Source Region Parser Experiment Notes

Current Parser Experiment

The parser works directly with CodePointIndex and CodePointSpan; rich Span conversion is intentionally left to rendering or reporting code.
Structured errors are currently enough to identify the failing region and the syntactic fact that failed, without committing to message text.
Empty documents are valid because the document grammar is a sequence of expressions.

SourceText.getSpan(CodePointSpan) is the key boundary operation: parsers can stay low-level, while diagnostics can opt into line and column information later.
Zero-width EOF spans are important for parse errors such as an unclosed list. The existing eofSpan support fits this well.
Parser clients still need a few tiny local span helpers, such as current-code-point span and EOF cursor span.
renderSpan returning structured LineView data works well for UI rendering because the application can choose its own DOM and styling without reimplementing line slicing.