Control.Monad.ST.UnsafePerform

Documentation

unsafePerformST :: ST s a -> a

This is like unsafePerformIO, but for the ST monad. Highly unsafe, only use when you really know what you're doing. The same precations as for unsafePerformIO apply:

If the ST computation wrapped in unsafePerformST 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 unsafePerformST) is indeterminate. Furthermore, when using unsafePerformST 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 unsafePerformST. If the call is inlined, the S/T 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.
• Make sure that the either you switch off let-floating (-fno-full-laziness), or that the call to unsafePerformST cannot float outside a lambda. For example, if you say: f x = unsafePerformST (newSTRef []) you may get only one reference cell shared between all calls to f. Better would be f x = unsafePerformST (newSTRef [x]) because now it can't float outside the lambda.

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