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The Other Prelude

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(class system extension proposal updates, and various other issues raised)
Line 75: Line 75:
 
-- this leaves little left for the actual Monad class
 
-- this leaves little left for the actual Monad class
 
class Applicative m => Monad m where
 
class Applicative m => Monad m where
-- 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
 
 
x >>= f = join (map f x)
 
x >>= f = join (map f x)
 
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.,
      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 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