attac-t Skill: Decide Test Merit
"A test that cannot fail is a comment that costs a second."
craft-test decides whether a behaviour deserves a test. This decides whether a test you already
have deserves its line.
The Decision
Keep it when:
- Breaking the code it guards turns it red
- It is the only check that would notice
- It names the mechanism where a neighbour names only the outcome
- It pins something documented — a code, a path, a format
Cut it when:
- A neighbour fails first, for the same reason
- It walks the same branch with different inputs
- It asserts a thing exists, and the next check reads that thing
- It passes without the mechanism it claims to prove ever running
The Heuristic
Ask: "If I delete this line, what goes red?"
Nothing → it was never a test.
The Quick Test
| Ask | Answer | Then |
|---|---|---|
| Break the code on purpose — does it fail? | No | It is not guarding. Fix it or cut it |
| Delete it — is every deliberate break still caught? | Yes | Cut it |
| Does another check fail first, same cause? | Yes | Cut the later one |
| Would anything else notice? | No | Keep it, whatever it costs |
Aim The Break
A break must change the lines you meant and no others. Prove it before you trust it: apply it, count the changed lines, and check the result still parses.
A break that fires in two places tests neither. One that must fire in several says so on its own line — otherwise the next reader cannot tell design from accident.
Moving code silently unaims every break that named it. A harness worth having reports the break did not apply rather than passing.
Test a break through the harness that runs it. Never by retyping its expression. A backslash does not always survive being retyped, so a working break reads as broken and gets "fixed" — the harness holds the only copy you can trust.
Passing For The Wrong Reason
Worse than a missing test, because every instrument scores it as coverage — including the person who wrote it. A gap is known. A false negative is not.
The tell: you cannot name the line that would have to change to turn it red.
Two Altitudes, Not Three
Split behaviour from mechanism only when the behaviour check can pass without the mechanism running. One extra check, not a family — a third, restating either, is triplication reading as thoroughness.
When The Failure Cannot Be Forced
Some shapes will not arrange portably: a permission the filesystem does not record, a race, a clock. Keep the guard and say so.
A guard with an untestable case is honest. A guard with a break that proves nothing is not — it reads as covered, which is worse than reading as absent.
The Anti-Patterns
| Don't | Do | Why |
|---|---|---|
| Count checks | Count what only they catch | Suites grow checks faster than coverage |
| Cut on a hunch | Cut, then re-run the breaks | The audit decides, not taste |
| Keep it "just in case" | Name the case, or cut it | An unnamed case is not a case |
| Assert existence, then read | Read it | The read already fails on absence |
| Trust a green suite never seen red | Break it on purpose | Green is evidence only if red was possible |