[Git][ghc/ghc][wip/document-system-io] Documentation: Improve documentation for symbols exported from System.IO
Jade (@Jade)
gitlab at gitlab.haskell.org
Thu May 9 16:10:25 UTC 2024
Jade pushed to branch wip/document-system-io at Glasgow Haskell Compiler / GHC
Commits:
0b4d3e06 by Jade at 2024-05-09T18:15:10+02:00
Documentation: Improve documentation for symbols exported from System.IO
- - - - -
8 changed files:
- libraries/base/src/GHC/IO/Handle.hs
- libraries/base/src/GHC/IO/StdHandles.hs
- libraries/base/src/System/IO.hs
- libraries/ghc-internal/src/GHC/Internal/IO/Handle.hs
- libraries/ghc-internal/src/GHC/Internal/IO/Handle/Text.hs
- libraries/ghc-internal/src/GHC/Internal/IO/Handle/Types.hs
- libraries/ghc-internal/src/GHC/Internal/IO/StdHandles.hs
- libraries/ghc-internal/src/GHC/Internal/System/IO.hs
Changes:
=====================================
libraries/base/src/GHC/IO/Handle.hs
=====================================
@@ -73,4 +73,4 @@ module GHC.IO.Handle
hPutBufNonBlocking
) where
-import GHC.Internal.IO.Handle
\ No newline at end of file
+import GHC.Internal.IO.Handle
=====================================
libraries/base/src/GHC/IO/StdHandles.hs
=====================================
@@ -26,4 +26,4 @@ module GHC.IO.StdHandles
withFileBlocking
) where
-import GHC.Internal.IO.StdHandles
\ No newline at end of file
+import GHC.Internal.IO.StdHandles
=====================================
libraries/base/src/System/IO.hs
=====================================
@@ -14,7 +14,10 @@
--
module System.IO
- (-- * The IO monad
+ (-- * Examples
+ -- $stdio_examples
+
+ -- * The IO monad
IO,
fixIO,
-- * Files and handles
@@ -199,3 +202,18 @@ import GHC.Internal.System.IO
-- It follows that an attempt to write to a file (using 'writeFile', for
-- example) that was earlier opened by 'readFile' will usually result in
-- failure with 'GHC.Internal.System.IO.Error.isAlreadyInUseError'.
+
+-- $stdio_examples
+-- Note: Some of the examples in this module do not work "as is" in ghci.
+-- This is because using 'stdin' in combination with lazy IO
+-- does not work well in interactive mode.
+--
+-- Lines starting with @>@ indicate 'stdin' and @^D@ signales EOF.
+--
+-- ==== __Example__
+--
+-- >>> foo
+-- > input
+-- output
+-- > input^D
+-- output
=====================================
libraries/ghc-internal/src/GHC/Internal/IO/Handle.hs
=====================================
@@ -74,6 +74,7 @@ import GHC.Internal.Real
import GHC.Internal.Data.Maybe
import GHC.Internal.Data.Typeable
+
-- ---------------------------------------------------------------------------
-- Closing a handle
@@ -468,6 +469,9 @@ hTell handle =
-- handle. Each of these operations returns `True' if the handle has
-- the specified property, and `False' otherwise.
+-- | @'hIsOpen' hdl@ returns whether the handle is open.
+-- If the 'haType' of @hdl@ is 'ClosedHandle' or 'SemiClosedHandle' this returns 'False'
+-- and 'True' otherwise.
hIsOpen :: Handle -> IO Bool
hIsOpen handle =
withHandle_ "hIsOpen" handle $ \ handle_ -> do
@@ -476,6 +480,9 @@ hIsOpen handle =
SemiClosedHandle -> return False
_ -> return True
+-- | @'hIsOpen' hdl@ returns whether the handle is closed.
+-- If the 'haType' of @hdl@ is 'ClosedHandle' this returns 'True'
+-- and 'False' otherwise.
hIsClosed :: Handle -> IO Bool
hIsClosed handle =
withHandle_ "hIsClosed" handle $ \ handle_ -> do
@@ -493,6 +500,7 @@ hIsClosed handle =
return (not (ho || hc))
-}
+-- | @'hIsReadable' hdl@ returns whether it is possible to read from the handle.
hIsReadable :: Handle -> IO Bool
hIsReadable (DuplexHandle _ _ _) = return True
hIsReadable handle =
@@ -502,6 +510,7 @@ hIsReadable handle =
SemiClosedHandle -> ioe_semiclosedHandle
htype -> return (isReadableHandleType htype)
+-- | @'hIsWritable' hdl@ returns whether it is possible to write to the handle.
hIsWritable :: Handle -> IO Bool
hIsWritable (DuplexHandle _ _ _) = return True
hIsWritable handle =
@@ -524,6 +533,7 @@ hGetBuffering handle =
-- of a semi-closed handle to be queried. -- sof 6/98
return (haBufferMode handle_) -- could be stricter..
+-- | @'hIsSeekable' hdl@ returns whether it is possible to 'hSeek' with the given handle.
hIsSeekable :: Handle -> IO Bool
hIsSeekable handle =
withHandle_ "hIsSeekable" handle $ \ handle_ at Handle__{..} -> do
@@ -775,4 +785,3 @@ showHandle' filepath is_duplex h =
where
def :: Int
def = bufSize buf
-
=====================================
libraries/ghc-internal/src/GHC/Internal/IO/Handle/Text.hs
=====================================
@@ -484,8 +484,9 @@ getSomeCharacters handle_ at Handle__{..} buf at Buffer{..} =
-- | The 'hGetContents'' operation reads all input on the given handle
-- before returning it as a 'String' and closing the handle.
--
+-- This is a strict version of 'hGetContents'
+--
-- @since base-4.15.0.0
-
hGetContents' :: Handle -> IO String
hGetContents' handle = do
es <- wantReadableHandle "hGetContents'" handle (strictRead handle)
@@ -565,7 +566,7 @@ lazyBuffersToString CRLF = loop '\0' where
--
-- This operation may fail with:
--
--- * 'isFullError' if the device is full; or
+-- * 'isFullError' if the device is full.
--
-- * 'isPermissionError' if another system resource limit would be exceeded.
@@ -623,16 +624,28 @@ hPutcBuffered handle_ at Handle__{..} c = do
-- | Computation 'hPutStr' @hdl s@ writes the string
-- @s@ to the file or channel managed by @hdl at .
--
+-- Note that 'hPutStr' is not concurrency safe unless the 'BufferMode' of
+-- @hdl@ is set to 'LineBuffering' or 'BlockBuffering':
+--
+-- >>> let f = forkIO . hPutStr stdout
+-- >>> in do hSetBuffering stdout NoBuffering; f "This is a longer string"; f ":D"; f "Hello Haskell"; pure ()
+-- This: HDiesl lao lHoansgkeerl lstring
+--
+-- >>> let f = forkIO . hPutStr stdout
+-- >>> in do hSetBuffering stdout LineBuffering; f "This is a longer string"; f ":D"; f "Hello Haskell"; pure ()
+-- This is a longer string:DHello Haskell
+--
-- This operation may fail with:
--
--- * 'isFullError' if the device is full; or
+-- * 'isFullError' if the device is full.
--
-- * 'isPermissionError' if another system resource limit would be exceeded.
-
hPutStr :: Handle -> String -> IO ()
hPutStr handle str = hPutStr' handle str False
-- | The same as 'hPutStr', but adds a newline character.
+--
+-- This operation may fail with the same errors, and has the same issues with concurrency, as 'hPutStr'!
hPutStrLn :: Handle -> String -> IO ()
hPutStrLn handle str = hPutStr' handle str True
@@ -1176,4 +1189,3 @@ illegalBufferSize handle fn sz =
InvalidArgument fn
("illegal buffer size " ++ showsPrec 9 sz [])
Nothing Nothing)
-
=====================================
libraries/ghc-internal/src/GHC/Internal/IO/Handle/Types.hs
=====================================
@@ -162,21 +162,29 @@ data HandleType
| AppendHandle
| ReadWriteHandle
+-- | @'isReadableHandleType' hdlType@ returns 'True' if
+-- @hdlType@ is one of 'ReadHandle' and 'ReadWriteHandle'.
isReadableHandleType :: HandleType -> Bool
isReadableHandleType ReadHandle = True
isReadableHandleType ReadWriteHandle = True
isReadableHandleType _ = False
+-- | @'isWritableHandleType' hdlType@ returns 'True' if
+-- @hdlType@ is one of 'AppendHandle', 'WriteHandle' and 'ReadWriteHandle'.
isWritableHandleType :: HandleType -> Bool
isWritableHandleType AppendHandle = True
isWritableHandleType WriteHandle = True
isWritableHandleType ReadWriteHandle = True
isWritableHandleType _ = False
+-- | @'isReadWriteHandleType' hdlType@ returns 'True' if
+-- @hdlType@ is 'ReadWriteHandle'.
isReadWriteHandleType :: HandleType -> Bool
isReadWriteHandleType ReadWriteHandle{} = True
isReadWriteHandleType _ = False
+-- | @'isAppendHandleType' hdlType@ returns 'True' if
+-- @hdlType@ is 'AppendHandle'.
isAppendHandleType :: HandleType -> Bool
isAppendHandleType AppendHandle = True
isAppendHandleType _ = False
@@ -450,4 +458,3 @@ instance Show Handle where
showHandle :: FilePath -> String -> String
showHandle file = showString "{handle: " . showString file . showString "}"
-
=====================================
libraries/ghc-internal/src/GHC/Internal/IO/StdHandles.hs
=====================================
@@ -29,72 +29,122 @@ import GHC.Internal.IO.IOMode
import GHC.Internal.IO.Handle.Types
import qualified GHC.Internal.IO.Handle.FD as POSIX
+
+-- windows only imports
#if defined(mingw32_HOST_OS)
import GHC.Internal.IO.SubSystem
import qualified GHC.Internal.IO.Handle.Windows as Win
import GHC.Internal.IO.Handle.Internals (hClose_impl)
+#endif
+-- | 'stdin' is a handle managing the programs standard input.
stdin :: Handle
+#if defined(mingw32_HOST_OS)
stdin = POSIX.stdin <!> Win.stdin
+#else
+stdin = POSIX.stdin
+#endif
+-- | 'stdout' is a handle managing the programs standard output.
stdout :: Handle
+#if defined(mingw32_HOST_OS)
stdout = POSIX.stdout <!> Win.stdout
+#else
+stdout = POSIX.stdout
+#endif
+-- | 'stderr' is a handle managing the programs standard error.
stderr :: Handle
+#if defined(mingw32_HOST_OS)
stderr = POSIX.stderr <!> Win.stderr
+#else
+stderr = POSIX.stderr
+#endif
-openFile :: FilePath -> IOMode -> IO Handle
+-- | The computation @'openFile' path mode@ returns a file handle that can be
+-- used to interact with the file.
+--
+-- The handle is open in text mode with 'System.IO.localeEncoding'.
+-- You can change the encoding with 'System.IO.hSetEncoding'.
+openFile
+ :: FilePath -- ^ The path to the file that should be opened
+ -> IOMode -- ^ The mode in which the file should be opened
+ -> IO Handle
+#if defined(mingw32_HOST_OS)
openFile = POSIX.openFile <!> Win.openFile
+#else
+openFile = POSIX.openFile
+#endif
+-- | The computation @'openBinaryFile' path mode@ returns a file handle that can be
+-- used to interact with the binary file.
+--
+-- This is different from 'openFile' as in that it does not use any file encoding.
+openBinaryFile
+ :: FilePath -- ^ The path to the binary file that should be opened
+ -> IOMode -- ^ The mode in which the binary file should be opened
+ -> IO Handle
+#if defined(mingw32_HOST_OS)
+openBinaryFile = POSIX.openBinaryFile <!> Win.openBinaryFile
+#else
+openBinaryFile = POSIX.openBinaryFile
+#endif
+
+-- | The computation @'withFile' path mode action@ opens the file and runs @action@
+-- with the obtained handle before closing the file.
+--
+-- Even when an exception is raised within the 'action', the file will still be closed.
+-- This is why @'withFile' path mode act@ is preferable to
+--
+-- @'openFile' path mode >>= (\\hdl -> act hdl >>= 'System.IO.hClose' hdl)@
+--
+-- See also: 'System.IO.bracket'
+withFile
+ :: FilePath -- ^ The path to the file that should be opened
+ -> IOMode -- ^ The mode in which the file should be opened
+ -> (Handle -> IO r) -- ^ The action to run with the obtained handle
+ -> IO r
+#if defined(mingw32_HOST_OS)
-- TODO: implement as for POSIX
-withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
withFile = POSIX.withFile <!> wf
where
wf path mode act = bracket (Win.openFile path mode) hClose_impl act
+#else
+withFile = POSIX.withFile
+#endif
-openBinaryFile :: FilePath -> IOMode -> IO Handle
-openBinaryFile = POSIX.openBinaryFile <!> Win.openBinaryFile
-
+-- | The computation @'withBinaryFile' path mode action@ opens the binary file
+-- and runs @action@ with the obtained handle before closing the binary file.
+--
+-- This is different from 'withFile' as in that it does not use any file encoding.
+--
+-- Even when an exception is raised within the 'action', the file will still be closed.
+-- This is why @'withBinaryFile' path mode act@ is preferable to
+--
+-- @'openBinaryFile' path mode >>= (\\hdl -> act hdl >>= 'System.IO.hClose' hdl)@
+--
+-- See also: 'System.IO.bracket'
withBinaryFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
+#if defined(mingw32_HOST_OS)
withBinaryFile = POSIX.withBinaryFile <!> wf
where
wf path mode act = bracket (Win.openBinaryFile path mode) hClose_impl act
+#else
+withBinaryFile = POSIX.withBinaryFile
+#endif
openFileBlocking :: FilePath -> IOMode -> IO Handle
+#if defined(mingw32_HOST_OS)
openFileBlocking = POSIX.openFileBlocking <!> Win.openFileBlocking
+#else
+openFileBlocking = POSIX.openFileBlocking
+#endif
withFileBlocking :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
+#if defined(mingw32_HOST_OS)
withFileBlocking = POSIX.withFileBlocking <!> wf
where
wf path mode act = bracket (Win.openFileBlocking path mode) hClose_impl act
-
#else
-
-stdin :: Handle
-stdin = POSIX.stdin
-
-stdout :: Handle
-stdout = POSIX.stdout
-
-stderr :: Handle
-stderr = POSIX.stderr
-
-openFile :: FilePath -> IOMode -> IO Handle
-openFile = POSIX.openFile
-
-withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
-withFile = POSIX.withFile
-
-openBinaryFile :: FilePath -> IOMode -> IO Handle
-openBinaryFile = POSIX.openBinaryFile
-
-withBinaryFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
-withBinaryFile = POSIX.withBinaryFile
-
-openFileBlocking :: FilePath -> IOMode -> IO Handle
-openFileBlocking = POSIX.openFileBlocking
-
-withFileBlocking :: FilePath -> IOMode -> (Handle -> IO r) -> IO r
withFileBlocking = POSIX.withFileBlocking
-
#endif
=====================================
libraries/ghc-internal/src/GHC/Internal/System/IO.hs
=====================================
@@ -257,24 +257,49 @@ import GHC.Internal.Text.Read
import GHC.Internal.IO.StdHandles
import GHC.Internal.Show
import GHC.Internal.MVar
-
--- -----------------------------------------------------------------------------
+-----------------------------------------------------------------------------
-- Standard IO
-- | Write a character to the standard output device
--- (same as 'hPutChar' 'stdout').
-
+--
+-- 'putChar' is implemented as @'hPutChar' 'stdout'@.
+--
+-- This operation may fail with the same errors as 'hPutChar'.
+--
+-- ==== __Examples__
+--
+-- Note that the following do not put a newline.
+--
+-- >>> putChar 'x'
+-- x
+--
+-- >>> putChar '\0042'
+-- *
putChar :: Char -> IO ()
putChar c = hPutChar stdout c
-- | Write a string to the standard output device
--- (same as 'hPutStr' 'stdout').
-
+--
+-- 'putStr' is implemented as @'hPutStr' 'stdout'@.
+--
+-- This operation may fail with the same errors, and has the same issues with concurrency, as 'hPutStr'!
+--
+-- ==== __Examples__
+--
+-- Note that the following do not put a newline.
+--
+-- >>> putStr "Hello, World!"
+-- Hello, World!
+--
+-- >>> putStr "\0052\0042\0050"
+-- 4*2
+--
putStr :: String -> IO ()
putStr s = hPutStr stdout s
-- | The same as 'putStr', but adds a newline character.
-
+--
+-- This operation may fail with the same errors, and has the same issues with concurrency, as 'hPutStr'!
putStrLn :: String -> IO ()
putStrLn s = hPutStrLn stdout s
@@ -284,94 +309,242 @@ putStrLn s = hPutStrLn stdout s
-- converts values to strings for output using the 'show' operation and
-- adds a newline.
--
--- For example, a program to print the first 20 integers and their
+-- 'print' is implemented as @'putStrLn' '.' 'show'@
+--
+-- This operation may fail with the same errors, and has the same issues with concurrency, as 'hPutStr'!
+--
+-- ==== __Examples__
+--
+-- >>> print [1, 2, 3]
+-- [1,2,3]
+--
+-- Be careful when using 'print' for outputting strings,
+-- as this will invoke 'show' and cause strings to be printed
+-- with quotation marks and non-ascii symbols escaped.
+--
+-- >>> print "λ :D"
+-- "\995 :D"
+--
+-- A program to print the first 8 integers and their
-- powers of 2 could be written as:
--
--- > main = print ([(n, 2^n) | n <- [0..19]])
-
+-- >>> print [(n, 2^n) | n <- [0..8]]
+-- [(0,1),(1,2),(2,4),(3,8),(4,16),(5,32),(6,64),(7,128),(8,256)]
print :: Show a => a -> IO ()
print x = putStrLn (show x)
--- | Read a character from the standard input device
--- (same as 'hGetChar' 'stdin').
-
+-- | Read a single character from the standard input device.
+--
+-- 'getChar' is implemented as @'hGetChar' 'stdin'@.
+--
+-- This operation may fail with the same errors as 'hGetChar'.
+--
+-- ==== __Examples__
+--
+-- >>> getChar
+-- a'a'
+--
+-- >>> getChar
+-- >
+-- '\n'
getChar :: IO Char
getChar = hGetChar stdin
--- | Read a line from the standard input device
--- (same as 'hGetLine' 'stdin').
-
+-- | Read a line from the standard input device.
+--
+-- 'getLine' is implemented as @'hGetLine' 'stdin'@.
+--
+-- This operation may fail with the same errors as 'hGetLine'.
+--
+-- ==== __Examples__
+--
+-- >>> getLine
+-- > Hello World!
+-- "Hello World!"
+--
+-- >>> getLine
+-- >
+-- ""
getLine :: IO String
getLine = hGetLine stdin
-- | The 'getContents' operation returns all user input as a single string,
--- which is read lazily as it is needed
--- (same as 'hGetContents' 'stdin').
-
+-- which is read lazily as it is needed.
+--
+-- 'getContents' is implemented as @'hGetContents' 'stdin'@.
+--
+-- This operation may fail with the same errors as 'hGetContents'.
+--
+-- ==== __Examples__
+--
+-- >>> getContents >>= putStr
+-- > aaabbbccc :D
+-- aaabbbccc :D
+-- > I hope you have a great day
+-- I hope you have a great day
+-- > ^D
+--
+-- >>> getContents >>= print . length
+-- > abc
+-- > <3
+-- > def ^D
+-- 11
getContents :: IO String
getContents = hGetContents stdin
-- | The 'getContents'' operation returns all user input as a single string,
-- which is fully read before being returned
--- (same as 'hGetContents'' 'stdin').
+--
+-- 'getContents'' is implemented as @'hGetContents'' 'stdin'@.
+--
+-- This operation may fail with the same errors as 'hGetContents''.
+--
+-- ==== __Examples__
+--
+-- >>> getContents' >>= putStr
+-- > aaabbbccc :D
+-- > I hope you have a great day
+-- aaabbbccc :D
+-- I hope you have a great day
+--
+-- >>> getContents' >>= print . length
+-- > abc
+-- > <3
+-- > def ^D
+-- 11
--
-- @since base-4.15.0.0
-
getContents' :: IO String
getContents' = hGetContents' stdin
--- | The 'interact' function takes a function of type @String->String@
--- as its argument. The entire input from the standard input device is
--- passed to this function as its argument, and the resulting string is
--- output on the standard output device.
-
+-- | @'interact' f@ takes the entire input from 'stdin' and applies @f@ to it.
+-- The resulting string is written to the 'stdout' device.
+--
+-- Note that this operation is lazy, which allows to produce output
+-- even before all input has been consumed.
+--
+-- This operation may fail with the same errors as 'getContents' and 'putStr'.
+--
+-- ==== __Examples__
+--
+-- >>> interact (\str -> str ++ str)
+-- > hi :)
+-- hi :)
+-- > ^D
+-- hi :)
+--
+-- >>> interact (const ":D")
+-- :D
+--
+-- >>> interact (show . words)
+-- > hello world!
+-- > I hope you have a great day
+-- > ^D
+-- ["hello","world!","I","hope","you","have","a","great","day"]
interact :: (String -> String) -> IO ()
interact f = do s <- getContents
putStr (f s)
-- | The 'readFile' function reads a file and
-- returns the contents of the file as a string.
+--
-- The file is read lazily, on demand, as with 'getContents'.
-
+--
+-- This operation may fail with the same errors as 'hGetContents' and 'openFile'.
+--
+-- ==== __Examples__
+--
+-- >>> readFile "~/hello_world"
+-- "Greetings!"
+--
+-- >>> take 5 <$> readFile "/dev/zero"
+-- "\NUL\NUL\NUL\NUL\NUL"
readFile :: FilePath -> IO String
readFile name = openFile name ReadMode >>= hGetContents
-- | The 'readFile'' function reads a file and
-- returns the contents of the file as a string.
--- The file is fully read before being returned, as with 'getContents''.
+--
+-- This is identical to 'readFile', but the file is fully read before being returned,
+-- as with 'getContents''.
--
-- @since base-4.15.0.0
-
readFile' :: FilePath -> IO String
-- There's a bit of overkill here—both withFile and
-- hGetContents' will close the file in the end.
readFile' name = withFile name ReadMode hGetContents'
--- | The computation 'writeFile' @file str@ function writes the string @str@,
+-- | The computation @'writeFile' file str@ function writes the string @str@,
-- to the file @file at .
+--
+-- This operation may fail with the same errors as 'hPutStr' and 'withFile'.
+--
+-- ==== __Examples__
+--
+-- >>> writeFile "hello" "world" >> readFile "hello"
+-- "world"
+--
+-- >>> writeFile "~/" "D:"
+-- *** Exception: ~/: withFile: inappropriate type (Is a directory)
writeFile :: FilePath -> String -> IO ()
writeFile f txt = withFile f WriteMode (\ hdl -> hPutStr hdl txt)
--- | The computation 'appendFile' @file str@ function appends the string @str@,
+-- | The computation @'appendFile' file str@ function appends the string @str@,
-- to the file @file at .
--
-- Note that 'writeFile' and 'appendFile' write a literal string
-- to a file. To write a value of any printable type, as with 'print',
-- use the 'show' function to convert the value to a string first.
--
--- > main = appendFile "squares" (show [(x,x*x) | x <- [0,0.1..2]])
-
+-- This operation may fail with the same errors as 'hPutStr' and 'withFile'.
+--
+-- ==== __Examples__
+--
+-- The following example could be more efficently written by acquiring a handle
+-- instead with 'openFile' and using the computations capable of writing to handles
+-- such as 'hPutStr'.
+--
+-- >>> let fn = "hello_world"
+-- >>> in writeFile fn "hello" >> appendFile fn " world!" >> (readFile fn >>= putStrLn)
+-- "hello world!"
+--
+-- >>> let fn = "foo"; output = readFile' fn >>= putStrLn
+-- >>> in output >> appendFile fn (show [1,2,3]) >> output
+-- this is what's in the file
+-- this is what's in the file[1,2,3]
appendFile :: FilePath -> String -> IO ()
appendFile f txt = withFile f AppendMode (\ hdl -> hPutStr hdl txt)
-- | The 'readLn' function combines 'getLine' and 'readIO'.
-
+--
+-- This operation may fail with the same errors as 'getLine' and 'readIO'.
+--
+-- ==== __Examples__
+--
+-- >>> fmap (+ 5) readLn
+-- > 25
+-- 30
+--
+-- >>> readLn :: IO String
+-- > this is not a string literal
+-- *** Exception: user error (Prelude.readIO: no parse)
readLn :: Read a => IO a
readLn = getLine >>= readIO
-- | The 'readIO' function is similar to 'read' except that it signals
-- parse failure to the 'IO' monad instead of terminating the program.
-
+--
+-- This operation may fail with:
+--
+-- * 'GHC.Internal.System.IO.Error.isUserError' if there is no unambiguous parse.
+--
+-- ==== __Examples__
+--
+-- >>> fmap (+ 1) (readIO "1")
+-- 2
+--
+-- >>> readIO "not quite ()" :: IO ()
+-- *** Exception: user error (Prelude.readIO: no parse)
readIO :: Read a => String -> IO a
readIO s = case (do { (x,t) <- reads s ;
("","") <- lex t ;
@@ -380,7 +553,7 @@ readIO s = case (do { (x,t) <- reads s ;
[] -> ioError (userError "Prelude.readIO: no parse")
_ -> ioError (userError "Prelude.readIO: ambiguous parse")
--- | The Unicode encoding of the current locale
+-- | The encoding of the current locale.
--
-- This is the initial locale encoding: if it has been subsequently changed by
-- 'GHC.Internal.IO.Encoding.setLocaleEncoding' this value will not reflect that change.
@@ -393,21 +566,22 @@ localeEncoding = initLocaleEncoding
-- This operation may fail with:
--
-- * 'GHC.Internal.System.IO.Error.isEOFError' if the end of file has been reached.
-
hReady :: Handle -> IO Bool
hReady h = hWaitForInput h 0
-- | Computation 'hPrint' @hdl t@ writes the string representation of @t@
--- given by the 'shows' function to the file or channel managed by @hdl@
+-- given by the 'show' function to the file or channel managed by @hdl@
-- and appends a newline.
--
--- This operation may fail with:
+-- This operation may fail with the same errors as 'hPutStrLn'
--
--- * 'GHC.Internal.System.IO.Error.isFullError' if the device is full; or
+-- ==== __Examples__
--
--- * 'GHC.Internal.System.IO.Error.isPermissionError' if another system resource limit
--- would be exceeded.
-
+-- >>> hPrint stdout [1,2,3]
+-- [1,2,3]
+--
+-- >>> hPrint stdin [4,5,6]
+-- *** Exception: <stdin>: hPutStr: illegal operation (handle is not open for writing)
hPrint :: Show a => Handle -> a -> IO ()
hPrint hdl = hPutStrLn hdl . show
@@ -415,9 +589,50 @@ hPrint hdl = hPutStrLn hdl . show
-- ---------------------------------------------------------------------------
-- fixIO
--- | The implementation of 'GHC.Internal.Control.Monad.Fix.mfix' for 'IO'. If the function
--- passed to 'fixIO' inspects its argument, the resulting action will throw
--- 'FixIOException'.
+-- | The implementation of 'Control.Monad.Fix.mfix' for 'IO'.
+--
+-- This operation may fail with:
+--
+-- * 'FixIOException' if the function passed to 'fixIO' inspects its argument.
+--
+-- ==== __Examples__
+--
+-- the IO-action is only executed once. The recursion is only on the values.
+--
+-- >>> take 3 <$> fixIO (\x -> putStr ":D" >> (:x) <$> readLn @Int)
+-- :D
+-- 2
+-- [2,2,2]
+--
+-- If we are strict in the value, just as with 'Data.Function.fix', we do not get termination:
+--
+-- >>> fixIO (\x -> putStr x >> pure ('x' : x))
+-- * hangs forever *
+--
+-- We can tie the knot of a structure within 'IO' using 'fixIO':
+--
+-- @
+-- data Node = MkNode Int (IORef Node)
+--
+-- foo :: IO ()
+-- foo = do
+-- p \<- fixIO (\p -> newIORef (MkNode 0 p))
+-- q <- output p
+-- r <- output q
+-- _ <- output r
+-- pure ()
+--
+-- output :: IORef Node -> IO (IORef Node)
+-- output ref = do
+-- MkNode x p <- readIORef ref
+-- print x
+-- pure p
+-- @
+--
+-- >>> foo
+-- 0
+-- 0
+-- 0
fixIO :: (a -> IO a) -> IO a
fixIO k = do
m <- newEmptyMVar
@@ -465,7 +680,6 @@ fixIO k = do
-- @O_EXCL@ flags are used to prevent this attack, but note that
-- @O_EXCL@ is sometimes not supported on NFS filesystems, so if you
-- rely on this behaviour it is best to use local filesystems only.
---
openTempFile :: FilePath -- ^ Directory in which to create the file
-> String -- ^ File name template. If the template is \"foo.ext\" then
-- the created file will be \"fooXXX.ext\" where XXX is some
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/0b4d3e06298d0ec75ed0b89f5401285ab1de1055
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/0b4d3e06298d0ec75ed0b89f5401285ab1de1055
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