Difference between revisions of "Unsafe functions"

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A colleague [...] asked me today whether I know how to use <code>unsafePerformIO</code> safely. And I realized I have no idea. [...]
A colleague [...] asked me today whether I know how to use <code>unsafePerformIO</code> safely. And I realized I have no idea.
 
  
 
<tt>[https://discourse.haskell.org/t/using-unsafeperformio-safely/4146 Richard Eisenberg.]</tt>
 
<tt>[https://discourse.haskell.org/t/using-unsafeperformio-safely/4146 Richard Eisenberg.]</tt>
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* <hask>unsafeIOToST :: IO a -> ST s a</hask>
 
* <hask>unsafeIOToST :: IO a -> ST s a</hask>
 
* <hask>unsafeIOToSTM :: IO a -> STM a</hask>
 
* <hask>unsafeIOToSTM :: IO a -> STM a</hask>
* <hask>unsafeFreeze, unsafeThaw</hask>
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* <hask>unsafeFreeze</hask>, <hask>unsafeThaw</hask>
 
* <hask>unsafeCoerce# :: a -> b</hask>
 
* <hask>unsafeCoerce# :: a -> b</hask>
 
* <hask>seq :: a -> b -> b</hask>
 
* <hask>seq :: a -> b -> b</hask>
  
Unsafe functions can break type safety (unsafeCoerce#, unsafePerformIO),
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Unsafe functions can break:
interfere with lazy IO (unsafeInterleaveIO), or break [http://en.wikipedia.org/wiki/Parametricity parametricity] (<code>seq</code>).
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* type safety (<code>unsafeCoerce#</code>, <code>unsafePerformIO</code>),
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* [https://okmij.org/ftp/Haskell/index.html#lazyIO-not-True equational reasoning] (<code>unsafeInterleaveIO</code>),
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* or [http://en.wikipedia.org/wiki/Parametricity parametricity] (<code>seq</code>).
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Their use (except in the case of <code>seq</code>) would require some
 
Their use (except in the case of <code>seq</code>) would require some
 
kind of assurance on the part of the programmer that what they're doing
 
kind of assurance on the part of the programmer that what they're doing

Latest revision as of 06:14, 11 August 2022

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),

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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