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

[Don Stewart]

Notably, extensible exceptions use dynamics, in conjunction with type
classes and existentials.

A number of solutions to the 'expression problem' involve dynamics.

bugfact:
> It would be interesting to see when you HAVE to use dynamics, e.g. when no
> other solution is possible in Haskell...
> 
> Right now if I use it, it feels that I'm doing so because I'm too new to
> Haskell.
> 
> 
> On Thu, Feb 12, 2009 at 7:53 PM, Lennart Augustsson <lennart at augustsson.net>
> wrote:
> 
>     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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