[Haskell-cafe] Is using Data.Dynamic considered a no-go?

[Lennart Augustsson]

You're quite right.  You should only be allowed to derive Typeable.
(Which could be arranged by hiding the methods of typeable.)

On Thu, Feb 12, 2009 at 6:24 PM, Jonathan Cast
<jonathanccast at 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
>
>
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