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

[Jonathan Cast]

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