelude*they*enhance*the Prelude. - NotJustMaybe - Instead of writing inside a specific monad (i.e. Maybe) write functions generalized on (Monad m)=> where possible.