[Template-haskell] How to get list of instances of a given class?

[Simon Peyton-Jones]

| Is it possible to publish a list of what is possible to
| access/introspect, and what is not possible? In 6.2.2? In 6.4.1?

It's hard for me to do that.  Except to say: look at the Info type 
	
http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell/L
anguage-Haskell-TH-Syntax.html#t%3AInfo

which tells you what reify returns.

But I'm entirely willing to answer qns.  

| Just for the future: from the original Template Haskell paper [2], I
| got the impression that the ultimate goal was to be able to access the
| whole tree of objects internally known to the compiler at the moment
| when the TH constructs are involved, wasn't it? However I understand
| that the priority is low.

The priority is low because few people ask, and because few people
volunteer.  It's a shame because it's a bit chicken-and-egg-ish.  If TH
was more complete, more people might use it.  

Many of the things that need doing do not require new design work --
they just involve filling out the interface -- and hence could be done
by someone other than me!


A particularly easy thing to do is to fill out the Wiki

Simon