Simon Marlow
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. Regards, Malcolm