X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Data%2FMaybe.hs;h=2f98c70e6eee655405db49719c330f2db790eb25;hb=HEAD;hp=3f1ffad76562cb7ab74e796fff2fc264e8539f43;hpb=d9e5fa673b75cdffbcd0e85cdcc98d706acbb29a;p=ghc-base.git

diff --git a/Data/Maybe.hs b/Data/Maybe.hs
index 3f1ffad..2f98c70 100644
--- a/Data/Maybe.hs
+++ b/Data/Maybe.hs
@@ -1,16 +1,15 @@
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# LANGUAGE CPP, NoImplicitPrelude, DeriveGeneric #-}
+
 -----------------------------------------------------------------------------
--- 
+-- |
 -- Module      :  Data.Maybe
 -- Copyright   :  (c) The University of Glasgow 2001
--- License     :  BSD-style (see the file libraries/core/LICENSE)
+-- License     :  BSD-style (see the file libraries/base/LICENSE)
 -- 
 -- Maintainer  :  libraries@haskell.org
--- Stability   :  experimental
+-- Stability   :  stable
 -- Portability :  portable
 --
--- $Id: Maybe.hs,v 1.3 2001/07/03 14:13:32 simonmar Exp $
---
 -- The Maybe type, and associated operations.
 --
 -----------------------------------------------------------------------------
@@ -18,29 +17,56 @@
 module Data.Maybe
    (
      Maybe(Nothing,Just)-- instance of: Eq, Ord, Show, Read,
-			--		Functor, Monad, MonadPlus
+                        --              Functor, Monad, MonadPlus
 
-   , maybe		-- :: b -> (a -> b) -> Maybe a -> b
+   , maybe              -- :: b -> (a -> b) -> Maybe a -> b
 
-   , isJust		-- :: Maybe a -> Bool
-   , isNothing		-- :: Maybe a -> Bool
-   , fromJust		-- :: Maybe a -> a
-   , fromMaybe		-- :: a -> Maybe a -> a
+   , isJust             -- :: Maybe a -> Bool
+   , isNothing          -- :: Maybe a -> Bool
+   , fromJust           -- :: Maybe a -> a
+   , fromMaybe          -- :: a -> Maybe a -> a
    , listToMaybe        -- :: [a] -> Maybe a
-   , maybeToList	-- :: Maybe a -> [a]
-   , catMaybes		-- :: [Maybe a] -> [a]
-   , mapMaybe		-- :: (a -> Maybe b) -> [a] -> [b]
+   , maybeToList        -- :: Maybe a -> [a]
+   , catMaybes          -- :: [Maybe a] -> [a]
+   , mapMaybe           -- :: (a -> Maybe b) -> [a] -> [b]
    ) where
 
 #ifdef __GLASGOW_HASKELL__
-import {-# SOURCE #-} GHC.Err ( error )
 import GHC.Base
+import GHC.Generics (Generic)
 #endif
 
+#ifdef __NHC__
+import Prelude
+import Prelude (Maybe(..), maybe)
+import Maybe
+    ( isJust
+    , isNothing
+    , fromJust
+    , fromMaybe
+    , listToMaybe
+    , maybeToList
+    , catMaybes
+    , mapMaybe
+    )
+#else
+
+#ifndef __HUGS__
 -- ---------------------------------------------------------------------------
 -- The Maybe type, and instances
 
-data  Maybe a  =  Nothing | Just a	deriving (Eq, Ord)
+-- | The 'Maybe' type encapsulates an optional value.  A value of type
+-- @'Maybe' a@ either contains a value of type @a@ (represented as @'Just' a@), 
+-- or it is empty (represented as 'Nothing').  Using 'Maybe' is a good way to 
+-- deal with errors or exceptional cases without resorting to drastic
+-- measures such as 'error'.
+--
+-- The 'Maybe' type is also a monad.  It is a simple kind of error
+-- monad, where all errors are represented by 'Nothing'.  A richer
+-- error monad can be built using the 'Data.Either.Either' type.
+
+data  Maybe a  =  Nothing | Just a
+  deriving (Eq, Ord, Generic)
 
 instance  Functor Maybe  where
     fmap _ Nothing       = Nothing
@@ -54,41 +80,65 @@ instance  Monad Maybe  where
     Nothing  >>  _      = Nothing
 
     return              = Just
-    fail _		= Nothing
+    fail _              = Nothing
 
 -- ---------------------------------------------------------------------------
 -- Functions over Maybe
 
+-- | The 'maybe' function takes a default value, a function, and a 'Maybe'
+-- value.  If the 'Maybe' value is 'Nothing', the function returns the
+-- default value.  Otherwise, it applies the function to the value inside
+-- the 'Just' and returns the result.
 maybe :: b -> (a -> b) -> Maybe a -> b
 maybe n _ Nothing  = n
 maybe _ f (Just x) = f x
+#endif  /* __HUGS__ */
 
+-- | The 'isJust' function returns 'True' iff its argument is of the
+-- form @Just _@.
 isJust         :: Maybe a -> Bool
 isJust Nothing = False
 isJust _       = True
 
+-- | The 'isNothing' function returns 'True' iff its argument is 'Nothing'.
 isNothing         :: Maybe a -> Bool
 isNothing Nothing = True
 isNothing _       = False
 
+-- | The 'fromJust' function extracts the element out of a 'Just' and
+-- throws an error if its argument is 'Nothing'.
 fromJust          :: Maybe a -> a
 fromJust Nothing  = error "Maybe.fromJust: Nothing" -- yuck
 fromJust (Just x) = x
 
+-- | The 'fromMaybe' function takes a default value and and 'Maybe'
+-- value.  If the 'Maybe' is 'Nothing', it returns the default values;
+-- otherwise, it returns the value contained in the 'Maybe'.
 fromMaybe     :: a -> Maybe a -> a
 fromMaybe d x = case x of {Nothing -> d;Just v  -> v}
 
+-- | The 'maybeToList' function returns an empty list when given
+-- 'Nothing' or a singleton list when not given 'Nothing'.
 maybeToList            :: Maybe a -> [a]
 maybeToList  Nothing   = []
 maybeToList  (Just x)  = [x]
 
+-- | The 'listToMaybe' function returns 'Nothing' on an empty list
+-- or @'Just' a@ where @a@ is the first element of the list.
 listToMaybe           :: [a] -> Maybe a
 listToMaybe []        =  Nothing
 listToMaybe (a:_)     =  Just a
- 
+
+-- | The 'catMaybes' function takes a list of 'Maybe's and returns
+-- a list of all the 'Just' values. 
 catMaybes              :: [Maybe a] -> [a]
 catMaybes ls = [x | Just x <- ls]
 
+-- | The 'mapMaybe' function is a version of 'map' which can throw
+-- out elements.  In particular, the functional argument returns
+-- something of type @'Maybe' b@.  If this is 'Nothing', no element
+-- is added on to the result list.  If it just @'Just' b@, then @b@ is
+-- included in the result list.
 mapMaybe          :: (a -> Maybe b) -> [a] -> [b]
 mapMaybe _ []     = []
 mapMaybe f (x:xs) =
@@ -97,3 +147,4 @@ mapMaybe f (x:xs) =
   Nothing -> rs
   Just r  -> r:rs
 
+#endif /* else not __NHC__ */