proposal: add 'unsafeCoerce'
simonmarhaskell at gmail.com
Mon Nov 20 08:40:55 EST 2006
Ross Paterson wrote:
> On Mon, Nov 20, 2006 at 04:51:29PM +1100, Donald Bruce Stewart wrote:
>>So the uses fall into 2 categories:
>> * FFI binding and raw pointer/foreign data manipulation
>> * type equalities known, but not expressible statically
>>Now, can we say something portable about these uses?
> The second group, perhaps, but the first group (unsafeCoerce#) depend
> on GHC's internal representations.
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. I *think*
the following cases are guaranteed to work:
* 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
* casting an unboxed type to another unboxed type of the same size.
The difficult cases arise when you want to cast a value to a type other than its
real type, such as you need to do in the implementation of Dynamic for example.
I doubt that we could provide a portable implementation of Dynamic.
Casting boxed types to unboxed types or vice versa is pretty much guaranteed to
end in tears.
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