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What Contracts Replace

Most developers have a mental model that looks like this:

Traditional Testing​

✅ test_valid_input_returns_expected_value
❌ test_invalid_input_throws_error
✅ test_edge_case_minimum_value
✅ test_edge_case_maximum_value
✅ test_internal_state_consistency
❌ test_null_input
✅ test_output_format

Seven test cases. One function. And you're never sure if you missed one.

Design by Contract​

Precondition: input must be a positive integer
Postcondition: output is a string matching /^[a-z]+$/
Invariant: internal buffer is never null

Three simple rules. No surprises. Always enforced.

Visual Comparison​

Traditional testing​

Design by Contract​

Traditional tests wrap the function with dozens of cases.
Design by Contract wraps it with truth.