[johnnyt]

Adds Predicator.AST with prewalk/3, postwalk/3 and reduce/3 over the positioned AST. Tooling that reads or rewrites a parsed tree no longer hand-writes a clause per node kind.

What it does

• Same shape as the standard library's Macro walks: prewalk(ast, acc, fun) and postwalk(ast, acc, fun) take fun.(node, acc) -> {node, acc} and return {ast, acc}; reduce(ast, acc, fun) takes fun.(node, acc) -> acc, visits pre-order and rewrites nothing.
• Walks everything t:Predicator.Parser.visitable/0 names: expressions, statements, blocks and programs.
• Children are visited in source order. The trailing position slot of every node is carried untouched, whether it is a {line, column}, a span or nil.
• New module and one docs sentence; no existing function changes what it answers, and Predicator.Visitor is unchanged.

Rules for the three shapes that are not plain children

• An object literal's key (t:Predicator.Parser.object_key/0) is outside the ast/0 and visitable/0 unions. The walks treat it as part of its entry: it is never handed to the callback and is carried through untouched; each entry's value is visited in entry order. Pinned by two tests (no key is ever seen; a rewrite leaves the key as it was).
• :duration holds {integer, unit} pairs, which are data. It has no children.
• :relative_date's first element is a node (its duration) and is walked as a child.

A tuple with an unknown tag raises FunctionClauseError; the sync test below is what keeps that from happening for a real node kind.

Sync test

test/predicator/ast_clause_coverage_test.exs reads the visitable/0 union from the compiled parser beam (Code.Typespec.fetch_types/1) and the map_children/3 clause heads from lib/predicator/ast.ex, and compares them both ways, so adding a node kind with no children clause turns it red (and so does a clause with no node kind). It follows the model of visitor_clause_coverage_test.exs. It is added to gate.sabotage.test_roots in .claude/wurk.json, as that file's convention asks of a binding test.

Provenance

The traversal's shape (Macro's, children in source order, programs and statement types walked) was decided by the conductor under a standing consent, 2026-10-12.

Gate and tier

• Full mix quality green on this head: Format, Compile, Doc links, Dependencies, Docs, Credo, Tests, Dialyzer. Doctor, Gettext and Sobelow report skipped (not installed), the expected state for this repo.
• Tier: size. The diff adds more than 300 lines outside docs (the module, its tests and the sync test), so it takes a cold pass rather than the in-turn review.
• No ISA or corpus change; no conformance/ file moved.
• Changelog: changelog.d/px-ybw0.md, ### Added, one line.

Sabotage (sync test and order tests)

Mutation	Test that failed	How
Renamed the :duration clause head to :durationx in ast.ex	Predicator.ASTClauseCoverageTest "map_children/3 covers exactly the constructors of Parser.visitable/0, both ways" (also walk tests that reach a duration)	assertion (missing and extra)
Added a {:sabotage_probe, ast(), position()} member to t:Predicator.Parser.ast/0	the same sync test	assertion (missing)
Walked right before left in the comparison clause	Predicator.ASTTest "binary nodes visit left then right" and "prewalk visits a parent before its children, postwalk after"	assertion

Each file was restored byte-equal from a copy and the build forced before the next check.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
lib/predicator/ast.ex	100.00% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.