Add Representable0 instances to Either and Maybe.

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

diff --git a/Data/Maybe.hs b/Data/Maybe.hs
index 69dd75f..e6039f2 100644 (file)
--- a/Data/Maybe.hs
+++ b/Data/Maybe.hs
@@ -1,4 +1,5 @@
-{-# OPTIONS_GHC -fno-implicit-prelude #-}
+{-# LANGUAGE CPP, NoImplicitPrelude, DeriveRepresentable #-}
+
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Maybe
@@ -16,23 +17,23 @@
 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 (Representable0)
 #endif
 
 #ifdef __NHC__
@@ -44,7 +45,7 @@ import Maybe
     , fromJust
     , fromMaybe
     , listToMaybe
-    , maybeToList 
+    , maybeToList
     , catMaybes
     , mapMaybe
     )
@@ -64,8 +65,8 @@ import Maybe
 -- 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)
+data  Maybe a  =  Nothing | Just a
+  deriving (Eq, Ord, Representable0)
 
 instance  Functor Maybe  where
     fmap _ Nothing       = Nothing
@@ -79,7 +80,7 @@ instance  Monad Maybe  where
     Nothing  >>  _      = Nothing
 
     return              = Just
-    fail _             = Nothing
+    fail _              = Nothing
 
 -- ---------------------------------------------------------------------------
 -- Functions over Maybe
@@ -87,7 +88,7 @@ instance  Monad Maybe  where
 -- | 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 that.
+-- the 'Just' and returns the result.
 maybe :: b -> (a -> b) -> Maybe a -> b
 maybe n _ Nothing  = n
 maybe _ f (Just x) = f x
@@ -136,8 +137,8 @@ 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' a@, then @a@ is
--- added on to the result.
+-- 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) =