Menene Design by Contract?

Question

Accepted Answer

**Design by Contract (DbC)** yana dauki alaƙa tsakanin hanya da wanda ya kira ta azaman yada juna na tsarin, wanda aka bayyana ta abubuwa guda uku: **prekondishi** (abin da mai kiran dole ya tabbatar), **postkondishi** (abin da hanya ta tabbatar a jiya), da **invariants** (abin da ya ci gari ne a koyaushe ga abuja).

## Abubuwan aljiha guda uku

```text
PRECONDITION  → caller's duty: inputs/state the method requires to run correctly
POSTCONDITION → method's duty: what it promises on return (if precondition held)
INVARIANT     → always-true property of the object, before and after every method
```

## In code

```python
class Account:
    def __init__(self, balance):
        self._balance = balance
        self._check_invariant()

def withdraw(self, amount):
        assert amount > 0, "PRE: amount must be positive"        # precondition
        assert amount <= self._balance, "PRE: sufficient funds"  # precondition
        old = self._balance
        self._balance -= amount
        assert self._balance == old - amount, "POST: balance reduced"  # postcond
        self._check_invariant()

def _check_invariant(self):
        assert self._balance >= 0, "INVARIANT: balance never negative"
```

Idan *prekondishi* ya kasa, **mai kiran** ne ke da kudio. Idan *postkondishi* ko *invariant* ya kasa, **hanya** ce ke da kudio — kwantat suna nuna kudio da daidai.

## Relationship to LSP

DbC ta ƙara Liskov daidai: wani nau'i na jijiya na iya **rage prekondishi** da **ƙarfafa postkondishi**, amma ba a jiya ba. Wannan ita ce daidai daidaita, wanda aka bayyana azaman kwantat.

## Trade-offs

```text
✓ Bugs surface at the boundary, with clear blame   ✓ Contracts double as documentation
⚠️ Runtime checks cost performance (often disabled in prod via assertions)
⚠️ Not all languages enforce it natively (Eiffel does; most use asserts/validation)
```

## Me yake matuhi

Kwantat suna canza zancen da ba a fayyace ba ("kumia bai kamata su zama maidowa ba, ina tunanin") zuwa alatu ne bayyane, za'a iya bincika — kuma suna nuna kudio daidai lokacin da abin ya rushe.

Suna bada tushe ga Liskov mabadili da ƙarfafa API gida: kwantat na hanya ita ce ainihin tarayya, gaskiya fi jirgin nishadi nau'i kaɗai.