The Pipeline Closes
Part of a series on #49 — TypedDataSet native. Previous: Fetch Is Just Push.
The as never Dies
The single worst line in blocks-ui was in fetchSource:
sink.apply({ type: "snapshot", dataset: data as never });
Raw JSON from r.json(), cast to bypass the type system entirely, shoved into a SnapshotEvent that claims to carry TypedDataSet. No TypedRow accessors. No branded ColumnIds. Every downstream consumer forced to cast back: this.dataSet as any, Array.isArray(data), entries.dataSet as LedgerEntry[].
We replaced it with the extraction pipeline that pages already had — extractDataSet takes the raw JSON, tabulates it into columns and rows (inferring schema from object keys when columns aren’t declared, using explicit ExternalColumnDef when they are), and produces real TypedDataSet with working cell(), number(), text(), date() accessors. The pages session had already done the heavy lifting: ExtractionDef type narrowing, fromRows() factory, SnapshotEvent.totalRows, the DataReceiver.dataSet: TypedDataSet | undefined contract fix, and the full table redesign.
On the blocks-ui side, fetchSource gained columns, dataPath, and totalPath options. DataSourceAdapter and DataSourceMixin propagate the correct type. createSourceFactory passes the options through.
list-pane — First Consumer
list-pane was the proving ground. Before: it received unknown from DataSourceMixin, did const data = this.dataSet as any, checked Array.isArray(data) or data.items, stored rows in private state, and built ColumnDef arrays with getValue extractors. After: it passes this.dataSet directly to <pages-table>. The _rows, _totalRows, _lastDataSet state variables and the willUpdate extraction logic are gone. The component shrank from 125 lines to 100.
One surprise: the extraction pipeline throws EMPTY_RESULT on empty arrays. An endpoint returning [] produces sink.error instead of an empty TypedDataSet. This is intentional for YAML data sources (where empty data signals misconfiguration) but wrong for REST APIs where zero results is normal. Filed as a garden entry — the upstream fix is straightforward but lives in pages-data.
Five Consumers Left
trust-score-panel, audit-trail-viewer, work-item-inbox, notification-inbox, and subscription-list all follow the same migration pattern: replace ColumnDef with columnConfig + columnRenderers, replace <pages-data-table> with <pages-table>, delete the manual extraction from unknown. trust-score-panel and audit-trail-viewer have existing bugs (wrong property names on ColumnDef, wrong element property) that get fixed as part of the migration.