module GHC.IOBase where
import GHC.ST
+import GHC.Arr -- to derive Ix class
+import GHC.Enum -- to derive Enum class
import GHC.STRef
import GHC.Base
import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
If the I\/O computation wrapped in 'unsafePerformIO'
performs side effects, then the relative order in which those side
effects take place (relative to the main I\/O trunk, or other calls to
-'unsafePerformIO') is indeterminate.
-
-However, it is less well known that
+'unsafePerformIO') is indeterminate. You have to be careful when
+writing and compiling modules that use 'unsafePerformIO':
+
+ * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
+ that calls 'unsafePerformIO'. If the call is inlined,
+ the I\/O may be performed more than once.
+
+ * Use the compiler flag @-fno-cse@ to prevent common sub-expression
+ elimination being performed on the module, which might combine
+ two side effects that were meant to be separate. A good example
+ is using multiple global variables (like @test@ in the example below).
+
+ * Make sure that the either you switch off let-floating, or that the
+ call to 'unsafePerformIO' cannot float outside a lambda. For example,
+ if you say:
+ @
+ f x = unsafePerformIO (newIORef [])
+ @
+ you may get only one reference cell shared between all calls to @f@.
+ Better would be
+ @
+ f x = unsafePerformIO (newIORef [x])
+ @
+ because now it can't float outside the lambda.
+
+It is less well known that
'unsafePerformIO' is not type safe. For example:
> test :: IORef [a]
'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
When passed a value of type @IO a@, the 'IO' will only be performed
when the value of the @a@ is demanded. This is used to implement lazy
-file reading, see 'IO.hGetContents'.
+file reading, see 'System.IO.hGetContents'.
-}
{-# NOINLINE unsafeInterleaveIO #-}
unsafeInterleaveIO :: IO a -> IO a
-- |The type of exceptions. Every kind of system-generated exception
-- has a constructor in the 'Exception' type, and values of other
-- types may be injected into 'Exception' by coercing them to
--- 'Dynamic' (see the section on Dynamic Exceptions).
+-- 'Dynamic' (see the section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
--
-- For backwards compatibility with Haskell 98, 'IOError' is a type synonym
-- for 'Exception'.
= ArithException ArithException
-- ^Exceptions raised by arithmetic
-- operations. (NOTE: GHC currently does not throw
- -- 'ArithException's).
+ -- 'ArithException's except for 'DivideByZero').
| ArrayException ArrayException
-- ^Exceptions raised by array-related
-- operations. (NOTE: GHC currently does not throw
-- fails. The 'String' argument contains the
-- location of the assertion in the source program.
| AsyncException AsyncException
- -- ^Asynchronous exceptions (see section on Asynchronous Exceptions).
+ -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
| BlockedOnDeadMVar
-- ^The current thread was executing a call to
-- 'takeMVar' that could never return, because there are no other
-- deadlocked. The 'Deadlock' exception is
-- raised in the main thread only (see also: "Control.Concurrent").
| DynException Dynamic
- -- ^Dynamically typed exceptions (see section on Dynamic Exceptions).
+ -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
| ErrorCall String
-- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
-- argument of 'ErrorCall' is the string passed to 'error' when it was
(case fn of
Nothing -> id
Just name -> showString "\nFile: " . showString name)
+
+-- -----------------------------------------------------------------------------
+-- IOMode type
+
+data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
+ deriving (Eq, Ord, Ix, Enum, Read, Show)
\end{code}