Unsafe functions

A colleague [...] asked me today whether I know how to use unsafePerformIO safely. And I realized I have no idea.

Richard Eisenberg.

There are a number of unsafe functions in the libraries.

• unsafePerformIO :: IO a -> a
• unsafeInterleaveIO :: IO a -> IO a
• unsafeInterleaveST :: ST s a -> ST s a
• unsafeIOToST :: IO a -> ST s a
• unsafeIOToSTM :: IO a -> STM a
• unsafeFreeze, unsafeThaw
• unsafeCoerce# :: a -> b
• seq :: a -> b -> b

Unsafe functions can break type safety (unsafeCoerce#, unsafePerformIO), interfere with lazy IO (unsafeInterleaveIO), or break parametricity (seq). Their use (except in the case of seq) would require some kind of assurance on the part of the programmer that what they're doing is safe.

unsafe is also a keyword which can be used in a foreign import declaration.

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