Instances for data types

[George Russell]

Sven Panne wrote:
 > Playing around with System.Posix.Terminal, I realized that there are no Read,
 > Show, Eq, Enum, ... instances for the data types defined there (TerminalState,
 > TerminalMode, ...). This is not a very difficult problem to solve in one's
 > own program, but it's not very handy, either.

Two other cases where I have to roll my own Typeable instance are
    Data.FiniteMap.FiniteMap
and
    HaXml's Element type (and friends)

This is all very well but will cause problems if someone else imports
one of my packages and another one which defines conflicting instances
of Typeable.  Or if later the standard libraries or some library I use
change to implement their own Typeable instance.