dict: deal with unknown keys

dictionary/dictionary.go

@@ -1,11 +1,15 @@
 package dictionary
 
+import "errors"
+
 type Dictionary map[string]string
 
-func (d Dictionary) Search(word string) string {
+var ErrNotFound = errors.New("could not find the word you were looking for")
+
+func (d Dictionary) Search(word string) (string, error) {
 	result, ok := d[word]
 	if !ok {
-		return ""
+		return "", ErrNotFound
 	}
-	return result
+	return result, nil
 }

dictionary/dictionary_test.go

@@ -3,7 +3,7 @@ package dictionary
 import "testing"
 
 func assertStrings(t testing.TB, got, want string) {
-    t.Helper()
+	t.Helper()
 
 	if got != want {
 		t.Errorf("got %q want %q given %q", got, want, "test")
@@ -13,8 +13,20 @@ func assertStrings(t testing.TB, got, want string) {
 func TestSearch(t *testing.T) {
 	dictionary := Dictionary{"test": "this is just a test"}
 
-	got := dictionary.Search("test")
-	want := "this is just a test"
+	t.Run("known word", func(t *testing.T) {
+		got, _ := dictionary.Search("test")
+		want := "this is just a test"
 
-    assertStrings(t, got, want)
+		assertStrings(t, got, want)
+	})
+
+	t.Run("unknown word", func(t *testing.T) {
+		_, err := dictionary.Search("undefined")
+		want := "could not find the word you were looking for"
+		if err == nil {
+			t.Fatal("expected to get an error")
+		}
+
+		assertStrings(t, err.Error(), want)
+	})
 }