poker: also using Result for the tested function

exercises/poker/example.rs

@@ -7,11 +7,11 @@ use counter::Counter;
 ///
 /// Note the type signature: this function should return _the same_ reference to
 /// the winning hand(s) as were passed in, not reconstructed strings which happen to be equal.
-pub fn winning_hands<'a>(hands: &[&'a str]) -> Option<Vec<&'a str>> {
+pub fn winning_hands<'a>(hands: &[&'a str]) -> Result<Vec<&'a str>, &'static str> {
     let mut hands = hands
         .into_iter()
         .map(|source| Hand::try_from(*source))
-        .collect::<Result<Vec<_>, _>>().ok()?;
+        .collect::<Result<Vec<_>, _>>()?;
     hands.sort_by(|a, b| a.partial_cmp(b).unwrap_or(Ordering::Less));
     hands.last().map(|last| {
         hands
@@ -21,6 +21,7 @@ pub fn winning_hands<'a>(hands: &[&'a str]) -> Option<Vec<&'a str>> {
             .map(|hand| hand.source)
             .collect()
     })
+    .ok_or("No winning hand")
 }
 
 #[derive(Debug, PartialEq, Eq, PartialOrd, Clone, Copy, Hash)]

exercises/poker/src/lib.rs

@@ -2,6 +2,6 @@
 ///
 /// Note the type signature: this function should return _the same_ reference to
 /// the winning hand(s) as were passed in, not reconstructed strings which happen to be equal.
-pub fn winning_hands<'a>(hands: &[&'a str]) -> Option<Vec<&'a str>> {
+pub fn winning_hands<'a>(hands: &[&'a str]) -> Result<Vec<&'a str>, &'static str> {
     unimplemented!("Out of {:?}, which hand wins?", hands)
 }

exercises/poker/tests/poker.rs

@@ -26,6 +26,18 @@ fn test_single_hand_always_wins() {
     test(&["4S 5S 7H 8D JC"], &["4S 5S 7H 8D JC"])
 }
 
+#[test]
+#[ignore]
+fn test_empty_input() {
+    assert!(&winning_hands(&[""]).is_err())
+}
+
+#[test]
+#[ignore]
+fn test_not_enough_cards() {
+    assert!(&winning_hands(&["4S 5S 7H 8D"]).is_err())
+}
+
 #[test]
 #[ignore]
 fn test_highest_card_of_all_hands_wins() {