Summary

M5c implements $eval(expr [, context]) — parse a string as a JSONata expression and evaluate it at runtime, faithfully to jsonata-js v2.2.1 (outer-scope variable visibility, D3120/D3121 error codes).

• wants_env hook — $eval is the first builtin that needs a live environment (to evaluate a newly-parsed AST so it can see builtins, outer-scope bound variables, and custom functions). Added an opt-in hook to M.apply: a builtin with wants_env = true receives (env, input, ...args). The default dispatch path is byte-unchanged when the flag is absent.
• $eval (functions/eval.lua) — lazy-requires the parser + evaluator (breaks the load cycle); undefined → undefined; pcall parse → D3120; focus-or-current-input selection; evaluates against the caller's live env directly (so ($x := 5; $eval('$x')) → 5); pcall evaluate → D3121.
• D3120/D3121 error templates added (errors.lua), with separate parse-vs-evaluate pcall boundaries.

Results

• Official jsonata suite: 1137 → 1147 / 1682 (+10, 68.0% → 68.6%), zero regressions. The function-eval group is fully green.
• 416/416 unit tests green.

Verification

An adversarial fidelity review against a jsonata-js v2.2.1 oracle (~50 paired cases) confirmed error-code fidelity is exact (D3120/D3121 boundary + inner-error rewrap of $assert/$error into D3121 all match — this is suite-invisible, as the suite runner checks only that an error was raised, not its code). It found two in-scope divergences, both fixed in 465c605:

• '1+1' ~> $eval crashed because env wasn't threaded into the ~>/partial-application apply paths (the "wire the hook into all apply paths" lesson from M5a) — now threaded.
• A non-sequence array focus carried the internal cons flag and corrupted path indexing — now rebuilt clean.

Deferred/logged (pre-existing, engine-wide, not $eval-specific): $$ root context broken; 1/0→inf vs null; deep-nested cons-array indexing; error-message text fidelity.

Test Plan

• busted full unit suite — 416/416
• scripts/run-suite.sh — 1147/1682, zero regressions
• Adversarial fidelity review vs jsonata-js v2.2.1 oracle (~50 paired cases)

🤖 Generated with Claude Code