Explicitly disallow non-string LDflex expressions.

They currently trigger infinite rerenders,
because the passed object is always different:
#15

src/hooks/useLDflex.js

@@ -11,7 +11,10 @@ const none = {};
  * Evaluates the Solid LDflex expression.
  * Returns an array of [result, pending, error].
  */
-export default function useLDflex(expression, listMode = false) {
+export default function useLDflex(expression = '', listMode = false) {
+  if (typeof expression !== 'string')
+    throw new Error('Only LDflex string expressions are currently supported.');
+
   // The user's WebID and recent updates might influence the evaluation
   const webId = useWebId();
   const latestUpdate = useLiveUpdate();

test/hooks/useLDflex-test.js

@@ -52,11 +52,9 @@ describe('useLDflex', () => {
     expect(evaluator.evaluate).toHaveBeenCalledTimes(1);
   });
 
-  it('does not re-evaluate a Promise when a user logs in', () => {
-    renderHook(() => useLDflex(Promise.resolve(), true));
-    evaluator.evaluate.mockClear();
-    auth.mockWebId('https://example.org/profile#me');
-    expect(evaluator.evaluate).toHaveBeenCalledTimes(0);
+  it('does not yet support non-string LDflex expressions', () => {
+    expect(() => useLDflex(Promise.resolve()))
+      .toThrow('Only LDflex string expressions are currently supported.');
   });
 
   it('re-evaluates when the UpdateContext changes', () => {