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The Other Prelude

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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.,
      mplus
      is replaced by
      (++)
      )
    • whole English words and camelCase for functions (e.g.,
      orElse
      but not
      fmap
      )

4 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.

5 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?

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

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
 
(.) = 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 = (<*>) -- 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

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