feat: improve worksheet

src/model/taxonomy.rs

@@ -423,17 +423,53 @@ pub enum Format {
     MultipleResponse,
     /// Two options, True and False.
     TrueFalse,
+    /// A free-text answer the student writes rather than selects.
+    ///
+    /// It carries no options and is not machine-scored. What a grader marks it
+    /// against, and what a solutions document shows, lives in the item's
+    /// [`crate::item::Solution`]: a model answer and, when the item is worth more
+    /// than a point, a rubric. This is the format for "explain", "derive", and
+    /// "estimate" prompts that a set of distractors would trivialize.
+    OpenResponse,
 }
 
 impl Format {
+    /// Every response format.
+    pub const ALL: [Format; 4] = [
+        Format::SingleBestAnswer,
+        Format::MultipleResponse,
+        Format::TrueFalse,
+        Format::OpenResponse,
+    ];
+
     /// The QTI question type Canvas expects for this format.
     pub fn qti_type(self) -> &'static str {
         match self {
             Format::SingleBestAnswer => "multiple_choice_question",
             Format::MultipleResponse => "multiple_answers_question",
             Format::TrueFalse => "true_false_question",
+            Format::OpenResponse => "essay_question",
         }
     }
+
+    /// The snake_case token used in YAML.
+    pub fn as_str(self) -> &'static str {
+        match self {
+            Format::SingleBestAnswer => "single_best_answer",
+            Format::MultipleResponse => "multiple_response",
+            Format::TrueFalse => "true_false",
+            Format::OpenResponse => "open_response",
+        }
+    }
+
+    /// Whether items in this format present selectable options.
+    ///
+    /// `false` only for [`Format::OpenResponse`]. Validation, assembly, and the
+    /// exporters branch on this rather than on the variant, so the day a second
+    /// free-text format is added it inherits the no-options handling for free.
+    pub fn has_options(self) -> bool {
+        !matches!(self, Format::OpenResponse)
+    }
 }
 
 /// How strongly an item is expected to separate strong from weak students.
@@ -633,4 +669,18 @@ mod tests {
             Status::InReview
         );
     }
+
+    #[test]
+    fn open_response_is_the_only_format_without_options() {
+        assert_eq!(
+            serde_json::from_str::<Format>("\"open_response\"").unwrap(),
+            Format::OpenResponse
+        );
+        assert_eq!(Format::OpenResponse.as_str(), "open_response");
+        assert_eq!(Format::OpenResponse.qti_type(), "essay_question");
+        assert!(!Format::OpenResponse.has_options());
+        for f in Format::ALL {
+            assert_eq!(f.has_options(), f != Format::OpenResponse);
+        }
+    }
 }