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The Other Prelude

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(Imported Prelude(id, const), now it compiles.)
(comment fmt, added Class Heirarchy Proposal (commented-out since invalid))
Line 22: Line 22:
== Open Issues ==
== Open Issues ==
-
* Should <hask>Functor</hask> imply <hask>Monad</hask> 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?
* 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 <hask>Integer</hask> instead of <hask>Int</hask>?
+
* Should Prelude functions use <hask>Integer</hask> instead of <hask>Int</hask>? Maybe <hask>Integral n => n</hask> or <hask>Ix i => i</hask> in some cases?
* Should <hask>String</hask> be a class rather than a type synonym?
* Should <hask>String</hask> be a class rather than a type synonym?
* The current proposal lacks a well thought <hask>fail</hask> mechanism. Should it be integrated into <hask>MonadZero</hask>, or have a class of his own, or remain in the <hask>Monad</hask> class?
* The current proposal lacks a well thought <hask>fail</hask> mechanism. Should it be integrated into <hask>MonadZero</hask>, or have a class of his own, or remain in the <hask>Monad</hask> class?
Line 30: Line 29:
== Reality ==
== 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.
+
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 ==
== The Code ==
Line 36: Line 35:
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.
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,
+
The imaginary Prelude as it stands:
=== <hask>TheOtherPrelude.hs</hask> ===
=== <hask>TheOtherPrelude.hs</hask> ===
Line 43: Line 42:
module TheOtherPrelude where
module TheOtherPrelude where
-
import Prelude (id, const) -- hide everything
+
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.
-
-- 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
class Functor f where
map :: (a -> b) -> f a -> f b
map :: (a -> b) -> f a -> f b
(.) :: (Functor f) => (a -> b) -> f a -> f b
(.) :: (Functor f) => (a -> b) -> f a -> f b
-
(.) = map -- defined outside the class as an infix synonym for map.
+
(.) = map
-
-- The following has been shamelessly copied from the
 
-
-- Functor hierarchy proposal page (see links below).
 
class (Functor p) => Applicative p where
class (Functor p) => Applicative p where
-- Minimal complete definition: return and (<*>).
-- Minimal complete definition: return and (<*>).
Line 63: Line 62:
(>>) :: p a -> p b -> p b -- when the second is independent of the first
(>>) :: p a -> p b -> p b -- when the second is independent of the first
-
-- Default definition:
+
pa >> pb = map (const id) pa <*> pb
-
f >> g = (map (const id) f) <*> g
+
--map f pa = return f <*> pa -- see Class system extension proposal, below
apply :: (Applicative p) => p (a -> b) -> p a -> p b
apply :: (Applicative p) => p (a -> b) -> p a -> p b
-
apply = (<*>) -- defined outside the class as a prefix synonym for (<*>).
+
apply = (<*>)
class (Applicative m) => Monad m where
class (Applicative m) => Monad m where
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join :: m (m a) -> m a -- combining levels of structure
join :: m (m a) -> m a -- combining levels of structure
-
-- Default definitions:
+
ma >>= k = join (map k ma)
-
x >>= f = join (map f x)
+
join mma = mma >>= id
-
join x = x >>= 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 shamelessly copy from the MonadPlus reform proposal (link below) now.
+
-- We copy from the MonadPlus reform proposal (link below) now.
-- 'zero' will be used when pattern matching against refutable patterns in
-- 'zero' will be used when pattern matching against refutable patterns in
-- do-notation as well as to provide support for monad comprehensions.
-- do-notation as well as to provide support for monad comprehensions.
-
-- Should satisfy 'left zero': zero >>= f = zero
+
 
class (Monad mz) => MonadZero mz where
class (Monad mz) => MonadZero mz where
 +
-- Should satisfy 'left zero': zero >>= f = zero
zero :: mz a
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
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
(++) :: 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
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
orElse :: mo a -> mo a -> mo a
</haskell>
</haskell>

Revision as of 17:00, 3 February 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. 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

  • 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

Retrieved from "http://www.haskell.org/haskellwiki/The_Other_Prelude"

Categories: Proposals | Mathematics | Code