On Wed, 2007-10-03 at 14:47 -0700, Justin Bailey wrote:

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 classic

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?

This already exists. The monad is called IO, the unsafePerformIO implementation is called id, and runUnsafe is called unsafePerformIO. I really don't see how another set of aliases buys anything. jcc