X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Data%2FMaybe.hs;h=0e8bef572ea0f17cf4317719fd49f472d6b2c3b1;hb=4b26136ab82fb1ff12e49477c4833a9586d368c5;hp=4b7b67e305445c9c4f3201fd7282d78a393b5ce0;hpb=f7a485978f04e84b086f1974b88887cc72d832d0;p=haskell-directory.git

diff --git a/Data/Maybe.hs b/Data/Maybe.hs
index 4b7b67e..0e8bef5 100644
--- a/Data/Maybe.hs
+++ b/Data/Maybe.hs
@@ -1,4 +1,4 @@
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Maybe
@@ -6,7 +6,7 @@
 -- License     :  BSD-style (see the file libraries/base/LICENSE)
 -- 
 -- Maintainer  :  libraries@haskell.org
--- Stability   :  experimental
+-- Stability   :  stable
 -- Portability :  portable
 --
 -- The Maybe type, and associated operations.
@@ -35,10 +35,37 @@ import {-# SOURCE #-} GHC.Err ( error )
 import GHC.Base
 #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)
 
 instance  Functor Maybe  where
     fmap _ Nothing       = Nothing
@@ -57,36 +84,60 @@ instance  Monad Maybe  where
 -- ---------------------------------------------------------------------------
 -- 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) =
@@ -95,3 +146,4 @@ mapMaybe f (x:xs) =
   Nothing -> rs
   Just r  -> r:rs
 
+#endif /* else not __NHC__ */