unsafe -text

package unsafe
SafeHaskell introduced the notion of safe and unsafe modules. In order to make as many as possible modules "safe", the well-known unsafe functions were moved to distinguished modules. This makes it hard to write packages that work with both old and new versions of GHC. This package provides a single module System.Unsafe that exports the unsafe functions from the base package. It provides them in a style ready for qualification, that is, you should import them by > import qualified System.Unsafe as Unsafe The package also contains a script called rename-unsafe.sh. It replaces all occurrences of the original identifiers with the qualified identifiers from this package. You still have to adapt the import commands. It uses the darcs-replace-rec script from the darcs-scripts package. Version 0.0
unsafeCoerce :: a -> b
base Unsafe.Coerce
unsafeForeignPtrToPtr :: ForeignPtr a -> Ptr a
base Foreign.ForeignPtr
This function extracts the pointer component of a foreign pointer. This is a potentially dangerous operations, as if the argument to unsafeForeignPtrToPtr is the last usage occurrence of the given foreign pointer, then its finalizer(s) will be run, which potentially invalidates the plain pointer just obtained. Hence, touchForeignPtr must be used wherever it has to be guaranteed that the pointer lives on - i.e., has another usage occurrence. To avoid subtle coding errors, hand written marshalling code should preferably use Foreign.ForeignPtr.withForeignPtr rather than combinations of unsafeForeignPtrToPtr and touchForeignPtr. However, the latter routines are occasionally preferred in tool generated marshalling code.
unsafeInterleaveIO :: IO a -> IO a
base System.IO.Unsafe
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 System.IO.hGetContents.
unsafeInterleaveST :: ST s a -> ST s a
base Control.Monad.ST, base Control.Monad.ST.Lazy
unsafeIOToST :: IO a -> ST s a
base Control.Monad.ST, base Control.Monad.ST.Lazy
unsafeLocalState :: IO a -> a
base Foreign.Marshal
Sometimes an external entity is a pure function, except that it passes arguments and/or results via pointers. The function unsafeLocalState permits the packaging of such entities as pure functions. The only IO operations allowed in the IO action passed to unsafeLocalState are (a) local allocation (alloca, allocaBytes and derived operations such as withArray and withCString), and (b) pointer operations (Foreign.Storable and Foreign.Ptr) on the pointers to local storage, and (c) foreign functions whose only observable effect is to read and/or write the locally allocated memory. Passing an IO operation that does not obey these rules results in undefined behaviour. It is expected that this operation will be replaced in a future revision of Haskell.
unsafePerformIO :: IO a -> a
base System.IO.Unsafe, base Foreign
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!
unsafeSTToIO :: ST s a -> IO a
base Control.Monad.ST
package unsafe-promises
An experimental library for creating promises that can be evaluated in pure code. Version
unsafeDrop :: Int -> ByteString -> ByteString
bytestring Data.ByteString.Unsafe
A variety of drop which omits the checks on n so there is an obligation on the programmer to provide a proof that 0 <= n <= length xs.
unsafeFinalize :: ByteString -> IO ()
bytestring Data.ByteString.Unsafe
Explicitly run the finaliser associated with a ByteString. References to this value after finalisation may generate invalid memory references. This function is unsafe, as there may be other ByteStrings referring to the same underlying pages. If you use this, you need to have a proof of some kind that all ByteStrings ever generated from the underlying byte array are no longer live.
unsafeForeignPtrToStorableArray :: Ix i => ForeignPtr e -> (i, i) -> IO (StorableArray i e)
array Data.Array.Unsafe, array Data.Array.Storable
Construct a StorableArray from an arbitrary ForeignPtr. It is the caller's responsibility to ensure that the ForeignPtr points to an area of memory sufficient for the specified bounds.
unsafeFreeze :: (Ix i, MArray a e m, IArray b e) => a i e -> m (b i e)
array Data.Array.MArray, array Data.Array.Unsafe
Converts an mutable array into an immutable array. The implementation may either simply cast the array from one type to the other without copying the array, or it may take a full copy of the array. Note that because the array is possibly not copied, any subsequent modifications made to the mutable version of the array may be shared with the immutable version. It is safe to use, therefore, if the mutable version is never modified after the freeze operation. The non-copying implementation is supported between certain pairs of array types only; one constraint is that the array types must have identical representations. In GHC, The following pairs of array types have a non-copying O(1) implementation of unsafeFreeze. Because the optimised versions are enabled by specialisations, you will need to compile with optimisation (-O) to get them. * IOUArray -> UArray * STUArray -> UArray * IOArray -> Array * STArray -> Array
unsafeHead :: ByteString -> Word8
bytestring Data.ByteString.Unsafe
A variety of head for non-empty ByteStrings. unsafeHead omits the check for the empty case, so there is an obligation on the programmer to provide a proof that the ByteString is non-empty.
unsafeIndex :: ByteString -> Int -> Word8
bytestring Data.ByteString.Unsafe
Unsafe ByteString index (subscript) operator, starting from 0, returning a Word8 This omits the bounds check, which means there is an accompanying obligation on the programmer to ensure the bounds are checked in some other way.
unsafeInit :: ByteString -> ByteString
bytestring Data.ByteString.Unsafe
A variety of init for non-empty ByteStrings. unsafeInit omits the check for the empty case. As with unsafeHead, the programmer must provide a separate proof that the ByteString is non-empty.
unsafeIOToSTM :: IO a -> STM a
base GHC.Conc.Sync, base GHC.Conc
Unsafely performs IO in the STM monad. Beware: this is a highly dangerous thing to do. * The STM implementation will often run transactions multiple times, so you need to be prepared for this if your IO has any side effects. * The STM implementation will abort transactions that are known to be invalid and need to be restarted. This may happen in the middle of unsafeIOToSTM, so make sure you don't acquire any resources that need releasing (exception handlers are ignored when aborting the transaction). That includes doing any IO using Handles, for example. Getting this wrong will probably lead to random deadlocks. * The transaction may have seen an inconsistent view of memory when the IO runs. Invariants that you expect to be true throughout your program may not be true inside a transaction, due to the way transactions are implemented. Normally this wouldn't be visible to the programmer, but using unsafeIOToSTM can expose it.
unsafeLast :: ByteString -> Word8
bytestring Data.ByteString.Unsafe
A variety of last for non-empty ByteStrings. unsafeLast omits the check for the empty case. As with unsafeHead, the programmer must provide a separate proof that the ByteString is non-empty.
unsafePackAddress :: Addr# -> IO ByteString
bytestring Data.ByteString.Unsafe
O(n) Pack a null-terminated sequence of bytes, pointed to by an Addr# (an arbitrary machine address assumed to point outside the garbage-collected heap) into a ByteString. A much faster way to create an Addr# is with an unboxed string literal, than to pack a boxed string. A unboxed string literal is compiled to a static char [] by GHC. Establishing the length of the string requires a call to strlen(3), so the Addr# must point to a null-terminated buffer (as is the case with string# literals in GHC). Use unsafePackAddressLen if you know the length of the string statically. An example: > literalFS = unsafePackAddress "literal"# This function is unsafe. If you modify the buffer pointed to by the original Addr# this modification will be reflected in the resulting ByteString, breaking referential transparency. Note this also won't work if you Add# has embedded '\0' characters in the string (strlen will fail).

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