options library like Google GFlags for Haskell

I'm interested in having something very similar to Google's flags library for Haskell.

Here is the small introduction to gflags that demonstrates why I love it: http://gflags.googlecode.com/svn/trunk/doc/gflags.html

I looked into the various getopt like libraries on Hackage and I haven't found one that matches the simplicity and flexibility of gflags.

Namely, I'd like to have these features:

generates --help (with default values mentioned in the help),
besides parsing the options given by the user, it should also err on unmatched options, so the user has a chance to note typos,
flags can be declared in any module easily (hopefully at the top-level, Template Haskell hackery acceptable if needed),
no need in main to call out to all the modules where I declared flags, instead the flags somehow register themselves at startup/linking/whatever time,
it's OK if main has to call a general initialization function, like in gflags'
```
google::ParseCommandLineFlags(&argc, &argv, true);
```
flags can be used purely (yeah, I think this is an appropriate usage of unsafePerformIO to make the API more simple).

After looking around without success, I played with the idea of doing this myself (and of course sharing it on Hackage). However, I have absolutely no idea for the implementation of the registration part. I need something similar to GCC's ((constructor)) attribute or to C++'s static initialization, but in Haskell. Standard top-level unsafePerformIO is not enough, because that is lazy, so it won't be called before main starts to run.

Solution

After investigating all the solutions on Hackage (thanks for all the tips!), I went ahead with Don's typeclass implementation idea and created a library named HFlags.

It's on hackage: http://hackage.haskell.org/package/hflags

I also have a blog post, describing it: http://blog.risko.hu/2012/04/ann-hflags-0.html