Nit: restore original signature of evaluateLines, map args in `dist…

…ance` impl

This is sort of subtle (and maybe a bit subjective). Hopefully the intent is clearer with the added JSDoc and updated argument names.

packages/xpath/src/functions/xforms/geo.ts

@@ -82,11 +82,22 @@ const INVALID_LINE: Line = {
 	end: INVALID_POINT,
 };
 
+/**
+ * Takes an {@link pointsArg} representing a serialized sequence of geopoints,
+ * producing an array of {@link Line}s.
+ *
+ * Corresponds to the semantics of _a single argument_ from either of the
+ * following functions, specified by ODK XForms:
+ *
+ * - {@link https://getodk.github.io/xforms-spec/#fn:area | area}
+ * - {@link https://getodk.github.io/xforms-spec/#fn:distance | distance}
+ */
 const evaluateLines = <T extends XPathNode>(
 	context: EvaluationContext<T>,
-	expression: readonly EvaluableArgument[]
+	pointsArg: EvaluableArgument
 ): Line[] => {
-	const points = expression.flatMap((el) => evaluatePoints(context, el));
+	const points = evaluatePoints(context, pointsArg);
+
 	if (points.length < 2) {
 		return [INVALID_LINE];
 	}
@@ -170,8 +181,8 @@ const geodesicArea = (lines: readonly Line[]): number => {
 export const area = new NumberFunction(
 	'area',
 	[{ arityType: 'required' }],
-	(context, [expression]) => {
-		const lines = evaluateLines(context, [expression!]);
+	(context, [pointsArg]) => {
+		const lines = evaluateLines(context, pointsArg!);
 
 		if (lines.some(isInvalidLine)) {
 			return NaN;
@@ -209,8 +220,10 @@ const sum = (values: readonly number[]) => {
 export const distance = new NumberFunction(
 	'distance',
 	[{ arityType: 'required' }, { arityType: 'variadic' }],
-	(context, args) => {
-		const lines = evaluateLines(context, args);
+	(context, pointsArgs) => {
+		const lines = pointsArgs.flatMap((pointsArg) => {
+			return evaluateLines(context, pointsArg);
+		});
 
 		if (lines.some(isInvalidLine)) {
 			return NaN;