public boolean eval(ContentFile<?> file) {
    // TODO: detect the case where a column is missing from the file using file's max field id.
    return new MetricsEvalVisitor().eval(file);
  }
@Test
  public void testNoNulls() {
    boolean shouldRead = new StrictMetricsEvaluator(SCHEMA, isNull("all_nulls")).eval(FILE);
    assertThat(shouldRead).as("Should match: all values are null").isTrue();

    shouldRead = new StrictMetricsEvaluator(SCHEMA, isNull("some_nulls")).eval(FILE);
    assertThat(shouldRead).as("Should not match: not all values are null").isFalse();

    shouldRead = new StrictMetricsEvaluator(SCHEMA, isNull("no_nulls")).eval(FILE);
    assertThat(shouldRead).as("Should not match: no values are null").isFalse();
  }