proposal: add 'unsafeCoerce'
Malcolm.Wallace at cs.york.ac.uk
Mon Nov 20 09:45:13 EST 2006
Simon Marlow <simonmarhaskell at gmail.com> wrote:
> >>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
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
> * 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
> * 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
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