# The Other Prelude

### From HaskellWiki

(Updated the link to "Not just Maybe") |
(Change "return" to "pure" to see whether someone complains.) |
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Line 65: | Line 65: | ||

class (Functor p) => Applicative p where |
class (Functor p) => Applicative p where |
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-- Minimal complete definition: return and (<@>). |
-- Minimal complete definition: return and (<@>). |
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− | return :: a -> p a -- value lifting |
+ | pure :: a -> p a -- value lifting |

(<@>) :: p (a -> b) -> p a -> p b -- lifted application |
(<@>) :: p (a -> b) -> p a -> p b -- lifted application |
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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 |

## Revision as of 14:20, 20 February 2009

## 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 Design Philosophy

### 4.1 Taking Typeclasses Seriously

Following Not just Maybe, functions should be generalized whenever possible. Of course, efficiency might be a concern but this is a fun project anyway.

- means the same thing asconcat. We propose we don't usejoinat all.concat
- is justconcatMap. That is, monadic functions are preferred over the same functions with different name.(>>=)

### 4.2 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

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 (<@>). pure :: 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 = (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

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! -- the order of arguments matches that of maybe and either. boolean x _ True = x boolean _ y False = 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 List function suggestions - Unlike
*The Other Prelude*they*enhance*the Prelude. - Not just Maybe - Instead of writing inside a specific monad (i.e. Maybe) write functions generalized on (Monad m)=> where possible.