better error message needed for missing package files

[duncan]

Here's a real user's confusion:

~ $ ghci
GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
Prelude> :module Network
Could not find module `Network':
  Use -v to see a list of the files searched for.
Prelude> Leaving GHCi.
~ $ ghc-pkg list network
/usr/lib/ghc-6.8.2/package.conf:
    network-2.1.0.0
/home/sjm/.ghc/i386-linux-6.8.2/package.conf:
    network-2.2.0.0

Turns out the problem is that the network-2.2.0.0 package that is registered locally is messed up, the .hi files are missing, probably accidentally deleted.

In this situation the error message is not very helpful. The current error message makes sense in the context of local files since there is is quite reasonable for them to not exist however it's no good at all for the case of a module from a package. We know that the package exports that module so the file had better exist and if it does not, it's not simply a case of a missing file, the package is totally borked. The user needs to know that the package is borked so that they can unregister or reinstall it. The current message does not alert them to the real problem.

The message should say which package (including version) claims to provide the module and that files are missing from the installed package and as such the package is unusably broken and should be reinstalled.

[duncan]

It would be great if ghc-pkg check would also stat all the files that we know we need for each package (ie the .hi files for all modules and the library .a files) and report any borked packages.

[simonmar]

Done:

Thu Jan 15 04:25:24 PST 2009  Simon Marlow <marlowsd@gmail.com>
  * More useful error message when a package .hi file cannot be found:

Thu Jan 15 04:21:43 PST 2009  Simon Marlow <marlowsd@gmail.com>
  * soup-up "ghc-pkg check"

[igloo]

Both merged.