code-comments

Chalk Code Comments — What Earns a Comment

Interpret MUST, MUST NOT, SHOULD, SHOULD NOT, MAY, etc. per RFC 2119.

Two kinds of comment

An interface comment says what a caller needs in order to use the thing without reading its body. An implementation comment says what the code inside it doesn't already say. Their defaults are opposite, and everything below is about implementation comments unless it says otherwise.

An interface comment on a public surface exists by default, and is judged on completeness. Parameters, return, errors, preconditions, units, ownership, thread-safety. A caller who has to read the body to find one of those has been failed.
A non-public surface takes implementation rules whatever its markup — a kdoc on a private function still faces the triggers.

The readers

An implementation comment's reader is a competent developer on this project, arriving at this line in a year, mid-investigation of a different bug. They did not read the commit that added it, do not know a change happened here, and will read this line and the twenty around it — nothing else.

Simulate that reader before there's a comment on the screen, not after.

What each clause rules out:

"in a year", "did not read the commit" — they have no referent for the change. "Now we…", "instead of…", "this handles the case we hit" say nothing to them.
"a different bug" — they want one fact and they're leaving. Orientation, summary and section-label comments cost them and give nothing.
"this line and the twenty around it" — nothing that depends on reading elsewhere. No "phase 2 of teardown".

An interface comment's reader is deciding whether to call this, from the signature and the doc alone, and will not read the body.

The triggers

An implementation comment MUST answer yes to one of these, asked of the code rather than of the comment. None applies → no comment. One per way the code under-determines what the reader does next.

Can a value here be absent or invalid, with nothing in the types saying so?
Is this state reachable from more than one thread, with the discipline that keeps it safe not visible here? A lock the caller holds, a happens-before, why a field is volatile.
Would a reader reorder these statements? Two adjacent lines whose order matters and whose names don't say so.
Is there a constant with no derivation? A timeout, a retry count, a buffer size, a threshold.
Is this a workaround for a defect outside this repo? Name it — the library and version, the spec clause, the issue.
Would a reader replace this with the obvious implementation? Say what the obvious one is and how it fails.
Must a caller do something the signature can't express? A precondition, a required close, an ordering between two calls.
Does something here read as an oversight? An empty catch, a discarded return, a branch that deliberately does nothing.
Would a reader delete this as unnecessary? A branch, a parameter or a supported case that nothing else in the code motivates.

A yes is necessary, not sufficient — the comment still has to pass the test.

The test

These are implementation-comment tests. An interface comment is judged on completeness for a caller who will not read the body, so a test asking what a reader of the surrounding code would get wrong does not apply to it.

Cover the comment, read the code, and name what the reader would get wrong. Nothing → delete it.

No clause of either test is yours to adjudicate — the reader's derivation decides, not your sense of what's subtle.

A comment MUST sit at a different level of detail from the code it describes — higher, saying what the code accomplishes, or lower, giving precision the code omits: units, ranges, boundary conditions, what "empty" means here. Same level as the code is restatement, and the red flag is a comment built from the identifiers beneath it.

A comment that fails MUST be deleted, not shortened. Reaching for a length budget instead is how justification survives a pass and comes back trimmed — so if you're rewriting a comment for the second time, apply the test rather than the budget.

Apply them to existing implementation comments too, and check an existing interface comment for completeness the same way you would a new one.

Where the rejected material goes

Design rationale MUST go in the commit body, not the source. Why the code is allowed to exist, why a surface is shaped as it is, why one option beat another. An answer to a question raised in review is the case to watch: it's neither repetition, step-narration nor history, so it passes every other rule here while being precisely what the reader never asks.
A comment about the change goes in the commit body — the reader has no referent for it. There it's read once, by someone who wants it. See "Transitions vs. current state" in chalk:voice.
Anything true beyond these twenty lines → the one place that owns it. A pattern's rationale goes at the pattern, once, not re-narrated at every site; a call site's oddity goes at that call site, not as a caller list on the function.

You're not writing to impress anyone

A comment defending a choice advertises diligence — the same move as ranking your own material — and it reads as care, which is exactly why it survives review.

Reviewing the comments in a diff

A code review MUST cover the diff's comments as well as its code.

Per comment in the diff:

Decide which kind it is first, from the surface's reach.
Interface comment → check completeness, and never report a deletion for failing a trigger it was never subject to.
Implementation comment → apply the triggers, then the test.
If nothing, report a deletion.
If it's misfiled rather than wrong, say where it goes — commit body, canonical place, call site.
Scrutinise the confident ones hardest. A comment restating a decision in assured prose is the one a reviewer waves through.

Markup

Most comments are one line and stay one line. // volatile — reads race with the flush thread doesn't want a bullet.
A comment with real structure takes a mindmap, at a higher threshold than prose, with markup depending on whether the language's tooling renders it — see the code-comment rules in chalk:mindmap.
Sentence-per-line, per chalk:voice.