:: IO String -> String package:rebase

This is the "back door" into the IO monad, allowing IO computation to be performed at any time. For this to be safe, the IO computation should be free of side effects and independent of its environment. 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. Furthermore, when using unsafePerformIO to cause side-effects, you should take the following precautions to ensure the side effects are performed as many times as you expect them to be. Note that these precautions are necessary for GHC, but may not be sufficient, and other compilers may require different precautions:

• 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 (-fno-full-laziness), 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]
test = unsafePerformIO $ newIORef []

main = do
writeIORef test [42]
bang <- readIORef test
print (bang :: [Char])

This program will core dump. This problem with polymorphic references is well known in the ML community, and does not arise with normal monadic use of references. There is no easy way to make it impossible once you use unsafePerformIO. Indeed, it is possible to write coerce :: a -> b with the help of unsafePerformIO. So be careful! WARNING: If you're looking for "a way to get a String from an 'IO String'", then unsafePerformIO is not the way to go. Learn about do-notation and the <- syntax element before you proceed.

This version of unsafePerformIO is more efficient because it omits the check that the IO is only being performed by a single thread. Hence, when you use unsafeDupablePerformIO, there is a possibility that the IO action may be performed multiple times (on a multiprocessor), and you should therefore ensure that it gives the same results each time. It may even happen that one of the duplicated IO actions is only run partially, and then interrupted in the middle without an exception being raised. Therefore, functions like bracket cannot be used safely within unsafeDupablePerformIO.

The largest element of a non-empty structure. This function is non-total and will raise a runtime exception if the structure happens to be empty. A structure that supports random access and maintains its elements in order should provide a specialised implementation to return the maximum in faster than linear time.

Examples

Basic usage:

>>> maximum [1..10]
10

>>> maximum []
*** Exception: Prelude.maximum: empty list

>>> maximum Nothing
*** Exception: maximum: empty structure

WARNING: This function is partial for possibly-empty structures like lists.

The least element of a non-empty structure. This function is non-total and will raise a runtime exception if the structure happens to be empty. A structure that supports random access and maintains its elements in order should provide a specialised implementation to return the minimum in faster than linear time.

Examples

Basic usage:

>>> minimum [1..10]
1

>>> minimum []
*** Exception: Prelude.minimum: empty list

>>> minimum Nothing
*** Exception: minimum: empty structure

WARNING: This function is partial for possibly-empty structures like lists.