SYB makes very heavy use of Typeable as well, although not, as I
recall dynamics as such.
Cheers,
Sterl.
On Feb 12, 2009, at 3:40 PM, Don Stewart wrote:
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
<lenn...@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
<jonathancc...@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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