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

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 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 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 > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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