Simplify primitives for short cut fusion
Currently, there appear to be two production primitives,
augment (although I may have missed some). There are multiple consumption primitives (at least:
all). The rule sets for some producers seem to forget to handle
augment, and the
augment/augment rule is omitted as "true, but not, I think, useful".
A number of other functions are instead rewritten into
foldr forms, and then written back if they don't fuse.
What to do:
Personally, I'd be very tempted to start by saying
build g = augment g  (or possibly even
build g = extend  g  if I can ever get that idea to work) and cut the problem in half. The main problem I see with this is if other people are importing
GHC.Base, writing things with
build, and expecting them to fuse. One way to deal with this, perhaps, is to hack a special rule into the RULES compiler to recognize
GHC.Base.build on the LHS of RULES and replace it with the appropriate
augment form, emitting a warning.
Where to go after that: the question remains whether it's best in general to rewrite a form to
foldr to make it fuse, or to fuse directly with
augment. The answer presumably depends, at least in part, on whether there are additional
foldr-based rules we may want to add that would take advantage of the effort put into the translation back from the
foldr form. I would conjecture that most such rules we could want would go beyond what the RULES system actually can do. That said, if we can find a way to use
foldr forms without the horrible pain of translating back from them, that would be a very good thing.