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

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

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