How do I create dynamic exceptions

[Bayley, Alistair]

Exceptions, again...

The docs for Control.Exception say (for Dynamic exceptions): "When using
dynamic exceptions it is advisable to define a new datatype to use for your
exception type, to avoid possible clashes with dynamic exceptions used in
other libraries."

So I went with that and created a datatype for my exceptions:

> data MyException = MkExc Int String

... but now it seems I have to install this as an instance of Typeable, and
I don't see how to do that.

> instance Typeable MyException where
>   typeOf ? = ?


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