Expand SMT decidable fragment: match, ADTs, decreases (#13)#176
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Add Z3 translation for match expressions, ADT constructors, and decreases clauses, promoting contracts from Tier 3 (runtime) to Tier 1 (statically verified). Verification coverage goes from 88/96 (91.7%) to 92/96 (95.8%) across all 15 examples. Phase A: Translate MatchExpr, ConstructorCall, NullaryConstructor to Z3 using algebraic datatype sorts with nested If-chains. Phase B: Verify Nat decreases via direct < comparison at recursive call sites with path-condition tracking. Phase C: Verify ADT decreases via structural rank functions with ForAll axioms for recursive constructor fields. Co-Authored-By: Claude <noreply@anthropic.invalid>
Document the tier model and verification stats (92/96 contracts, 95.8% static) with a breakdown of the 4 remaining Tier 3 contracts and what the Tier 1 decidable fragment covers. Co-Authored-By: Claude <noreply@anthropic.invalid>
- README.md: Mark #13 as done (v0.0.51), annotate #45 as partially done - E522 rationale: Remove "pattern matching" from unsupported examples (match expressions are now Tier 1) - E525 rationale: Replace "not yet implemented" with accurate description of what IS verified (self-recursive Nat/ADT) and what still falls back (mutual recursion, untranslatable measures) Co-Authored-By: Claude <noreply@anthropic.invalid>
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Summary
MatchExpr,ConstructorCall, andNullaryConstructorto Z3 usingz3.Datatypesorts with nestedIf-chains for match arms and recognizer/accessor-based pattern bindings<comparison for Nat measures, structural rank functions withForAllaxioms for ADT measuresContracts promoted to Tier 1
ensures(@Nat.result >= 0)onlengthdecreases(@Nat.0)decreases(@List<Int>.0)onlengthdecreases(@List<Int>.0)onsumRemaining 4 Tier 3 contracts
Implementation
vera/smt.py(+376 lines): ADT sort creation, match/constructor translation, rank functions, per-sort length functionsvera/verifier.py(+276 lines): Constructor registration in_register_data, ADT parameter declarations,_verify_decreaseswith recursive call collection and path-condition trackingtests/test_verifier.py(+357 lines): 16 new tests across 3 new test classes (match/ADT verification, Nat decreases, ADT decreases)Test plan
pytest tests/ -v)mypy vera/)python scripts/check_examples.py)python scripts/check_version_sync.py)Closes #13
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