0035. What `public` means on an attribute
0035. What public means on an attribute
Status
Proposed
Date
2026-10-04
Deciders
open; the operator rules when the first transport is designed
Context
The attribute contract on main has a public flag (packages/compiler/src/contracts.ts), and the example post.mx uses it. The flag is recorded in the model as Ash records public?, and nothing in v1 reads it (roadmap, M1 and M2). What it means for a transport is open. Ash, the Elixir framework Mesh is modelled on, has public? on attributes (and on relationships, calculations, aggregates and actions), default false: “Whether it appears over public interfaces. Attributes are private by default” (Ash features, section 2.2). The default is an Ash 3.0 reversal, made because “It was too easy to add an attribute and not realize that you had exposed it over your api” (Ash strengths and weaknesses, section 4). Mesh’s goals include “private unless exposed” (research synthesis, section 14, goal 4). One caution from the ecosystem: in ash_ai, which exposes actions as agent tools, public or private decides which fields come back, not who may call; policies are the security boundary (Ash ecosystem packages, sections 8 and “Implications for Mesh”).
Since the operator ruled that Mesh copies Ash’s DSL for v1 (ADR-0034), Ash’s public? semantics carry more weight than before: a Mesh public that differs from Ash’s needs a stated reason.
There is no transport in v1 (ADR-0005), so nothing is “public” yet. The flag is not a build error; it is recorded and ignored.
Decision
Not decided. Recommendation: Option A, with the exposure rule decided together with the first transport. It blocks nothing in v1; it blocks the first transport and the typed client. Until then the flag has no effect.
Options considered
Option A: public means “may leave through a transport”
Plan revision 2, D10, by the roadmap author: in-process callers get the full record; a transport returns only public attributes (plan revision 2 (superseded), section 8, D10).
| Dimension | Assessment |
|---|---|
| Complexity | Low: a filter in each transport |
| Cost | Nothing before the first transport |
| Fit with Ash | Close: Ash’s flag concerns public interfaces |
| Risk | Each transport must apply the filter; a forgotten one leaks |
Pros: matches Ash and the 3.0 lesson; trusted in-process code can read everything.
Cons: untestable until a transport exists, so the rule is unverified; the filter lives in adapters, outside core.
Option B: public is part of the typed result for all callers
| Dimension | Assessment |
|---|---|
| Complexity | Medium: generated result types differ from the stored record |
| Cost | Moderate, in M2 and M5 emitters |
| Fit with Ash | Weaker: Ash describes public? as exposure “over public interfaces”; whether in-process callers see private fields was not checked |
| Risk | The application’s own code cannot read private fields without a second API |
Pros: enforced by the compiler in every call, with or without a transport.
Cons: needs a second path for internal reads, which Mesh would invent.
Option C: Drop the flag until a transport exists
| Dimension | Assessment |
|---|---|
| Complexity | Lowest |
| Cost | Another edit to the contracts and the fixture |
| Fit with Ash | Loses a copied name |
| Risk | Reintroduction later is a vocabulary change |
Pros: no half-meaning on main.
Cons: contradicts the copy-Ash direction; the flag costs nothing while it is recorded and unread.
Trade-off analysis
A keeps Ash’s meaning and costs nothing now. B changes what a call returns. C undoes a copied name for no gain, since the recorded flag does nothing today.
Consequences
If A: transport authors own the filter, and the conformance tests for a transport must include it. Revisit when the first transport is designed (e.g. the command-line adapter for agents, ADR-0027); the roadmap does not order the after-v1 items.
Action items
- M1: record
publicin the model, as Ash recordspublic?; nothing reads it. - After v1, with the first transport: rule on this ADR and implement the filter.