[project @ 2005-02-21 11:36:07 by simonmar]

[ghc-base.git] / Data / Maybe.hs

diff --git a/Data/Maybe.hs b/Data/Maybe.hs
index 195daab..0e8bef5 100644 (file)
--- 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.
@@ -84,37 +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) =