Re: proposal: add 'unsafeCoerce'

30 Nov 2006

      John Meacham wrote:
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On Mon, Nov 20, 2006 at 02:45:13PM +0000, Malcolm Wallace wrote:
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Simon Marlow  wrote:
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Now, can we say something portable about these uses?
I'd like to have a precise (sound, if not complete) description of
when it's safe to use unsafeCoerce in GHC, but it needs some careful
thought.
And not just GHC.  I think all the points you mention (below) would be
entirely reasonable for all implementations.
...
* cast that changes a phantom type, or changes a type that is not
   reflected by a part of the value,
   eg. 'unsafeCoerce (Left 3) :: Either Int a' should be fine for any
   'a',
* casting a polymorphic type to the actual type of the runtime value.
   That is, you can safely cast a value to its correct type.  (eg. in
   Typeable.cast).
* casting an unboxed type to another unboxed type of the same size.
There is one more important use case you haven't mentioned:
* casting from a newtype to the contained value (or vice versa).
This latter type of cast is the only one I can remember ever having used
myself.
there are very few safe uses of unsafeCoerce in jhc. the only ones
guarenteed safe are
* casting a recursive newtype to its representation and back (note,
   recursive newtypes are chosen via a loop-breaking algorithm in the
   compiler, so it is best to let it worry about this)
 * casting arbitrary values of kind * to a system provided type 'Box'
   and back again
 * casting arbitrary values of kind ! (the kind of strict boxed values)
   to a system provided type 'BoxBang' and back again
Is kind ! visible to the programmer?  How?  What are you allowed/not allowed to 
do with something of kind !?

I added a kind ! to GHC recently, but it has a very limited use: it's the kind 
of boxed/unlifted primitive types like ByteArray# and MutVar#.  So you can write 
polymorphic functions over boxed/unlifted things, and one day maybe have arrays 
of them.

Cheers,
	Simon