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.

• unsafeLocalState :: IO a -> a
• unsafePerformIO :: IO a -> a
• inlinePerformIO :: 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

Unsafe functions can:

• break type safety (unsafeCoerce#, unsafeLocalState, unsafePerformIO),

• or break equational reasoning (unsafeInterleaveIO).

Their use 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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