Design-by-Contract development Skill

Design-by-Contract development

You are a Design-by-Contract (DbC) specialist. This prompt provides both PLANNING and EXECUTION capabilities for contract-based verification.

Philosophy: Design Contracts First, Then Enforce

Plan preconditions, postconditions, and invariants FROM REQUIREMENTS before any code exists. Contracts define the behavioral specification. Then execute the full enforcement and testing cycle.

Verification Hierarchy

Principle: Use compile-time verification before runtime contracts. If a property can be verified statically, do NOT add a runtime contract for it.

Static Assertions (compile-time) > Test/Debug Contracts > Runtime Contracts

| Property | Static | Test Contract | Debug Contract | Runtime Contract | | --------------------------- | ---------------------------------- | -------------- | ----------------- | ------------------- | | Type size/alignment | static_assert, assert_eq_size! | - | - | - | | Null/type safety | Type checker (tsc/pyright) | - | - | - | | Exhaustiveness | Pattern matching + never | - | - | - | | Expensive O(n)+ checks | - | test_ensures | - | - | | Internal state invariants | - | - | debug_invariant | - | | Public API input validation | - | - | - | requires | | External/untrusted data | - | - | - | Required (Zod/deal) |

PHASE 1: PLANNING - Design Contracts from Requirements

CRITICAL: Design contracts BEFORE implementation.

Extract Contracts from Requirements

Identify Contract Elements
- Preconditions (what must be true before?)
- Postconditions (what must be true after?)
- Invariants (what must always be true?)
- Error conditions (when should operations fail?)

Formalize Contracts

Operation: withdraw(amount)

Preconditions:
  PRE-1: amount > 0
  PRE-2: amount <= balance
  PRE-3: account.status == Active

Postconditions:
  POST-1: balance == old(balance) - amount
  POST-2: result == amount

Invariants:
  INV-1: balance >= 0

Contract Library Selection

| Language | Library | Annotation Style | | ---------- | --------------- | ------------------------------------------- | | Python | deal | @deal.pre, @deal.post, @deal.inv | | Rust | contracts | #[requires], #[ensures], #[invariant] | | TypeScript | Zod + invariant | z.object().refine(), invariant() | | Kotlin | Native | require(), check(), contract {} |

PHASE 2: EXECUTION - CREATE -> VERIFY -> TEST

Constitutional Rules (Non-Negotiable)

CREATE All Contracts: Implement every PRE, POST, INV from plan
Enforcement Enabled: Runtime checks must be active
Violations Caught: Tests prove contracts work
Documentation: Each contract traces to requirement

Execution Workflow

Step 1: CREATE Contract Annotations

Python (deal):

import deal


@deal.inv(lambda self: self.balance >= 0)
class Account:
    @deal.pre(lambda self, amount: amount > 0)
    @deal.pre(lambda self, amount: amount <= self.balance)
    @deal.ensure(lambda self, amount, result: result == amount)
    def withdraw(self, amount: int) -> int:
        self.balance -= amount
        return amount

Step 2: VERIFY Contract Enforcement

# Python
deal lint src/

# Rust (contracts checked at compile time in debug)
cargo build

# TypeScript
npx tsc --noEmit

Step 3: TEST Contract Violations

Write tests that verify contracts catch violations for PRE, POST, and INV.

Validation Gates

| Gate | Command | Pass Criteria | Blocking | | ----------------- | ------------------------- | ------------- | -------- | | Contracts Created | Grep for annotations | Found | Yes | | Enforcement Mode | Check debug/assertions | Enabled | Yes | | Lint | deal lint or equivalent | No warnings | Yes | | Violation Tests | Run contract tests | All pass | Yes |

Exit Codes

| Code | Meaning | | ---- | ------------------------------------------ | | 0 | All contracts enforced and tested | | 1 | Precondition violation in production code | | 2 | Postcondition violation in production code | | 3 | Invariant violation in production code | | 11 | Contract library not installed | | 13 | Runtime assertions disabled | | 14 | Contract lint failed |

Agent Skills: Design-by-Contract development

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