Provably Correct Software Construction

Build software that is correct by design using formal verification.

Core Methodology

1. Design-by-Contract (DbC)

• Action: Define Preconditions, Postconditions, and Class Invariants.
• Convention: Reject defensive programming. If a precondition is specified, the supplier must assume it is true. Do not add redundant checks.
• Exception Handling: Restore class invariants before resumption or triggering an "organized panic".
• Rollback Rule: When restoring state in an exception handler, use established property setters or call the class invariant check immediately after restoration to ensure a validated state.

2. Hoare Logic & wp Calculus

• Hoare Triples: Apply {P} C {Q} to specify code blocks.
• Backward Construction: Use wp(C, Q) to derive the weakest precondition from your postcondition.
• Assignment Rule: wp("x := E", R) = R[x -> E].
• Sequential Composition: wp("S1; S2", R) = wp(S1, wp(S2, R)).

Implementation Workflow

1. Define Postcondition (Q): State the exact logical goal.

2. Derive Precondition (P): Calculate the minimum required state using wp.

3. Loop Verification (Total Correctness):

   • Invariant (I): Must hold before, during, and after the loop. (I AND NOT B) => Q.
   • Variant (v): Strictly decreasing non-negative integer function ensuring termination.
   • Checks: Prove Initialization (P => I), Preservation ({I AND B} Body {I}), and Termination (v decreases, I => v >= 0).
   • Runtime Enforcement: Embed assert statements within the loop body to verify the Invariant and Variant at each iteration.

4. Annotate: Use language-native assertions (e.g., assert in Python, @requires/@ensures in Eiffel/JML) to document the contract.

Worked Example: Array Binary Search

def binary_search(array: list[int], target: int) -> int:
    """
    Precondition: array is sorted.
    Postcondition: returns mid s.t. array[mid] == target, else -1.
    """
    # DbC Precondition
    assert all(array[i] <= array[i+1] for i in range(len(array)-1))

    low, high = 0, len(array) - 1
    # Invariant I: target in array iff target in array[low...high]
    # Variant v: high - low + 1
    
    while low <= high:
        # Runtime Invariant & Variant Verification
        v_old = high - low + 1
        assert (target not in array) or (target in array[low:high+1])
        assert v_old >= 0

        mid = (low + high) // 2
        if array[mid] == target:
            return mid # Postcondition Q reached
        elif array[mid] < target:
            low = mid + 1
        else:
            high = mid - 1
        
        # v decreases, I is preserved
        v_new = high - low + 1
        assert v_new < v_old
            
    return -1 # (I AND low > high) => target not in array

References

• Hoare Logic
• Weakest Precondition
• Design-by-Contract