[Haskell-cafe] Why not assign a type to unsafePerformIO?

Justin Bailey jgbailey at gmail.com
Wed Oct 3 17:47:00 EDT 2007

One of the "holes" in real-world Haskell is you never know if a
library/function is calling unsafePerformIO and you have to trust the
library author. I recognize the necessity of the function, but should it
announce itself? unsafePerformIO has this type:

  unsafePerformIO :: IO a -> a

Would there be any value to making it have a type that can be stripped off,
like some other monads? For example, providing a "runUnsafe" or similar:

  data UnsafePerformIO a = Unsafe a

  runUnsafe :: UnsafePerformIO a -> a
  runUnsafe (Unsafe o) = o

and changing unsafePerformIO to have the type:

  unsafePerformIO :: IO a -> UnsafePerformIO a

It seems it would be valuable to have functions announce when they use
unsafePerformIO, but additionally allow it to be stripped off. So the

  launchMissiles :: a -- Uses unsafePerfomIO!

Would become

  launchMissiles :: UnsafePerformIO a

Which could be stripped off it you wanted:

  evilDictatator :: a
  evilDictator = runUnsafe $ launchMissiles

But doesn't have to be:

  incompetentDictator :: a
  incompetentDictator = launchMissiles -- Doesn't type check!

I doubt this is original - does it buy anything?

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