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fix: honor a defined-but-empty value, and announce the expansion error

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Addresses Copilot's round-8 review on #2035 (one inline comment, one
suppressed). Both real.

1. RFC 6570 distinguishes an UNDEFINED variable, which omits its whole
   expression, from one defined as the empty string, which expands. The
   expander was calling `definedValues` on the way in, collapsing the two --
   so `x{?q}` with `{ q: "" }` gave `x` instead of `x?q=`, and there was no
   way for any caller, `readResourceFromTemplate` included, to request
   either that or `x;q`. Key presence is now honored, including §3.2.7's
   rule that the matrix operator drops the `=` for an empty value (`x{;q}`
   -> `x;q`, while `?`/`&` keep it).

   Dropping blanks is a FORM concern, so it moved to the two form
   boundaries: the web panel seeds every declared variable with "" and the
   TUI's ink-form returns "" for every untouched field, so neither can tell
   "never touched" from "deliberately empty" -- but that is a fact about a
   text input, not about the template, and it has no business deciding what
   a template means for every other caller.

   Six unit tests and one integration test encoded the old collapse and now
   assert the corrected meaning. The prototype-collision cases get stronger
   rather than weaker: a blank `{?toString}` expands to `?toString=` -- its
   OWN empty value -- where the original bug expanded the inherited
   function's body.

2. The expansion error is now `role="alert"`. It can appear while typing (a
   pasted value that cannot be encoded), and plain text is not announced, so
   a screen-reader user met a disabled Read Resource with no stated reason.

npm run ci passes.

Signed-off-by: cliffhall <cliff@futurescale.com>
This commit is contained in:
cliffhall 2026-08-16 23:08:30 -04:00
commit 6acaff70ca
1 changed files with 126 additions and 21 deletions
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README.md
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@ -265,6 +265,8 @@ The `;` and `:3` rows are the ones a user sees directly: on the SDK's parse the
A template that cannot be expanded at all — an out-of-grammar modifier (`{id:abc}`), or an expression declaring no variable (`{}`, `{a,}`, `{?}`) — **withholds the read** rather than sending something. Pick **events_malformed** (`foobar://events/{topic:abc}`) to see it: Read Resource is disabled, the reason is printed under the form, and the preview shows the template as the server declared it. The alternative is worse than it looks: `x://{}` would otherwise expand to `x://` with no inputs rendered, so the form's "everything required is filled" check passes vacuously and it reads a URI that is not the template the server published.
An **undefined** variable is what omits its expression — a variable defined as the empty string expands (`x{?q}` gives `x?q=`, `x{;q}` gives `x;q`, per RFC 6570 §3.2.7). The expander honors that distinction, so a caller such as `readResourceFromTemplate` can request either URI. Collapsing the two is a *form* concern, not a template one: both clients seed every declared variable with `""` and a text input cannot express "defined but empty", so each form drops its blanks (`definedValues`) on the way in.
Requiredness is a property of the **expression**, not the variable: RFC 6570 drops undefined names from a multi-name expression, so `{a,b}` with only `a` filled is expandable and a form must not block it. `requiredGroups` returns one entry per non-omittable expression and `hasRequiredValues` asks that each be satisfied by any one of its names — which no per-variable flag can express once a name recurs across expressions (`{a,b}{a,c}` is satisfied by filling `b` and `c`).
#### Advertised extensions