design-by-contract

name	design-by-contract
description	Design-by-Contract (DbC) development - design contracts from requirements, then execute CREATE -> VERIFY -> TEST cycle. Use when implementing with formal preconditions, postconditions, and invariants using deal (Python), contracts (Rust), Zod (TypeScript), or Kotlin contracts.

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.

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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)

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PHASE 1: PLANNING - Design Contracts from Requirements

CRITICAL: Design contracts BEFORE implementation.

Extract Contracts from Requirements

1. 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?)

2. 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 {}

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

Constitutional Rules (Non-Negotiable)

1. CREATE All Contracts: Implement every PRE, POST, INV from plan
2. Enforcement Enabled: Runtime checks must be active
3. Violations Caught: Tests prove contracts work
4. 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