[project @ 2002-07-08 10:43:10 by simonmar]

[haskell-directory.git] / Data / Maybe.hs

diff --git a/Data/Maybe.hs b/Data/Maybe.hs
index 3f1ffad..32101b7 100644 (file)
--- a/Data/Maybe.hs
+++ b/Data/Maybe.hs
@@ -1,16 +1,14 @@
 {-# OPTIONS -fno-implicit-prelude #-}
 -----------------------------------------------------------------------------
--- 
+-- |
 -- 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
 -- Portability :  portable
 --
--- $Id: Maybe.hs,v 1.3 2001/07/03 14:13:32 simonmar Exp $
---
 -- The Maybe type, and associated operations.
 --
 -----------------------------------------------------------------------------
@@ -40,7 +38,18 @@ import GHC.Base
 -- ---------------------------------------------------------------------------
 -- 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