[Haskell-cafe] Error in Installing Data.deriveTH

[Albert Y. C. Lai]

Two precautions:

1. "ghc-pkg check" is incomplete: It ignores shadowing problems, which 
are like 99% of the problems in practice. Skip "ghc-pkg check", go 
straight to "ghc -v" for the real report card.

2. "ghc-pkg unregister" has a long-standing bug. I have filed it as

http://hackage.haskell.org/trac/ghc/ticket/5442

In practice, 99% of the time you use "--user", so the message for you is 
that before you order "ghc-pkg unregister --user blah", make sure you 
have a "blah" in the user database. Every time.

(In particular, I have seen someone ordering the same "--user" command 
twice for no good reason, and the first time succeeded, and therefore 
the second time happily went ahead to mess with the global database.)