formalize

Formalize and Check a Function

Drive a propose-contract -> check -> repair loop around one Python function. The verifier (edify check, backed by CrossHair) owns deduction. You own intent: what the function should guarantee. Verification proves code meets a contract; it does NOT prove the contract is the right one. That judgment is yours, and when intent is genuinely ambiguous you ASK rather than guess.

Procedure

1. Establish intent. Derive what the target function should guarantee from the conversation and the surrounding code. If the intended behavior is genuinely ambiguous (edge cases, error handling, empty inputs), use AskUserQuestion — do not invent a specification.

2. Write the contract. Add icontract decorators to the function: @require(lambda <args>: <precondition>) and @ensure(lambda <args>, result: <postcondition>). A target needs at least one pre/post-condition or CrossHair will not analyze it.

3. Check. Run:

   edify check <file-or-dotted-target> --json
   

   The result status is one of verified, refuted, or error.

4. Interpret — the core judgment:

   • verified — no counterexample within CrossHair's search budget. Report and stop. State the honest guarantee level: bounded path-exploration, not a total proof.
   • refuted — read the counterexample (falsifying input + location), then decide why it failed:
     • code bug — the implementation is wrong: fix the code.
     • spec bug — the contract was wrong or too strong: fix the contract.
     • intent ambiguity — the counterexample exposes a case the user has not decided (e.g. "what should this do on an empty list?"): STOP and ask. Then loop back to step 3.
   • error — surface it. A common cause is a target with no contract (CrossHair analyzed nothing); add a contract. Never report error as success.

5. Loop with a cap. Do at most a small fixed number of repair iterations (e.g. 5). If you hit the cap, STOP and report the honest state: the last counterexample and what remains unresolved. Never upgrade refuted or an error to "verified".

Disciplines

• Ambiguity about intent -> ask, never guess.
• Every counterexample gets an explicit code-bug vs spec-bug triage.
• No "verified" claim without a verified result from edify check.
• A verified result is bounded, not a soundness/termination proof — say so.

Backend

edify check wraps CrossHair (symbolic execution + Z3). It is the deduction oracle; this skill is the intent-holder. See docs/superpowers/specs/2026-06-08-invariant-guided-verify-loop-design.md.