# The Other Prelude

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Nmessenger (Talk | contribs) (Formatting, small code changes, moved method synonyms outside classes to enforce consistency (choices were arbitrary, change if you want)) |
Nmessenger (Talk | contribs) (Added signatures to sidestep MMR) |
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map :: (a -> b) -> f a -> f b |
map :: (a -> b) -> f a -> f b |
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+ | (.) :: (Functor f) => (a -> b) -> f a -> f b |
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(.) = map -- defined outside the class as an infix synonym for map. |
(.) = map -- defined outside the class as an infix synonym for map. |
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f >> g = (map (const id) f) <*> g |
f >> g = (map (const id) f) <*> g |
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+ | apply :: (Applicative p) => p (a -> b) -> p a -> p b |
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apply = (<*>) -- defined outside the class as a prefix synonym for (<*>). |
apply = (<*>) -- defined outside the class as a prefix synonym for (<*>). |
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## Revision as of 23:54, 19 January 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

- Should implyFunctoror the other way around?Monad
- 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?Int
- 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. May be the class system extension proposal can 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 () -- hide 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. class Functor f where map :: (a -> b) -> f a -> f b (.) :: (Functor f) => (a -> b) -> f a -> f b (.) = map -- defined outside the class as an infix synonym for map. -- The following has been shamelessly copied from the -- Functor hierarchy proposal page (see links below). 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 -- Default definition: f >> g = (map (const id) f) <*> g apply :: (Applicative p) => p (a -> b) -> p a -> p b apply = (<*>) -- defined outside the class as a prefix synonym for (<*>). 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 -- Default definitions: x >>= f = join (map f x) join x = x >>= id -- We shamelessly 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. -- Should satisfy 'left zero': zero >>= f = zero class (Monad mz) => MonadZero mz where zero :: mz 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 mp) => MonadPlus mp where (++) :: mp a -> mp a -> mp 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 mo) => MonadOr mo where 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.