ADR 0061: A rule that says nothing says nothing

Status: Accepted

Context

A rule returns the messages it wants shown, and no messages is an empty list. So this is what a person writes:

(value) => { if (value === "taken") return ["Already taken"]; }

There is no else, because there is nothing to say when the value is fine. It returns undefined, and the engine read .map off it — inside the computed every read of valid() goes through. createForm accepted the rule without a word and the first question thrown at the form threw back Cannot read properties of undefined (reading 'map'), from a stack pointing at field-record.ts while the mistake sat three files away in something the consumer wrote. After that the form could not be asked anything: not its validity, not through a renderer, not by a submit.

The asynchronous half of the same idiom failed more quietly. results.flat() kept the undefined, so every good value was marked invalid and the word “undefined” appeared next to the field.

ADR 0057 refused a wrong-shaped argument where it arrived. A rule is the same kind of argument with one difference that decides this record: what it answers cannot be known until it runs, so there is no door to refuse it at.

Decision

Nothing is nothing. A rule returning undefined or null has no messages. It is the ordinary case — the shape most people write — and reading it as “no errors” loses nothing, because a rule with something to say says it.

A bare string is one message. A rule with one thing to say, said without the list around it, reaches the person it was written for.

Anything else becomes a message that says so. A boolean, a number, an object, or a list holding one, cannot be guessed at: passing the value silently would let a rule someone wrote stop applying without a word, and that is the failure this record exists to prevent. The value is reported as unchecked, and the development channel names the shape.

The same reading applies to a synchronous rule and to what an asynchronous one resolves with, because it is one idiom written twice.

Consequences

A rule that returned false used to make the form unreadable and now marks the field invalid with "This value could not be checked." — developer-facing wording on a user-facing surface, the same trade ADR 0060 takes. A product that wants its own words writes a rule that returns them.

false is read as unreadable rather than as “invalid”. Somebody meaning “invalid, no message” gets a message they did not write. The alternative is guessing that false means invalid and true means valid, which is a second rule vocabulary nobody declared.

A rule returning nothing no longer fails loudly, so a genuine mistake — a rule that meant to return a list and fell off the end of a branch — now passes quietly. That is the price of supporting the idiom, and it is bounded: the rule was not applying either way, and before this it took the whole form down with it.

Alternatives rejected

Refuse at field() / addValidators. There is nothing to inspect: a function’s return value is not knowable at the door, and the list-of-functions check ADR 0057 added is already there.

Throw at the first bad answer, naming the rule. It is the loudest option and it keeps the failure mode this record removes — a form that cannot be read — while moving the message. A form that cannot answer valid() cannot be rendered, and a rendering framework catching that has nowhere useful to put it.

Read false as invalid and true as valid. It makes the boolean shapes work and introduces a second way to answer a rule, which every consumer and every adapter would then have to know about.

Verification

• battle-tests/adversarial/validation/a-validator-that-returned-nothing.battle.test.mjs — the four shapes, synchronous and asynchronous, with the explicit empty list as the control.
• battle-tests/adversarial/validation/a-validator-that-breaks.battle.test.mjs — a rule that throws, which is the neighbouring failure and is unchanged by this.
• packages/core/test/ — the workspace suite, where every built-in validator returns a list and so passes through unchanged.

Security and privacy

None directly. A message that is not a string is replaced rather than rendered, which closes one more route by which an object reaches a page as [object Object] — the same route ADR 0060 closed for a server’s answer.