Test-Driven Development (TDD)

Write the test first. Watch it fail. Write minimal code to pass.

When to Use

Always:

• New features

• Bug fixes

• Refactoring

• Behavior changes

Exceptions (ask first):

• Throwaway prototypes

• Generated code

• Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"

• Don't "adapt" it while writing tests

• Don't look at it

• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor Cycle

┌─────────────────────────────────────────────────────────────┐ │ │ │ RED ──────────► GREEN ──────────► REFACTOR │ │ │ │ │ │ │ │ │ │ │ │ ▼ ▼ ▼ │ │ Write Minimal Clean up │ │ failing code to (tests stay │ │ test pass green) │ │ │ │ │ │ │ └────────────────┴───────────────────┘ │ │ │ │ │ ▼ │ │ NEXT │ │ TEST │ └─────────────────────────────────────────────────────────────┘

RED - Write Failing Test

Write one minimal test showing what should happen.

Good:

#[test] fn retries_failed_operations_3_times() { let attempts = AtomicUsize::new(0); let operation = || { let count = attempts.fetch_add(1, Ordering::SeqCst); if count < 2 { Err(anyhow!("fail")) } else { Ok("success") } };

let result = retry_operation(operation, 3);

assert_eq!(result.unwrap(), "success");
assert_eq!(attempts.load(Ordering::SeqCst), 3);

}

Clear name, tests real behavior, one thing.

Bad:

#[test] fn retry_works() { let mock = MockFn::new() .returning_err(anyhow!("fail")) .returning_err(anyhow!("fail")) .returning_ok("success"); retry_operation(mock, 3); assert_eq!(mock.call_count(), 3); }

Vague name, tests mock not code.

Requirements:

• One behavior

• Clear name

• Real code (no mocks unless unavoidable)

Verify RED - Watch It Fail

MANDATORY. Never skip.

cargo test retry_operation -- --nocapture

Confirm:

• Test fails (not errors)

• Failure message is expected

• Fails because feature missing (not typos)

Test passes? You're testing existing behavior. Fix test.

Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test.

Good:

fn retry_operation<T, E, F>(mut f: F, max_attempts: usize) -> Result<T, E> where F: FnMut() -> Result<T, E>, { for i in 0..max_attempts { match f() { Ok(v) => return Ok(v), Err(e) if i == max_attempts - 1 => return Err(e), Err(_) => continue, } } unreachable!() }

Just enough to pass.

Bad:

fn retry_operation<T, E, F>( f: F, config: RetryConfig, // YAGNI ) -> Result<T, E> where F: FnMut() -> Result<T, E>, { // Exponential backoff, jitter, logging, metrics... }

Over-engineered.

Don't add features, refactor other code, or "improve" beyond the test.

Verify GREEN - Watch It Pass

MANDATORY.

cargo test retry_operation

Confirm:

• Test passes

• Other tests still pass

• Output pristine (no errors, warnings)

Test fails? Fix code, not test.

Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication

• Improve names

• Extract helpers

Keep tests green. Don't add behavior.

Repeat

Next failing test for next feature.

Good Tests

Quality Good Bad

Minimal One thing. "and" in name? Split it. test_validates_email_and_domain_and_whitespace

Clear Name describes behavior test1

Shows intent Demonstrates desired API Obscures what code should do

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing

• Might test implementation, not behavior

• Might miss edge cases you forgot

• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)

• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust.

Common Rationalizations

Excuse Reality

"Too simple to test" Simple code breaks. Test takes 30 seconds.

"I'll test after" Tests passing immediately prove nothing.

"Tests after achieve same goals" Tests-after = "what does this do?" Tests-first = "what should this do?"

"Already manually tested" Ad-hoc ≠ systematic. No record, can't re-run.

"Deleting X hours is wasteful" Sunk cost fallacy. Keeping unverified code is technical debt.

"Keep as reference, write tests first" You'll adapt it. That's testing after. Delete means delete.

"Need to explore first" Fine. Throw away exploration, start with TDD.

"Test hard = design unclear" Listen to test. Hard to test = hard to use.

"TDD will slow me down" TDD faster than debugging.

"Existing code has no tests" You're improving it. Add tests for existing code.

Red Flags - STOP and Start Over

• Code before test

• Test after implementation

• Test passes immediately

• Can't explain why test failed

• Tests added "later"

• Rationalizing "just this once"

• "I already manually tested it"

• "Tests after achieve the same purpose"

• "Keep as reference" or "adapt existing code"

• "Already spent X hours, deleting is wasteful"

• "TDD is dogmatic, I'm being pragmatic"

• "This is different because..."

All of these mean: Delete code. Start over with TDD.

Example: Bug Fix

Bug: Empty email accepted

RED

#[test] fn rejects_empty_email() { let result = validate_email(""); assert!(result.is_err()); assert_eq!(result.unwrap_err().to_string(), "Email required"); }

Verify RED

$ cargo test rejects_empty_email FAIL: called Result::unwrap_err() on an Ok value

GREEN

fn validate_email(email: &str) -> Result<(), ValidationError> { if email.trim().is_empty() { return Err(ValidationError::new("Email required")); } // ... Ok(()) }

Verify GREEN

$ cargo test rejects_empty_email PASS

REFACTOR

Extract validation for multiple fields if needed.

Verification Checklist

Before marking work complete:

• Every new function/method has a test

• Watched each test fail before implementing

• Each test failed for expected reason (feature missing, not typo)

• Wrote minimal code to pass each test

• All tests pass

• Output pristine (no errors, warnings)

• Tests use real code (mocks only if unavoidable)

• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Problem Solution

Don't know how to test Write wished-for API. Write assertion first. Ask for help.

Test too complicated Design too complicated. Simplify interface.

Must mock everything Code too coupled. Use dependency injection.

Test setup huge Extract helpers. Still complex? Simplify design.

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Final Rule

Production code → test exists and failed first Otherwise → not TDD

No exceptions.