Which extensions are you using that are not Haskell 98? I would be very interested to know what users would generally require from a refactorer.
I don't see myself as typical Haskell user yet, I'm way to new to the language to consider myself a real "user". Currently, I'm trying to learn arrows and Yampa (mainly to see how well it compares to my own dataflow/reactive stuff that was written in C#, C++ and assembler) I also needed functional dependencies, and usually my code does not compile without -fglasgow-exts, and I really don't know why :)
I agree with Neil, AST editors are generally ugly and hard to use. There is also the problem of laying out Haskell code. Everyone uses their own layout style and pretty printing ASTs is generally a bad thing to do in this context.
First of all, let's see if I get the concept of a "syntax directed editor" right. The idea is, that I (or my company), has a specific indentation rule, naming convention rule, etc... When I get code from someone else (in a syntax tree form ala XML), it will immediately show the text using my conventions. Furthermore, when I need to perform refactoring, a rename is just *one* change to the entire system, no matter how many other files use the name; no more merging for stupid renames. When diffing, whitespace, indentation, etc does not matter; the structure of the files is compared instead. A lot of metadata (for different views) can be attached to the syntax tree without cluttering my text files (like e.g. most version control systems do). I could go on like that, but the intentional programming website explains most of it. Cheers, Peter