# The Other Prelude

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== Open Issues == | == Open Issues == | ||

− | |||

* When the same function has an infix and a prefix implementation, should one of them be outside the class to enforce consistency? | * 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 <hask>Integer</hask> instead of <hask>Int</hask>? | + | * Should Prelude functions use <hask>Integer</hask> instead of <hask>Int</hask>? Maybe <hask>Integral n => n</hask> or <hask>Ix i => i</hask> in some cases? |

* Should <hask>String</hask> be a class rather than a type synonym? | * Should <hask>String</hask> be a class rather than a type synonym? | ||

* The current proposal lacks a well thought <hask>fail</hask> mechanism. Should it be integrated into <hask>MonadZero</hask>, or have a class of his own, or remain in the <hask>Monad</hask> class? | * The current proposal lacks a well thought <hask>fail</hask> mechanism. Should it be integrated into <hask>MonadZero</hask>, or have a class of his own, or remain in the <hask>Monad</hask> class? | ||

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== Reality == | == Reality == | ||

− | What we have here right now is not ready to be adopted by existing projects. | + | What we have here right now is not ready to be adopted by existing projects. The [[class system extension proposal]] might make a difference. |

== The Code == | == The Code == | ||

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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. | 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 imaginary Prelude as it stands | + | The imaginary Prelude as it stands: |

=== <hask>TheOtherPrelude.hs</hask> === | === <hask>TheOtherPrelude.hs</hask> === | ||

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module TheOtherPrelude where | module TheOtherPrelude where | ||

− | import Prelude (id, const) -- hide everything | + | import Prelude (id, const, flip) -- hide almost everything |

+ | |||

+ | -- The idea is to rename 'fmap'. | ||

+ | -- Both map :: (a -> b) -> [a] -> [b] (in []) | ||

+ | -- and (.) :: (a -> b) -> (e -> a) -> (e -> b) (in (->) e) | ||

+ | -- are good names, and are intuitively prefix and infix respectively. | ||

+ | -- 'map' is aliased as (.) below. | ||

− | |||

− | |||

− | |||

− | |||

class Functor f where | class Functor f where | ||

map :: (a -> b) -> f a -> f b | map :: (a -> b) -> f a -> f b | ||

(.) :: (Functor f) => (a -> b) -> f a -> f b | (.) :: (Functor f) => (a -> b) -> f a -> f b | ||

− | (.) = map | + | (.) = map |

− | |||

− | |||

class (Functor p) => Applicative p where | class (Functor p) => Applicative p where | ||

-- Minimal complete definition: return and (<*>). | -- Minimal complete definition: return and (<*>). | ||

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(>>) :: p a -> p b -> p b -- when the second is independent of the first | (>>) :: p a -> p b -> p b -- when the second is independent of the first | ||

− | + | pa >> pb = map (const id) pa <*> pb | |

− | + | --map f pa = return f <*> pa -- see Class system extension proposal, below | |

apply :: (Applicative p) => p (a -> b) -> p a -> p b | apply :: (Applicative p) => p (a -> b) -> p a -> p b | ||

− | apply = (<*>) | + | apply = (<*>) |

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

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join :: m (m a) -> m a -- combining levels of structure | join :: m (m a) -> m a -- combining levels of structure | ||

− | + | ma >>= k = join (map k ma) | |

− | + | join mma = mma >>= id | |

− | join | + | --mf <*> ma = mf >>= flip map ma -- see Class system extension proposal, below |

+ | --ma >> mb = ma >>= const mb | ||

+ | --map f ma = ma >>= return . f -- this depends on (.), which is map! Be careful. | ||

− | -- We | + | -- We copy from the MonadPlus reform proposal (link below) now. |

-- 'zero' will be used when pattern matching against refutable patterns in | -- 'zero' will be used when pattern matching against refutable patterns in | ||

-- do-notation as well as to provide support for monad comprehensions. | -- do-notation as well as to provide support for monad comprehensions. | ||

− | + | ||

class (Monad mz) => MonadZero mz where | class (Monad mz) => MonadZero mz where | ||

+ | -- Should satisfy 'left zero': zero >>= f = zero | ||

zero :: mz a | zero :: mz a | ||

− | |||

− | |||

− | |||

− | |||

class (MonadZero mp) => MonadPlus mp where | class (MonadZero mp) => MonadPlus mp where | ||

+ | -- 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) | ||

(++) :: mp a -> mp a -> mp a | (++) :: mp a -> mp a -> mp a | ||

− | |||

− | |||

− | |||

− | |||

− | |||

class (MonadZero mo) => MonadOr mo where | class (MonadZero mo) => MonadOr mo where | ||

+ | -- 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 | ||

orElse :: mo a -> mo a -> mo a | orElse :: mo a -> mo a -> mo a | ||

</haskell> | </haskell> |

## Revision as of 17:00, 3 February 2007

## Contents |

## 1 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.

## 2 Committee

This project has no committee whatsoever. Issues are discussed on the talk page.

## 3 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.*,is replaced bymplus)(++) - whole English words and camelCase for functions (
*e.g.*,but notorElse)fmap

- descriptive symbols for functions that are naturally infix (

## 4 The Hierarchy

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

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

## 5 Open Issues

- 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 instead ofInteger? MaybeIntorIntegral n => nin some cases?Ix i => i
- Should be a class rather than a type synonym?String
- The current proposal lacks a well thought mechanism. Should it be integrated intofail, or have a class of his own, or remain in theMonadZeroclass?Monad

## 6 Reality

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

## 7 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 imaginary Prelude as it stands:

### 7.1 TheOtherPrelude.hs

TheOtherPrelude.hs

module TheOtherPrelude where import Prelude (id, const, flip) -- hide almost everything -- The idea is to rename 'fmap'. -- Both map :: (a -> b) -> [a] -> [b] (in []) -- and (.) :: (a -> b) -> (e -> a) -> (e -> b) (in (->) e) -- are good names, and are intuitively prefix and infix respectively. -- 'map' is aliased as (.) below. class Functor f where map :: (a -> b) -> f a -> f b (.) :: (Functor f) => (a -> b) -> f a -> f b (.) = map class (Functor p) => Applicative p where -- Minimal complete definition: return and (<*>). return :: a -> p a -- value lifting (<*>) :: p (a -> b) -> p a -> p b -- lifted application (>>) :: p a -> p b -> p b -- when the second is independent of the first pa >> pb = map (const id) pa <*> pb --map f pa = return f <*> pa -- see Class system extension proposal, below apply :: (Applicative p) => p (a -> b) -> p a -> p b apply = (<*>) class (Applicative m) => Monad m where -- Minimal complete definition: one of join or (>>=). (>>=) :: m a -> (a -> m b) -> m b -- bind join :: m (m a) -> m a -- combining levels of structure ma >>= k = join (map k ma) join mma = mma >>= id --mf <*> ma = mf >>= flip map ma -- see Class system extension proposal, below --ma >> mb = ma >>= const mb --map f ma = ma >>= return . f -- this depends on (.), which is map! Be careful. -- We copy from the MonadPlus reform proposal (link below) now. -- 'zero' will be used when pattern matching against refutable patterns in -- do-notation as well as to provide support for monad comprehensions. class (Monad mz) => MonadZero mz where -- Should satisfy 'left zero': zero >>= f = zero zero :: mz a class (MonadZero mp) => MonadPlus mp where -- 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) (++) :: mp a -> mp a -> mp a class (MonadZero mo) => MonadOr mo where -- 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 orElse :: mo a -> mo a -> mo a

### 7.2 TheOtherPrelude/Utilities.hs

TheOtherPrelude/Utilities.hs

module TheOtherPrelude.Utilities where 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

## 8 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

## 9 See also

- Class system extension proposal - Makes this proposal worth reading at last
- Functor hierarchy proposal - Making implyMonad m(adopted byFunctor m
*The Other Prelude*). - If-then-else - Making a function (partially adopted byif
*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.