See long block comment in codegen.rs. In brief, I think we actually want
to compile to a trie with priority-intervals, a sort of hybrid of a
priority tree and a trie representing decisions keyed on match-ops
(PatternInsts).
The reasons are:
1. The lexicographic ordering that is fundamental to the FSM-building in
the Peepmatic view of the problem is sort of fundamentally limited
w.r.t. our notion of rule priorities. See the example in the block
comment.
2. While the FSM is nice for interpreter-based execution, when compiling
to a language with structured control flow, what we really want is a
tree; otherwise, if we want to form DAGs to share substructure, we
need something like a "diamond-recovery" algorithm that finds common
suffixes of *input match-op sequences*, and then we need to
incorporate something like phi-nodes in order to allow captures from
either side of the diamond to be used.
3. One of the main advantages of the automaton/transducer approach,
namely sharing suffixes of the *output* sequence (emitting partial
output at each state transition), is unfortunately not applicable if
we allow the overall function to be partial. Otherwise, there is
always the possibility that we fail at the last match op, so we
cannot allow any external constructors to be called until we reach
the final state anyway.
4. Pragmatically, I found I was having to significantly edit the
peepmatic_automata implementation to adapt to this use-case
(compilation to Rust), and it seemed more practical to design the
data structure we want than to try to shoehorn the existing thing
into the new problem.
WIP, hopefully working soon.
