Survey the abstraction's blind spots instead of stopping at the first - #62
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A proof halts at the first divergence it can reach. That is the right behaviour for a verdict and useless for a decision: it says nothing about how many more lie behind it, and the count is what decides whether sharpening the abstraction is worth attempting at all. The evidence to date is one candidate. `AMPERSAND UIDENT LBRACKET RBRACKET SEMICOLON EOF` is confirmed spurious -- an exhaustive 0..8 search found no ambiguity, and the candidate is five real tokens -- yet it survived the predecessor filter and survived raising the level from 3 to 4 at 2.4x the pairs. Whether that is one local blind spot or the first of hundreds changes what to build next, and nothing measured so far distinguishes the two. `--survey N` walks the whole abstract space and counts distinct sites, where a site is the stack pair and lookahead at which two parses first part ways, so one blind spot reached by many sentences counts once. Divergence is recorded where it is born rather than where it is inherited; counting inherited divergence would report path length instead. Up to N example sentences are printed. The mode is opt-in and the ordinary proof path is untouched -- same node key, same cost, same verdicts -- because surveying cannot stop early and is the more expensive question. Counts are reported as a floor when the pair budget or the timeout cut the walk short, since a partial walk cannot claim a total. A survey that finds no site and covered the space is a proof, and says so. The flag is plumbed through the Python CLI and exposed as a workflow input, so the count can be obtained from Actions rather than only from a local build. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01N4KejxPN4Q9uQ67dHwUDgj
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Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe ambiguity prover now supports survey mode. Survey mode traverses the abstract pair space, records divergence sites and examples, reports coverage, and is available through the CLI and workflow input. ChangesAmbiguity survey
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The new survey mode can miss divergences that occur first at EOF and incorrectly report an ambiguous grammar as proven unambiguous. Because this can produce a false correctness result, the PR is not merge-ready until the omission is fixed or explicitly accepted. Sequence Diagram(s)sequenceDiagram
participant User
participant ambiguity.py
participant ambiguity_search.ml
participant ProofReport
User->>ambiguity.py: run prove --survey N
ambiguity.py->>ambiguity_search.ml: pass --prove-survey N
ambiguity_search.ml->>ambiguity_search.ml: traverse abstract pair space
ambiguity_search.ml->>ProofReport: return sites, examples, and coverage
ProofReport-->>User: display survey verdict
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In `@tools/ambiguity_search.ml`:
- Around line 1034-1045: Update the EOF acceptance handling around
accepts_diverged to record the first EOF divergence in sites using (left, right,
"#"), while preserving the existing candidate and survey-example behavior.
Ensure completed surveys cannot report PROVEN UNAMBIGUOUS when divergence occurs
only on EOF, and add a regression grammar covering reduce/reduce divergence at
EOF.
In `@tools/test_prover.py`:
- Around line 395-467: Extend the survey helper and its command invocation to
accept a timeout parameter, then add a test using a conflict-free grammar with
timeout 0. Assert the truncated survey reports counts as a floor, omits “PROVEN
UNAMBIGUOUS,” and returns NOT_PROVEN, while preserving existing default-timeout
behavior for current tests.
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Two problems, one of them a false proof. EOF is not one of the terminals the site loop walks - it is the sentinel the joint outcomes take separately - so a pair that first parted ways on end of input incremented `accepting` while contributing no site. The site is now recorded for the "#" lookahead too. That gap was reachable as a wrong verdict because the survey printed PROVEN UNAMBIGUOUS on `sites = 0`. A grammar whose only divergence lives on EOF would have been reported as proven while ordinary proof mode returned an abstract candidate for the same pair. Sites are a diagnostic breakdown; whether any diverging pair reaches acceptance is the actual question, and `accepting` is what counts it, so the verdict is gated on that instead. Either fix alone would have closed this instance; gating on acceptance is what keeps a future gap in site accounting from printing a false theorem again. EOF_REDUCE_REDUCE covers it: `a` reduces to either `x` or `y` with nothing before end of input to separate them. It joins the ambiguous corpus, so proof mode must also never prove it. An incomplete survey is also pinned now. A walk cut short has counted nothing, so its zero is a floor rather than a total and must not read as a proof - the same rule the bounded search already follows. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01N4KejxPN4Q9uQ67dHwUDgj
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Adds
ambiguity prove K --survey N, which answers how many places the abstraction cannot separate two parses, instead of stopping at the first.Why
A proof halts at the first divergence it reaches. Correct for a verdict, useless for a decision — it says nothing about how many lie behind it, and that count is what decides what to build next.
Everything measured so far rests on one candidate:
AMPERSAND UIDENT LBRACKET RBRACKET SEMICOLON EOFThat candidate is confirmed spurious — an exhaustive
0..8search found no ambiguity and it is five real tokens — yet it survived the predecessor filter (#59) and survived raising the level, at 2.4× the pairs. The budget is not the constraint either: 36,942 of 7,895,160 pairs is 0.5%.Whether that is one local blind spot or the first of hundreds decides the next step, and nothing measured distinguishes them:
What a site is
The stack pair and lookahead at which two parses first part ways. One blind spot reached by a thousand sentences counts once.
Divergence is recorded where it is born, not where it is inherited — a node that is already diverged carries its ancestor's site, and counting it again at every step would report path length rather than the number of blind spots.
Design
Opt-in, and the ordinary proof path is untouched: same node key, same pair-space size, same verdicts, same cost. Surveying cannot stop early, so it is the more expensive question and does not become the default.
PROVEN UNAMBIGUOUSwith exit 0.Plumbed through the Python CLI (
--survey) and exposed as a workflow input, so the count can come from Actions rather than only from a local build.Reading the result
Which kind of site each one is shows in how its example behaves as the level rises. A bounded blind spot keeps its shape and vanishes at some level; one standing on unbounded stack correlation grows longer with every level and never vanishes, because defeating a deeper abstraction just takes a longer sentence.
EVEN_PALINDROMEin the corpus is the second kind, which is why no level proves it — and why "prove the whole grammar automatically" may be the wrong target.docs/ambiguity.mddocuments both readings.Tests
SurveyTests(engine-backed): a conflict-free grammar surveys to zero sites and still proves; an ambiguous one surveys to at least one; dangling-else surveys to more than one, which is the assertion that the mode does not silently behave like a proof;--prove-surveywithout--proveexits 2.SurveyFlagTests: the flag reaches the engine only when asked for.Verification
No OCaml toolchain in this session — devbox installs nixpkgs from
api.github.com, blocked by the sandbox egress policy — so CI compiles this first. Checked locally: both Python modules compile, the workflow YAML parses with the new input wired toSURVEY, andtools.test_ambiguity_cli+tools.test_proverpass (54 tests, engine-backed ones skipped without a build).🤖 Generated with Claude Code
https://claude.ai/code/session_01N4KejxPN4Q9uQ67dHwUDgj
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