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