# The Other Prelude

### From HaskellWiki

Uchchwhash (Talk | contribs) (class system extension proposal updates, and various other issues raised) |
CaleGibbard (Talk | contribs) |
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-- this leaves little left for the actual Monad class |
-- this leaves little left for the actual Monad class |
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class Applicative m => Monad m where |
class Applicative m => Monad m where |
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− | -- the binding operation, gist of a monad |
+ | (>>=) :: 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 |

− | -- throwing out the outer monad |
+ | -- Minimal complete instance: one of join or (>>=). |

− | join :: m (m a) -> m a |
+ | -- Default definitions: |

− | |||

− | -- intuitive definitions |
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x >>= f = join (map f x) |
x >>= f = join (map f x) |
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join x = x >>= id |
join x = x >>= id |

## Revision as of 01:21, 15 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. Haskell community discussed the issues here.

## 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 imaginery Prelude as it stands,

### 7.1 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

### 7.2 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

## 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.