libghc-base-compat-doc 0.8.2-2 / usr / share / doc / libghc-base-compat-doc / html / base-compat.txt

-- Hoogle documentation, generated by Haddock
-- See Hoogle, http://www.haskell.org/hoogle/


-- | A compatibility layer for base
--   
--   Provides functions available in later versions of <tt>base</tt> to a
--   wider range of compilers, without requiring you to use CPP pragmas in
--   your code. See the <a>README</a> for what is covered. Also see the
--   <a>changelog</a> for recent changes.
--   
--   Note that <tt>base-compat</tt> does not add any orphan instances.
--   There is a separate package <tt><a>base-orphans</a></tt> for that.
@package base-compat
@version 0.8.2

module Text.Read.Compat

-- | Parsing of <a>String</a>s, producing values.
--   
--   Derived instances of <a>Read</a> make the following assumptions, which
--   derived instances of <a>Show</a> obey:
--   
--   <ul>
--   <li>If the constructor is defined to be an infix operator, then the
--   derived <a>Read</a> instance will parse only infix applications of the
--   constructor (not the prefix form).</li>
--   <li>Associativity is not used to reduce the occurrence of parentheses,
--   although precedence may be.</li>
--   <li>If the constructor is defined using record syntax, the derived
--   <a>Read</a> will parse only the record-syntax form, and furthermore,
--   the fields must be given in the same order as the original
--   declaration.</li>
--   <li>The derived <a>Read</a> instance allows arbitrary Haskell
--   whitespace between tokens of the input string. Extra parentheses are
--   also allowed.</li>
--   </ul>
--   
--   For example, given the declarations
--   
--   <pre>
--   infixr 5 :^:
--   data Tree a =  Leaf a  |  Tree a :^: Tree a
--   </pre>
--   
--   the derived instance of <a>Read</a> in Haskell 2010 is equivalent to
--   
--   <pre>
--   instance (Read a) =&gt; Read (Tree a) where
--   
--           readsPrec d r =  readParen (d &gt; app_prec)
--                            (\r -&gt; [(Leaf m,t) |
--                                    ("Leaf",s) &lt;- lex r,
--                                    (m,t) &lt;- readsPrec (app_prec+1) s]) r
--   
--                         ++ readParen (d &gt; up_prec)
--                            (\r -&gt; [(u:^:v,w) |
--                                    (u,s) &lt;- readsPrec (up_prec+1) r,
--                                    (":^:",t) &lt;- lex s,
--                                    (v,w) &lt;- readsPrec (up_prec+1) t]) r
--   
--             where app_prec = 10
--                   up_prec = 5
--   </pre>
--   
--   Note that right-associativity of <tt>:^:</tt> is unused.
--   
--   The derived instance in GHC is equivalent to
--   
--   <pre>
--   instance (Read a) =&gt; Read (Tree a) where
--   
--           readPrec = parens $ (prec app_prec $ do
--                                    Ident "Leaf" &lt;- lexP
--                                    m &lt;- step readPrec
--                                    return (Leaf m))
--   
--                        +++ (prec up_prec $ do
--                                    u &lt;- step readPrec
--                                    Symbol ":^:" &lt;- lexP
--                                    v &lt;- step readPrec
--                                    return (u :^: v))
--   
--             where app_prec = 10
--                   up_prec = 5
--   
--           readListPrec = readListPrecDefault
--   </pre>
class Read a

-- | attempts to parse a value from the front of the string, returning a
--   list of (parsed value, remaining string) pairs. If there is no
--   successful parse, the returned list is empty.
--   
--   Derived instances of <a>Read</a> and <a>Show</a> satisfy the
--   following:
--   
--   <ul>
--   <li><tt>(x,"")</tt> is an element of <tt>(<a>readsPrec</a> d
--   (<a>showsPrec</a> d x ""))</tt>.</li>
--   </ul>
--   
--   That is, <a>readsPrec</a> parses the string produced by
--   <a>showsPrec</a>, and delivers the value that <a>showsPrec</a> started
--   with.
readsPrec :: Read a => Int -> ReadS a

-- | The method <a>readList</a> is provided to allow the programmer to give
--   a specialised way of parsing lists of values. For example, this is
--   used by the predefined <a>Read</a> instance of the <a>Char</a> type,
--   where values of type <a>String</a> should be are expected to use
--   double quotes, rather than square brackets.
readList :: Read a => ReadS [a]

-- | Proposed replacement for <a>readsPrec</a> using new-style parsers (GHC
--   only).
readPrec :: Read a => ReadPrec a

-- | Proposed replacement for <a>readList</a> using new-style parsers (GHC
--   only). The default definition uses <a>readList</a>. Instances that
--   define <a>readPrec</a> should also define <a>readListPrec</a> as
--   <a>readListPrecDefault</a>.
readListPrec :: Read a => ReadPrec [a]

-- | A parser for a type <tt>a</tt>, represented as a function that takes a
--   <a>String</a> and returns a list of possible parses as
--   <tt>(a,<a>String</a>)</tt> pairs.
--   
--   Note that this kind of backtracking parser is very inefficient;
--   reading a large structure may be quite slow (cf <a>ReadP</a>).
type ReadS a = String -> [(a, String)]

-- | equivalent to <a>readsPrec</a> with a precedence of 0.
reads :: Read a => ReadS a

-- | The <a>read</a> function reads input from a string, which must be
--   completely consumed by the input process.
read :: Read a => String -> a

-- | <tt><a>readParen</a> <a>True</a> p</tt> parses what <tt>p</tt> parses,
--   but surrounded with parentheses.
--   
--   <tt><a>readParen</a> <a>False</a> p</tt> parses what <tt>p</tt>
--   parses, but optionally surrounded with parentheses.
readParen :: Bool -> ReadS a -> ReadS a

-- | The <a>lex</a> function reads a single lexeme from the input,
--   discarding initial white space, and returning the characters that
--   constitute the lexeme. If the input string contains only white space,
--   <a>lex</a> returns a single successful `lexeme' consisting of the
--   empty string. (Thus <tt><a>lex</a> "" = [("","")]</tt>.) If there is
--   no legal lexeme at the beginning of the input string, <a>lex</a> fails
--   (i.e. returns <tt>[]</tt>).
--   
--   This lexer is not completely faithful to the Haskell lexical syntax in
--   the following respects:
--   
--   <ul>
--   <li>Qualified names are not handled properly</li>
--   <li>Octal and hexadecimal numerics are not recognized as a single
--   token</li>
--   <li>Comments are not treated properly</li>
--   </ul>
lex :: ReadS String
data Lexeme :: *

-- | Character literal
Char :: Char -> Lexeme

-- | String literal, with escapes interpreted
String :: String -> Lexeme

-- | Punctuation or reserved symbol, e.g. <tt>(</tt>, <tt>::</tt>
Punc :: String -> Lexeme

-- | Haskell identifier, e.g. <tt>foo</tt>, <tt>Baz</tt>
Ident :: String -> Lexeme

-- | Haskell symbol, e.g. <tt>&gt;&gt;</tt>, <tt>:%</tt>
Symbol :: String -> Lexeme

Number :: Number -> Lexeme
EOF :: Lexeme

-- | Parse a single lexeme
lexP :: ReadPrec Lexeme

-- | <tt>(parens p)</tt> parses "P", "(P0)", "((P0))", etc, where
--   <tt>p</tt> parses "P" in the current precedence context and parses
--   "P0" in precedence context zero
parens :: ReadPrec a -> ReadPrec a

-- | A possible replacement definition for the <a>readList</a> method (GHC
--   only). This is only needed for GHC, and even then only for <a>Read</a>
--   instances where <a>readListPrec</a> isn't defined as
--   <a>readListPrecDefault</a>.
readListDefault :: Read a => ReadS [a]

-- | A possible replacement definition for the <a>readListPrec</a> method,
--   defined using <a>readPrec</a> (GHC only).
readListPrecDefault :: Read a => ReadPrec [a]

-- | Parse a string using the <a>Read</a> instance. Succeeds if there is
--   exactly one valid result. A <a>Left</a> value indicates a parse error.
readEither :: Read a => String -> Either String a

-- | Parse a string using the <a>Read</a> instance. Succeeds if there is
--   exactly one valid result.
readMaybe :: Read a => String -> Maybe a

module System.Exit.Compat

-- | Write given error message to <a>stderr</a> and terminate with
--   <a>exitFailure</a>.
die :: String -> IO a


-- | Miscellaneous information about the system environment.
module System.Environment.Compat

-- | Computation <a>getArgs</a> returns a list of the program's command
--   line arguments (not including the program name).
getArgs :: IO [String]

-- | Computation <a>getProgName</a> returns the name of the program as it
--   was invoked.
--   
--   However, this is hard-to-impossible to implement on some non-Unix
--   OSes, so instead, for maximum portability, we just return the leafname
--   of the program as invoked. Even then there are some differences
--   between platforms: on Windows, for example, a program invoked as foo
--   is probably really <tt>FOO.EXE</tt>, and that is what
--   <a>getProgName</a> will return.
getProgName :: IO String

-- | Computation <a>getEnv</a> <tt>var</tt> returns the value of the
--   environment variable <tt>var</tt>. For the inverse, POSIX users can
--   use <a>putEnv</a>.
--   
--   This computation may fail with:
--   
--   <ul>
--   <li><a>isDoesNotExistError</a> if the environment variable does not
--   exist.</li>
--   </ul>
getEnv :: String -> IO String

-- | Return the value of the environment variable <tt>var</tt>, or
--   <tt>Nothing</tt> if there is no such value.
--   
--   For POSIX users, this is equivalent to <a>getEnv</a>.
lookupEnv :: String -> IO (Maybe String)

-- | <tt>setEnv name value</tt> sets the specified environment variable to
--   <tt>value</tt>.
--   
--   On Windows setting an environment variable to the <i>empty string</i>
--   removes that environment variable from the environment. For the sake
--   of compatibility we adopt that behavior. In particular
--   
--   <pre>
--   setEnv name ""
--   </pre>
--   
--   has the same effect as
--   
--   <pre>
--   <a>unsetEnv</a> name
--   </pre>
--   
--   If you don't care about Windows support and want to set an environment
--   variable to the empty string use <tt>System.Posix.Env.setEnv</tt> from
--   the <tt>unix</tt> package instead.
--   
--   Throws <a>IOException</a> if <tt>name</tt> is the empty string or
--   contains an equals sign.
setEnv :: String -> String -> IO ()

-- | <tt>unSet name</tt> removes the specified environment variable from
--   the environment of the current process.
--   
--   Throws <a>IOException</a> if <tt>name</tt> is the empty string or
--   contains an equals sign.
unsetEnv :: String -> IO ()

-- | <a>withArgs</a> <tt>args act</tt> - while executing action
--   <tt>act</tt>, have <a>getArgs</a> return <tt>args</tt>.
withArgs :: [String] -> IO a -> IO a

-- | <a>withProgName</a> <tt>name act</tt> - while executing action
--   <tt>act</tt>, have <a>getProgName</a> return <tt>name</tt>.
withProgName :: String -> IO a -> IO a

-- | <a>getEnvironment</a> retrieves the entire environment as a list of
--   <tt>(key,value)</tt> pairs.
--   
--   If an environment entry does not contain an <tt>'='</tt> character,
--   the <tt>key</tt> is the whole entry and the <tt>value</tt> is the
--   empty string.
getEnvironment :: IO [(String, String)]

module Prelude.Compat

module Numeric.Compat

-- | Show a signed <a>RealFloat</a> value using standard decimal notation
--   (e.g. <tt>245000</tt>, <tt>0.0015</tt>).
--   
--   This behaves as <a>showFFloat</a>, except that a decimal point is
--   always guaranteed, even if not needed.
showFFloatAlt :: RealFloat a => Maybe Int -> a -> ShowS

-- | Show a signed <a>RealFloat</a> value using standard decimal notation
--   for arguments whose absolute value lies between <tt>0.1</tt> and
--   <tt>9,999,999</tt>, and scientific notation otherwise.
--   
--   This behaves as <a>showFFloat</a>, except that a decimal point is
--   always guaranteed, even if not needed.
showGFloatAlt :: RealFloat a => Maybe Int -> a -> ShowS

module Foreign.Marshal.Utils.Compat

-- | Fill a given number of bytes in memory area with a byte value.
fillBytes :: Ptr a -> Word8 -> Int -> IO ()

module Foreign.Marshal.Array.Compat

-- | Like <a>mallocArray</a>, but allocated memory is filled with bytes of
--   value zero.
callocArray :: Storable a => Int -> IO (Ptr a)

-- | Like <a>callocArray0</a>, but allocated memory is filled with bytes of
--   value zero.
callocArray0 :: Storable a => Int -> IO (Ptr a)

module Foreign.Marshal.Alloc.Compat

-- | Like <a>malloc</a> but memory is filled with bytes of value zero.
calloc :: Storable a => IO (Ptr a)

-- | Llike <a>mallocBytes</a> but memory is filled with bytes of value
--   zero.
callocBytes :: Int -> IO (Ptr a)

module Foreign.Marshal.Compat

module Foreign.Compat

module Debug.Trace.Compat

-- | Like <a>trace</a> but returns the message instead of a third value.
traceId :: String -> String

-- | Like <a>traceShow</a> but returns the shown value instead of a third
--   value.
traceShowId :: Show a => a -> a

-- | Like <a>trace</a> but returning unit in an arbitrary monad. Allows for
--   convenient use in do-notation. Note that the application of
--   <a>trace</a> is not an action in the monad, as <a>traceIO</a> is in
--   the <a>IO</a> monad.
--   
--   <pre>
--   ... = do
--     x &lt;- ...
--     traceM $ "x: " ++ show x
--     y &lt;- ...
--     traceM $ "y: " ++ show y
--   </pre>
traceM :: Monad m => String -> m ()

-- | Like <a>traceM</a>, but uses <a>show</a> on the argument to convert it
--   to a <a>String</a>.
--   
--   <pre>
--   ... = do
--     x &lt;- ...
--     traceMShow $ x
--     y &lt;- ...
--     traceMShow $ x + y
--   </pre>
traceShowM :: (Show a, Monad m) => a -> m ()

module Data.Word.Compat

-- | Swap bytes in <a>Word16</a>.
byteSwap16 :: Word16 -> Word16

-- | Reverse order of bytes in <a>Word32</a>.
byteSwap32 :: Word32 -> Word32

-- | Reverse order of bytes in <a>Word64</a>.
byteSwap64 :: Word64 -> Word64

module Data.Version.Compat

-- | Construct tag-less <a>Version</a>
makeVersion :: [Int] -> Version

module Data.Monoid.Compat

-- | An infix synonym for <a>mappend</a>.
(<>) :: Monoid m => m -> m -> m

module Data.List.Compat

module Data.Functor.Compat

-- | The <a>Functor</a> class is used for types that can be mapped over.
--   Instances of <a>Functor</a> should satisfy the following laws:
--   
--   <pre>
--   fmap id  ==  id
--   fmap (f . g)  ==  fmap f . fmap g
--   </pre>
--   
--   The instances of <a>Functor</a> for lists, <a>Maybe</a> and <a>IO</a>
--   satisfy these laws.
class Functor (f :: * -> *)
fmap :: Functor f => (a -> b) -> f a -> f b

-- | Replace all locations in the input with the same value. The default
--   definition is <tt><a>fmap</a> . <a>const</a></tt>, but this may be
--   overridden with a more efficient version.
(<$) :: Functor f => a -> f b -> f a

-- | Flipped version of <a>&lt;$</a>.
--   
--   <h4><b>Examples</b></h4>
--   
--   Replace the contents of a <tt><tt>Maybe</tt> <tt>Int</tt></tt> with a
--   constant <tt>String</tt>:
--   
--   <pre>
--   &gt;&gt;&gt; Nothing $&gt; "foo"
--   Nothing
--   
--   &gt;&gt;&gt; Just 90210 $&gt; "foo"
--   Just "foo"
--   </pre>
--   
--   Replace the contents of an <tt><tt>Either</tt> <tt>Int</tt>
--   <tt>Int</tt></tt> with a constant <tt>String</tt>, resulting in an
--   <tt><tt>Either</tt> <tt>Int</tt> <tt>String</tt></tt>:
--   
--   <pre>
--   &gt;&gt;&gt; Left 8675309 $&gt; "foo"
--   Left 8675309
--   
--   &gt;&gt;&gt; Right 8675309 $&gt; "foo"
--   Right "foo"
--   </pre>
--   
--   Replace each element of a list with a constant <tt>String</tt>:
--   
--   <pre>
--   &gt;&gt;&gt; [1,2,3] $&gt; "foo"
--   ["foo","foo","foo"]
--   </pre>
--   
--   Replace the second element of a pair with a constant <tt>String</tt>:
--   
--   <pre>
--   &gt;&gt;&gt; (1,2) $&gt; "foo"
--   (1,"foo")
--   </pre>
($>) :: Functor f => f a -> b -> f b

-- | <tt><a>void</a> value</tt> discards or ignores the result of
--   evaluation, such as the return value of an <a>IO</a> action.
--   
--   <h4><b>Examples</b></h4>
--   
--   Replace the contents of a <tt><tt>Maybe</tt> <tt>Int</tt></tt> with
--   unit:
--   
--   <pre>
--   &gt;&gt;&gt; void Nothing
--   Nothing
--   
--   &gt;&gt;&gt; void (Just 3)
--   Just ()
--   </pre>
--   
--   Replace the contents of an <tt><tt>Either</tt> <tt>Int</tt>
--   <tt>Int</tt></tt> with unit, resulting in an <tt><tt>Either</tt>
--   <tt>Int</tt> '()'</tt>:
--   
--   <pre>
--   &gt;&gt;&gt; void (Left 8675309)
--   Left 8675309
--   
--   &gt;&gt;&gt; void (Right 8675309)
--   Right ()
--   </pre>
--   
--   Replace every element of a list with unit:
--   
--   <pre>
--   &gt;&gt;&gt; void [1,2,3]
--   [(),(),()]
--   </pre>
--   
--   Replace the second element of a pair with unit:
--   
--   <pre>
--   &gt;&gt;&gt; void (1,2)
--   (1,())
--   </pre>
--   
--   Discard the result of an <a>IO</a> action:
--   
--   <pre>
--   &gt;&gt;&gt; mapM print [1,2]
--   1
--   2
--   [(),()]
--   
--   &gt;&gt;&gt; void $ mapM print [1,2]
--   1
--   2
--   </pre>
void :: Functor f => f a -> f ()

module Data.Function.Compat

-- | <a>&amp;</a> is a reverse application operator. This provides
--   notational convenience. Its precedence is one higher than that of the
--   forward application operator <a>$</a>, which allows <a>&amp;</a> to be
--   nested in <a>$</a>.
(&) :: a -> (a -> b) -> b

module Data.Foldable.Compat

module Data.Either.Compat

-- | Return <a>True</a> if the given value is a <a>Left</a>-value,
--   <a>False</a> otherwise.
--   
--   <h4><b>Examples</b></h4>
--   
--   Basic usage:
--   
--   <pre>
--   &gt;&gt;&gt; isLeft (Left "foo")
--   True
--   
--   &gt;&gt;&gt; isLeft (Right 3)
--   False
--   </pre>
--   
--   Assuming a <a>Left</a> value signifies some sort of error, we can use
--   <a>isLeft</a> to write a very simple error-reporting function that
--   does absolutely nothing in the case of success, and outputs "ERROR" if
--   any error occurred.
--   
--   This example shows how <a>isLeft</a> might be used to avoid pattern
--   matching when one does not care about the value contained in the
--   constructor:
--   
--   <pre>
--   &gt;&gt;&gt; import Control.Monad ( when )
--   
--   &gt;&gt;&gt; let report e = when (isLeft e) $ putStrLn "ERROR"
--   
--   &gt;&gt;&gt; report (Right 1)
--   
--   &gt;&gt;&gt; report (Left "parse error")
--   ERROR
--   </pre>
isLeft :: Either a b -> Bool

-- | Return <a>True</a> if the given value is a <a>Right</a>-value,
--   <a>False</a> otherwise.
--   
--   <h4><b>Examples</b></h4>
--   
--   Basic usage:
--   
--   <pre>
--   &gt;&gt;&gt; isRight (Left "foo")
--   False
--   
--   &gt;&gt;&gt; isRight (Right 3)
--   True
--   </pre>
--   
--   Assuming a <a>Left</a> value signifies some sort of error, we can use
--   <a>isRight</a> to write a very simple reporting function that only
--   outputs "SUCCESS" when a computation has succeeded.
--   
--   This example shows how <a>isRight</a> might be used to avoid pattern
--   matching when one does not care about the value contained in the
--   constructor:
--   
--   <pre>
--   &gt;&gt;&gt; import Control.Monad ( when )
--   
--   &gt;&gt;&gt; let report e = when (isRight e) $ putStrLn "SUCCESS"
--   
--   &gt;&gt;&gt; report (Left "parse error")
--   
--   &gt;&gt;&gt; report (Right 1)
--   SUCCESS
--   </pre>
isRight :: Either a b -> Bool

module Data.Bool.Compat

-- | Case analysis for the <a>Bool</a> type. <tt><a>bool</a> x y p</tt>
--   evaluates to <tt>x</tt> when <tt>p</tt> is <a>False</a>, and evaluates
--   to <tt>y</tt> when <tt>p</tt> is <a>True</a>.
--   
--   This is equivalent to <tt>if p then y else x</tt>; that is, one can
--   think of it as an if-then-else construct with its arguments reordered.
--   
--   <h4><b>Examples</b></h4>
--   
--   Basic usage:
--   
--   <pre>
--   &gt;&gt;&gt; bool "foo" "bar" True
--   "bar"
--   
--   &gt;&gt;&gt; bool "foo" "bar" False
--   "foo"
--   </pre>
--   
--   Confirm that <tt><a>bool</a> x y p</tt> and <tt>if p then y else
--   x</tt> are equivalent:
--   
--   <pre>
--   &gt;&gt;&gt; let p = True; x = "bar"; y = "foo"
--   
--   &gt;&gt;&gt; bool x y p == if p then y else x
--   True
--   
--   &gt;&gt;&gt; let p = False
--   
--   &gt;&gt;&gt; bool x y p == if p then y else x
--   True
--   </pre>
bool :: a -> a -> Bool -> a

module Data.Bits.Compat

-- | Default implementation for <a>bit</a>.
--   
--   Note that: <tt>bitDefault i = 1 <a>shiftL</a> i</tt>
bitDefault :: (Bits a, Num a) => Int -> a

-- | Default implementation for <a>testBit</a>.
--   
--   Note that: <tt>testBitDefault x i = (x .&amp;. bit i) /= 0</tt>
testBitDefault :: (Bits a, Num a) => a -> Int -> Bool

-- | Default implementation for <a>popCount</a>.
--   
--   This implementation is intentionally naive. Instances are expected to
--   provide an optimized implementation for their size.
popCountDefault :: (Bits a, Num a) => a -> Int

-- | Attempt to convert an <a>Integral</a> type <tt>a</tt> to an
--   <a>Integral</a> type <tt>b</tt> using the size of the types as
--   measured by <a>Bits</a> methods.
--   
--   A simpler version of this function is:
--   
--   <pre>
--   toIntegral :: (Integral a, Integral b) =&gt; a -&gt; Maybe b
--   toIntegral x
--     | toInteger x == y = Just (fromInteger y)
--     | otherwise        = Nothing
--     where
--       y = toInteger x
--   </pre>
--   
--   This version requires going through <a>Integer</a>, which can be
--   inefficient. However, <tt>toIntegralSized</tt> is optimized to allow
--   GHC to statically determine the relative type sizes (as measured by
--   <a>bitSizeMaybe</a> and <a>isSigned</a>) and avoid going through
--   <a>Integer</a> for many types. (The implementation uses
--   <a>fromIntegral</a>, which is itself optimized with rules for
--   <tt>base</tt> types but may go through <a>Integer</a> for some type
--   pairs.)
toIntegralSized :: (Integral a, Integral b, Bits a, Bits b) => a -> Maybe b

module Control.Monad.Compat

-- | The <a>Monad</a> class defines the basic operations over a
--   <i>monad</i>, a concept from a branch of mathematics known as
--   <i>category theory</i>. From the perspective of a Haskell programmer,
--   however, it is best to think of a monad as an <i>abstract datatype</i>
--   of actions. Haskell's <tt>do</tt> expressions provide a convenient
--   syntax for writing monadic expressions.
--   
--   Instances of <a>Monad</a> should satisfy the following laws:
--   
--   <ul>
--   <li><pre><a>return</a> a <a>&gt;&gt;=</a> k = k a</pre></li>
--   <li><pre>m <a>&gt;&gt;=</a> <a>return</a> = m</pre></li>
--   <li><pre>m <a>&gt;&gt;=</a> (x -&gt; k x <a>&gt;&gt;=</a> h) = (m
--   <a>&gt;&gt;=</a> k) <a>&gt;&gt;=</a> h</pre></li>
--   </ul>
--   
--   Furthermore, the <a>Monad</a> and <a>Applicative</a> operations should
--   relate as follows:
--   
--   <ul>
--   <li><pre><a>pure</a> = <a>return</a></pre></li>
--   <li><pre>(<a>&lt;*&gt;</a>) = <a>ap</a></pre></li>
--   </ul>
--   
--   The above laws imply:
--   
--   <ul>
--   <li><pre><a>fmap</a> f xs = xs <a>&gt;&gt;=</a> <a>return</a> .
--   f</pre></li>
--   <li><pre>(<a>&gt;&gt;</a>) = (<a>*&gt;</a>)</pre></li>
--   </ul>
--   
--   and that <a>pure</a> and (<a>&lt;*&gt;</a>) satisfy the applicative
--   functor laws.
--   
--   The instances of <a>Monad</a> for lists, <a>Maybe</a> and <a>IO</a>
--   defined in the <a>Prelude</a> satisfy these laws.
class Applicative m => Monad (m :: * -> *)

-- | Sequentially compose two actions, passing any value produced by the
--   first as an argument to the second.
(>>=) :: Monad m => m a -> (a -> m b) -> m b

-- | Sequentially compose two actions, discarding any value produced by the
--   first, like sequencing operators (such as the semicolon) in imperative
--   languages.
(>>) :: Monad m => m a -> m b -> m b

-- | Inject a value into the monadic type.
return :: Monad m => a -> m a

-- | Fail with a message. This operation is not part of the mathematical
--   definition of a monad, but is invoked on pattern-match failure in a
--   <tt>do</tt> expression.
fail :: Monad m => String -> m a

-- | Monads that also support choice and failure.
class (Alternative m, Monad m) => MonadPlus (m :: * -> *)

-- | the identity of <a>mplus</a>. It should also satisfy the equations
--   
--   <pre>
--   mzero &gt;&gt;= f  =  mzero
--   v &gt;&gt; mzero   =  mzero
--   </pre>
mzero :: MonadPlus m => m a

-- | an associative operation
mplus :: MonadPlus m => m a -> m a -> m a

module Control.Concurrent.MVar.Compat

-- | Like <a>withMVar</a>, but the <tt>IO</tt> action in the second
--   argument is executed with asynchronous exceptions masked.
withMVarMasked :: MVar a -> (a -> IO b) -> IO b