Coerce the state token of prim operation and wrap it into a MonadPrim action.

Highly unsafe!

Since: 0.3.0

Coerce the state token of prim operation and wrap it into a MonadPrim action.

Highly unsafe!

Since: 0.3.0

Unwrap any MonadPrimBase action while coercing the state token

Highly unsafe!

Unwrap any MonadPrimBase action that does not return anything, while coercing the state token

Highly unsafe!

Convert a MonadPrimBase action to another MonadPrim while coercing the state token.

Highly unsafe!

Convert a MonadPrimBase action to IO while coercing the state token to RealWorld.

Highly unsafe!

Convert a MonadPrimBase action to ST while coercing the state token s.

Highly unsafe!

Convert an IO action to some MonadPrim while coercing the state token.

Highly unsafe!

It is similar to unsafeSTToIO, except resulting action can be any other MonadPrim action, therefore it is a lot more dangerous.

Convert an ST action to some MonadPrim while coercing the state token.

Highly unsafe!

A version of liftPrimBase that coerce the state token.

Highly unsafe!

Same as noDuplicate, except works in any MonadPrim.

Same as unsafeDupablePerformIO, except works not only with IO, but with other MonadPrimBase actions as well. Reading and writing values into memory is safe, as long as writing action is idempotent. On the other hand things like memory or resource allocation, exceptions handling are not safe at all, since supplied action can be run multiple times and a copy interrupted at will.

Take an IO and compute it as a pure value, while inlining the action itself.

Ridiculously unsafe!

This is even more unsafe then both unsafePerformIO and unsafeDupableInterleaveIO.

The only time it is really safe to use is on idempotent action that only read values from memory, but do note do any mutation, allocation and certainly not interaction with real world.

In bytestring it is known as accursedUnutterablePerformIO. Here are some resources that discuss it's unsafety:

• Stack overflow question
• Reddit discussion

Take an ST and compute it as a pure value, while inlining the action itself. Same as unsafeInlineIO.

Ridiculously unsafe!

Take any MonadPrimBase action and compute it as a pure value, while inlining the action. Same as unsafeInlineIO, but applied to any MonadPrimBase action.

Ridiculously unsafe!

Same as unsafeInterleaveIO, except works in any MonadPrimBase

Same as unsafeDupableInterleaveIO, except works in any MonadPrimBase

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!

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.

Since: base-4.4.0.0

unsafeInterleaveIO allows an 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 hGetContents.

unsafeDupableInterleaveIO allows an 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.

The computation may be performed multiple times by different threads, possibly at the same time. To ensure that the computation is performed only once, use unsafeInterleaveIO instead.