[Haskell] How to close a type class

[Marcin 'Qrczak' Kowalczyk]

oleg at pobox.com writes:

> Thus we have reduced the problem of excluding certain types from a
> typeclass to the problem of excluding all types from one particular
> typeclass: Fail. How can we prevent the user from adding instances to
> Fail?

By not exporting its name?

-- 
   __("<         Marcin Kowalczyk
   \__/       qrczak at knm.org.pl
    ^^     http://qrnik.knm.org.pl/~qrczak/