On Thu, Feb 12, 2009 at 6:24 PM, Jonathan Cast
<
jonathanccast@fastmail.fm> wrote:
> On Thu, 2009-02-12 at 19:04 +0100, Lennart Augustsson wrote:
>> They are not unsafe in the way unsafePerformIO is,
>
> I beg permission to demur:
>
> newtype Unsafe alpha = Unsafe { unUnsafe :: alpha }
> instance Typeable (Unsafe alpha) where
> typeOf _ = typeOf ()
>
> pseudoSafeCoerce :: alpha -> Maybe beta
> pseudoSafeCoerce = fmap unUnsafe . cast . Unsafe
>
> Note that
>
> pseudoSafeCoerce = Just . unsafeCoerce
>
>> but I regard them
>> as a last resort in certain situations.
>> Still, in those situations they are very useful.
>
> But I would agree with both of these. As long as you *derive* Typeable.
>
> jcc
>
>