X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FMonad.hs;h=2bbfc5714c79045e312f95be4fd5d5058999ca1a;hb=41e8fba828acbae1751628af50849f5352b27873;hp=734aae449cf5bba329ca7e797afecc6c1c7e8ca9;hpb=550bd82e4fa20eb3ac0613d5a8e145bf69e4d9cd;p=ghc-base.git

diff --git a/Control/Monad.hs b/Control/Monad.hs
index 734aae4..2bbfc57 100644
--- a/Control/Monad.hs
+++ b/Control/Monad.hs
@@ -1,4 +1,5 @@
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# LANGUAGE CPP, NoImplicitPrelude #-}
+
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Control.Monad
@@ -20,26 +21,33 @@ module Control.Monad
     , Monad((>>=), (>>), return, fail)
 
     , MonadPlus (   -- class context: Monad
-	  mzero     -- :: (MonadPlus m) => m a
-	, mplus     -- :: (MonadPlus m) => m a -> m a -> m a
-	)
+          mzero     -- :: (MonadPlus m) => m a
+        , mplus     -- :: (MonadPlus m) => m a -> m a -> m a
+        )
     -- * Functions
 
     -- ** Naming conventions
     -- $naming
 
-    -- ** Basic functions from the "Prelude"
+    -- ** Basic @Monad@ functions
 
     , mapM          -- :: (Monad m) => (a -> m b) -> [a] -> m [b]
     , mapM_         -- :: (Monad m) => (a -> m b) -> [a] -> m ()
+    , forM          -- :: (Monad m) => [a] -> (a -> m b) -> m [b]
+    , forM_         -- :: (Monad m) => [a] -> (a -> m b) -> m ()
     , sequence      -- :: (Monad m) => [m a] -> m [a]
     , sequence_     -- :: (Monad m) => [m a] -> m ()
     , (=<<)         -- :: (Monad m) => (a -> m b) -> m a -> m b
+    , (>=>)         -- :: (Monad m) => (a -> m b) -> (b -> m c) -> (a -> m c)
+    , (<=<)         -- :: (Monad m) => (b -> m c) -> (a -> m b) -> (a -> m c)
+    , forever       -- :: (Monad m) => m a -> m b
+    , void
 
     -- ** Generalisations of list functions
 
     , join          -- :: (Monad m) => m (m a) -> m a
     , msum          -- :: (MonadPlus m) => [m a] -> m a
+    , mfilter       -- :: (MonadPlus m) => (a -> Bool) -> m a -> m a
     , filterM       -- :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
     , mapAndUnzipM  -- :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
     , zipWithM      -- :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
@@ -56,7 +64,6 @@ module Control.Monad
     , unless        -- :: (Monad m) => Bool -> m () -> m ()
 
     -- ** Monadic lifting operators
-    -- $lifting
 
     , liftM         -- :: (Monad m) => (a -> b) -> (m a -> m b)
     , liftM2        -- :: (Monad m) => (a -> b -> c) -> (m a -> m b -> m c)
@@ -81,34 +88,49 @@ infixr 1 =<<
 -- -----------------------------------------------------------------------------
 -- Prelude monad functions
 
+-- | Same as '>>=', but with the arguments interchanged.
 {-# SPECIALISE (=<<) :: (a -> [b]) -> [a] -> [b] #-}
 (=<<)           :: Monad m => (a -> m b) -> m a -> m b
-f =<< x		= x >>= f
+f =<< x         = x >>= f
 
+-- | Evaluate each action in the sequence from left to right,
+-- and collect the results.
 sequence       :: Monad m => [m a] -> m [a] 
 {-# INLINE sequence #-}
 sequence ms = foldr k (return []) ms
-	    where
-	      k m m' = do { x <- m; xs <- m'; return (x:xs) }
+            where
+              k m m' = do { x <- m; xs <- m'; return (x:xs) }
 
+-- | Evaluate each action in the sequence from left to right,
+-- and ignore the results.
 sequence_        :: Monad m => [m a] -> m () 
 {-# INLINE sequence_ #-}
 sequence_ ms     =  foldr (>>) (return ()) ms
 
+-- | @'mapM' f@ is equivalent to @'sequence' . 'map' f@.
 mapM            :: Monad m => (a -> m b) -> [a] -> m [b]
 {-# INLINE mapM #-}
 mapM f as       =  sequence (map f as)
 
+-- | @'mapM_' f@ is equivalent to @'sequence_' . 'map' f@.
 mapM_           :: Monad m => (a -> m b) -> [a] -> m ()
 {-# INLINE mapM_ #-}
 mapM_ f as      =  sequence_ (map f as)
+
 #endif  /* __GLASGOW_HASKELL__ */
 
 -- -----------------------------------------------------------------------------
--- |The MonadPlus class definition
+-- The MonadPlus class definition
 
+-- | Monads that also support choice and failure.
 class Monad m => MonadPlus m where
-   mzero :: m a
+   -- | the identity of 'mplus'.  It should also satisfy the equations
+   --
+   -- > mzero >>= f  =  mzero
+   -- > v >> mzero   =  mzero
+   --
+   mzero :: m a 
+   -- | an associative operation
    mplus :: m a -> m a -> m a
 
 instance MonadPlus [] where
@@ -124,11 +146,13 @@ instance MonadPlus Maybe where
 -- -----------------------------------------------------------------------------
 -- Functions mandated by the Prelude
 
+-- | @'guard' b@ is @'return' ()@ if @b@ is 'True',
+-- and 'mzero' if @b@ is 'False'.
 guard           :: (MonadPlus m) => Bool -> m ()
 guard True      =  return ()
 guard False     =  mzero
 
--- This subsumes the list-based filter function.
+-- | This generalizes the list-based 'filter' function.
 
 filterM          :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
 filterM _ []     =  return []
@@ -137,12 +161,40 @@ filterM p (x:xs) =  do
    ys  <- filterM p xs
    return (if flg then x:ys else ys)
 
--- This subsumes the list-based concat function.
+-- | 'forM' is 'mapM' with its arguments flipped
+forM            :: Monad m => [a] -> (a -> m b) -> m [b]
+{-# INLINE forM #-}
+forM            = flip mapM
+
+-- | 'forM_' is 'mapM_' with its arguments flipped
+forM_           :: Monad m => [a] -> (a -> m b) -> m ()
+{-# INLINE forM_ #-}
+forM_           = flip mapM_
+
+-- | This generalizes the list-based 'concat' function.
 
 msum        :: MonadPlus m => [m a] -> m a
 {-# INLINE msum #-}
 msum        =  foldr mplus mzero
 
+infixr 1 <=<, >=>
+
+-- | Left-to-right Kleisli composition of monads.
+(>=>)       :: Monad m => (a -> m b) -> (b -> m c) -> (a -> m c)
+f >=> g     = \x -> f x >>= g
+
+-- | Right-to-left Kleisli composition of monads. @('>=>')@, with the arguments flipped
+(<=<)       :: Monad m => (b -> m c) -> (a -> m b) -> (a -> m c)
+(<=<)       = flip (>=>)
+
+-- | @'forever' act@ repeats the action infinitely.
+forever     :: (Monad m) => m a -> m b
+forever a   = a >> forever a
+
+-- | @'void' value@ discards or ignores the result of evaluation, such as the return value of an 'IO' action.
+void :: Functor f => f a -> f ()
+void = fmap (const ())
+
 -- -----------------------------------------------------------------------------
 -- Other monad functions
 
@@ -158,7 +210,7 @@ join x            =  x >>= id
 mapAndUnzipM      :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c])
 mapAndUnzipM f xs =  sequence (map f xs) >>= return . unzip
 
--- | The 'zipWithM' function generalises 'zipWith' to arbitrary monads.
+-- | The 'zipWithM' function generalizes 'zipWith' to arbitrary monads.
 zipWithM          :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c]
 zipWithM f xs ys  =  sequence (zipWith f xs ys)
 
@@ -168,17 +220,19 @@ zipWithM_ f xs ys =  sequence_ (zipWith f xs ys)
 
 {- | The 'foldM' function is analogous to 'foldl', except that its result is
 encapsulated in a monad. Note that 'foldM' works from left-to-right over
-the list arguments. This could be an issue where '(>>)' and the `folded
+the list arguments. This could be an issue where @('>>')@ and the `folded
 function' are not commutative.
 
 
->	foldM f a1 [x1, x2, ..., xm ]
+>       foldM f a1 [x1, x2, ..., xm]
+
 ==  
->	do
->	  a2 <- f a1 x1
->	  a3 <- f a2 x2
->	  ...
->	  f am xm
+
+>       do
+>         a2 <- f a1 x1
+>         a3 <- f a2 x2
+>         ...
+>         f am xm
 
 If right-to-left evaluation is required, the input list should be reversed.
 -}
@@ -187,18 +241,22 @@ foldM             :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a
 foldM _ a []      =  return a
 foldM f a (x:xs)  =  f a x >>= \fax -> foldM f fax xs
 
+-- | Like 'foldM', but discards the result.
 foldM_            :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m ()
 foldM_ f a xs     = foldM f a xs >> return ()
 
+-- | @'replicateM' n act@ performs the action @n@ times,
+-- gathering the results.
 replicateM        :: (Monad m) => Int -> m a -> m [a]
 replicateM n x    = sequence (replicate n x)
 
+-- | Like 'replicateM', but discards the result.
 replicateM_       :: (Monad m) => Int -> m a -> m ()
 replicateM_ n x   = sequence_ (replicate n x)
 
 {- | Conditional execution of monadic expressions. For example, 
 
->	when debug (putStr "Debugging\n")
+>       when debug (putStr "Debugging\n")
 
 will output the string @Debugging\\n@ if the Boolean value @debug@ is 'True',
 and otherwise do nothing.
@@ -212,60 +270,81 @@ when p s          =  if p then s else return ()
 unless            :: (Monad m) => Bool -> m () -> m ()
 unless p s        =  if p then return () else s
 
-{- $lifting
-
-The monadic lifting operators promote a function to a monad.
-The function arguments are scanned left to right. For example, 
-
->	liftM2 (+) [0,1] [0,2] = [0,2,1,3]
->	liftM2 (+) (Just 1) Nothing = Nothing
-
--}
-
+-- | Promote a function to a monad.
 liftM   :: (Monad m) => (a1 -> r) -> m a1 -> m r
-liftM2  :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
-liftM3  :: (Monad m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
-liftM4  :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r
-liftM5  :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r
-
 liftM f m1              = do { x1 <- m1; return (f x1) }
+
+-- | Promote a function to a monad, scanning the monadic arguments from
+-- left to right.  For example,
+--
+-- >    liftM2 (+) [0,1] [0,2] = [0,2,1,3]
+-- >    liftM2 (+) (Just 1) Nothing = Nothing
+--
+liftM2  :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
 liftM2 f m1 m2          = do { x1 <- m1; x2 <- m2; return (f x1 x2) }
+
+-- | Promote a function to a monad, scanning the monadic arguments from
+-- left to right (cf. 'liftM2').
+liftM3  :: (Monad m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r
 liftM3 f m1 m2 m3       = do { x1 <- m1; x2 <- m2; x3 <- m3; return (f x1 x2 x3) }
+
+-- | Promote a function to a monad, scanning the monadic arguments from
+-- left to right (cf. 'liftM2').
+liftM4  :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r
 liftM4 f m1 m2 m3 m4    = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; return (f x1 x2 x3 x4) }
+
+-- | Promote a function to a monad, scanning the monadic arguments from
+-- left to right (cf. 'liftM2').
+liftM5  :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r
 liftM5 f m1 m2 m3 m4 m5 = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; x5 <- m5; return (f x1 x2 x3 x4 x5) }
 
 {- | In many situations, the 'liftM' operations can be replaced by uses of
 'ap', which promotes function application. 
 
->	return f `ap` x1 `ap` ... `ap` xn
+>       return f `ap` x1 `ap` ... `ap` xn
 
 is equivalent to 
 
->	liftMn f x1 x2 ... xn
+>       liftMn f x1 x2 ... xn
 
 -}
 
 ap                :: (Monad m) => m (a -> b) -> m a -> m b
 ap                =  liftM2 id
 
+
+-- -----------------------------------------------------------------------------
+-- Other MonadPlus functions
+
+-- | Direct 'MonadPlus' equivalent of 'filter'
+-- @'filter'@ = @(mfilter:: (a -> Bool) -> [a] -> [a]@
+-- applicable to any 'MonadPlus', for example
+-- @mfilter odd (Just 1) == Just 1@
+-- @mfilter odd (Just 2) == Nothing@
+
+mfilter :: (MonadPlus m) => (a -> Bool) -> m a -> m a
+mfilter p ma = do
+  a <- ma
+  if p a then return a else mzero
+
 {- $naming
 
 The functions in this library use the following naming conventions: 
 
-* A postfix `M' always stands for a function in the Kleisli category:
-  @m@ is added to function results (modulo currying) and nowhere else.
-  So, for example, 
+* A postfix \'@M@\' always stands for a function in the Kleisli category:
+  The monad type constructor @m@ is added to function results
+  (modulo currying) and nowhere else.  So, for example, 
 
 >  filter  ::              (a ->   Bool) -> [a] ->   [a]
 >  filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
 
-* A postfix `_' changes the result type from @(m a)@ to @(m ())@.
-  Thus (in the "Prelude"): 
+* A postfix \'@_@\' changes the result type from @(m a)@ to @(m ())@.
+  Thus, for example: 
 
 >  sequence  :: Monad m => [m a] -> m [a] 
 >  sequence_ :: Monad m => [m a] -> m () 
 
-* A prefix `m' generalises an existing function to a monadic form.
+* A prefix \'@m@\' generalizes an existing function to a monadic form.
   Thus, for example: 
 
 >  sum  :: Num a       => [a]   -> a