The Other Prelude

Categories: Proposals | Mathematics | Code

Contents 1 Call For Contribution 2 Committee 3 Naming Conventions 4 Design Philosophy 4.1 Taking Typeclasses Seriously 4.2 The Hierarchy 5 Open Issues 6 Reality 7 The Code 7.1 TheOtherPrelude.hs 7.2 TheOtherPrelude/Utilities.hs 8 How To Use 9 See also

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 Design Philosophy

4.1 Taking Typeclasses Seriously

Following NotJustMaybe (unfortunately I can't find the page), functions should be generalized whenever possible. Of course, efficiency might be a concern but this is a fun project anyway.

• concat means the same thing as join. We propose we don't use concat at all.
• concatMap is just (>>=). 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.

• 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

• 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? Maybe Integral n => n or Ix i => i in some cases?
• 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. 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 (<*>).
  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
 
  pa >> pb = map (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!
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 Monad m imply Functor m (adopted by The Other Prelude).
• If-then-else - Making if a function (partially adopted by 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.
• NotJustMaybe - Instead of writing inside a specific monad (i.e. Maybe) write functions generalized on (Monad m)=> where possible.

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