rose-ash/plans/maude-on-sx.md

Maude-on-SX: rewriting as primitive

Equational logic + term rewriting as the only computational primitive. Every other guest in the set reduces ultimately to lambda terms or stack frames; Maude (Clavel et al.) reduces to rewrite rules over equational classes modulo theories (associativity, commutativity, identity). Implementing it forces the substrate to articulate its reduction semantics — currently implicit in the CEK machine and the JIT.

The chisel: reduction step. Different from Idris's evidence chisel and from Probabilistic's trace chisel. Maude asks: "what is one step of computation?" Maude's answer (apply a rewrite rule, modulo equational theories) is more general than CEK's transition. Making both consistent is informative — either the substrate has room for them to coexist, or one is a special case of the other.

What this exposes about the substrate:

• Whether SX's pattern matching (lib/guest/match.sx) extends to equational matching — matching modulo associativity, commutativity, identity.
• Whether the substrate has a notion of "normal form" or just "result of evaluation."
• Whether term-graph sharing matters at the value-level.
• Whether confluence (different rewrite orders → same result) can be checked or just hoped for.
• Whether order-sorted signatures (subsorts, polymorphism via inheritance) fit SX's value taxonomy.

End-state goal: Maude 3 functional + system modules — sorts, subsorts, equations, conditional equations, rewrite rules, equational matching modulo assoc/comm/id, simple strategy language. Not the full LTL model checker; a faithful core that runs idiomatic Maude programs and proves equational identities.

Ground rules

• Scope: lib/maude/** and plans/maude-on-sx.md only. Substrate gaps → sx-improvements.md.
• Consumes from lib/guest/: core/lex, core/pratt, core/ast, core/match (extended).
• Will propose a new sub-layer lib/guest/rewriting/ — equational matching beyond syntactic match, normal-form computation, confluence checking, term-graph rewriting. Second consumer: a Pure-on-SX plan, a CafeOBJ port, or a research term-rewriting playground.
• Branch: loops/maude. Standard worktree pattern.

Architecture sketch

Maude source text (functional / system / object modules)
    │
    ▼
lib/maude/parser.sx        — fmod ... endfm syntax, sort declarations,
    │                        equations, rewrite rules
    ▼
lib/maude/signatures.sx    — sort hierarchy, operator declarations with arities,
    │                        subsort relationships, kind inference
    ▼
lib/maude/matching.sx      — pattern matching MODULO equational theories
    │                        (assoc, comm, id) — generalises core/match.sx
    ▼
lib/maude/reduce.sx        — apply equations until normal form (confluent set)
    │
    ▼
lib/maude/rewrite.sx       — apply rewrite rules under a strategy (system modules)
    │
    ▼
lib/maude/runtime.sx       — module loading, reflection (META-LEVEL)

Semantic mappings

Maude construct	SX mapping
sort Nat .	declare sort: (declare-sort Nat)
subsort Nat < Int .	subsort relation: (declare-subsort Nat Int)
op _+_ : Nat Nat -> Nat [assoc comm id: 0] .	operator with equational attributes
eq X + 0 = X .	equation in the equational theory
ceq X + Y = Y if foo(X, Y) .	conditional equation
rl [step] : foo(X) => bar(X) .	rewrite rule (asymmetric, in system modules)
red TERM .	reduce term to normal form by equations
rew TERM .	apply rewrite rules under default strategy
META-LEVEL	reflection: terms representing terms

The novel substrate stress: equational matching. Pattern X + Y against 1 + 2 + 3 (where + is assoc comm) succeeds with multiple binding sets: (X=1, Y=2+3), (X=2, Y=1+3), (X=3, Y=1+2), etc. The matcher must enumerate solutions, not return the first.

Roadmap

Phase 1 — Parser + signatures

• Parser for fmod / endfm syntax, sort declarations, op declarations, equations.
• Sort hierarchy with subsort relations.
• Operator overloading by arity + sort.
• Tests: parse classic examples (peano nat, list of naturals).

Phase 2 — Syntactic equational reduction

• Apply equations left-to-right until no equation matches.
• Standard pattern matching (no equational theories yet — strict syntactic match).
• Tests: peano arithmetic, list manipulation, propositional logic simplifier.

Phase 3 — Equational matching (assoc / comm / id)

• Extend matching to handle assoc operators (flatten then match across permutations of subterm groups).
• Handle comm (try both argument orderings).
• Handle id: e (X * e ≡ X).
• Combinations: assoc comm id together.
• Returns all matches, not just first — caller drives.
• Tests: classic AC-matching examples (multiset rewriting, set theory, group equations).

Phase 4 — Conditional equations

• ceq L = R if Cond — apply only when Cond reduces to true.
• Recursion via the same reduce engine (terminating because Cond is shorter).
• Tests: gcd, sorting, conditional simplifications.

Phase 5 — System modules + rewrite rules

• mod ... endm syntax with rl rules.
• Rules apply asymmetrically (=> not =); fairness across rules.
• Default strategy: top-down, leftmost-outermost, first applicable rule.
• Tests: state-transition systems (puzzle solvers, protocol simulators).

Phase 6 — Strategy language

• Compose strategies: sequential ;, alternative |, iteration *, fixed-point.
• User-named strategies; strategy expressions as values.
• Tests: programs whose meaning depends on strategy choice.

Phase 7 — Reflection (META-LEVEL)

• Terms-as-data: META-LEVEL lets you encode/decode terms as Maude terms.
• Build proofs / programs that manipulate Maude programs.
• Tests: meta-circular interpretation, generic theorem helpers.

Phase 8 — Propose lib/guest/rewriting/

• Extract equational matching engine (the most reusable piece).
• Extract normal-form-by-equations infrastructure.
• Extract strategy combinators.
• Wait for second consumer before extracting.

lib/guest feedback loop

Consumes: core/lex, core/pratt, core/ast, core/match (with proposed extension for equational matching).

Stresses substrate: matching backtracking and enumeration (Maude's all-matches semantics is very different from Haskell-style first-match); whether SX values can carry sort metadata efficiently; term-graph sharing.

May propose: lib/guest/rewriting/ sub-layer — equational matching (extending core/match), normal-form-by-equations machinery, strategy combinators, confluence checking.

What it teaches: whether the substrate's reduction model has implicit assumptions (deterministic, leftmost-outermost, etc.) that a rewriting language exposes. If core/match.sx cleanly extends to equational matching via a configuration toggle, that's substrate-deep validation. If extending it requires fundamental rework, the substrate's matching abstraction was leaking.

References

• Clavel et al., "All About Maude — A High-Performance Logical Framework" (Springer, 2007).
• Maude Manual: https://maude.lcc.uma.es/
• "Term Rewriting and All That" (Baader & Nipkow, 1998) — theoretical foundation.
• Eker, "Associative-Commutative Rewriting on Large Terms" (RTA 2003) — for the matching algorithm.
• Pure language (Albrecht Gräf): https://agraef.github.io/pure-lang/ — practical functional rewriting.

Progress log

(awaiting Phase 1 — depends on substrate matching maturity from lib/guest/core/match.sx)

Blockers

(speculative — equational matching is algorithmically heavy and may surface JIT issues)