The Other Prelude

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.

Committee

This project has no committee whatsoever. Haskell community discussed the issues here.

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)

The Hierarchy

Although, not Haskell 98, hierarchical modules will definitely be in Haskell'. We take it for granted.

• TheOtherPrelude - Minimalistic module.
• TheOtherPrelude.Extension - Convenient definitions.

Open Issues

• Should Prelude functions use Integer instead of Int?

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,

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]] wiki page.
class Functor f => Applicative f where
  -- lifting a value
  return :: a -> f a

  -- should this be named 'ap'? is 'ap' a good name?
  -- can you come up with a better name?
  -- can it refactor the liftM* type gymnastics?
  (<*>) :: f (a -> b) -> f a -> f b   
  
-- this leaves little left for the actual Monad class
class (Applicative m) => Monad m where
  -- the binding operation, gist of a monad 
  (>>=) :: m a -> (a -> m b) -> m b

  -- throwing out the outer monad
  join :: m (m a) -> m a

  -- when the second is independent of the first
  -- included in the class in case performance can be enhanced
  (>>) :: m a -> m b -> m b

  -- intuitive definitions
  fa >> fb = (map (const id) fa) <*> fb    -- is there a better definition?
  x >>= f = join (map f x)
  join x = x >>= id

-- we shamelessly copy from the ''MonadPlus reform proposal'' now.

-- should satisfy 'left zero': zero >>= f = zero
-- zero will be used when pattern matching against refutable patterns in
-- do-notation as well as to provide support for monad comprehensions.
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

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

See also

• 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.
• Functor hierarchy proposal - Making Monad m imply Functor m (adopted by The Other Prelude).
• If-then-else - Making if a function.
• MissingH - Functions "missing" from the Haskell Prelude/libraries.
• MonadPlus reform proposal - Clarifies ambiguities around MonadPlus laws (adopted by The Other Prelude)