Unsafe functions
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A colleague [...] asked me today whether I know how to use unsafePerformIO
safely. And I realized I have no idea. [...]
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
),
- break equational reasoning (
unsafeInterleaveIO
),
- or weaken 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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