The Other Prelude

From HaskellWiki
Jump to navigation Jump to search

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.


This project has no committee whatsoever. Issues are discussed on the talk page.

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)

Design Philosophy

Taking Typeclasses Seriously

Following Not just Maybe, 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.

The Hierarchy

Although, not Haskell98, hierarchical modules are already in Haskell2010. 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.
  • TheOtherPrelude.Legacy - providing as much backwards compatibility as possible

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?


What we have here right now is not ready to be adopted by existing projects. The class system extension proposal might make a difference.

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 some time.

The imaginary Prelude as it stands:


{-# LANGUAGE NoImplicitPrelude #-}

module TheOtherPrelude where

import Prelude (id, const, flip, (.)) 
-- hide almost everything
-- in fact, we could do better, by just defining them here

-- 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

-- definitely a bad idea, sorry Cale!
-- (.) :: (Functor f) => (a -> b) -> f a -> f b
-- (.) = map

class (Functor p) => Applicative p where
  -- Minimal complete definition: return and (<@>).
  pure   :: a -> p a                 -- value lifting
                                     -- actually I think we should
                                     -- stick to return
                                     -- to make do notation work
  (<@>)  :: 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 = (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

class (Monad m) => MonadFail m where
  fail :: String -> m a


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!
-- the order of arguments matches that of maybe and either.
boolean x _ True = x
boolean _ y False = y

How To Use

-- ''The Other Prelude'' is an alternative, not a replacement.
-- So we need to hide everything from the Prelude
--import Prelude ()
-- Now that we have it,
{-# LANGUAGE NoImplicitPrelude #-}
-- 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