Add Serialize impl for LeafAnswer

src/machine/lib_machine/mod.rs

@@ -44,6 +44,28 @@ pub enum LeafAnswer {
     },
 }
 
+impl Serialize for LeafAnswer {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        match self {
+            LeafAnswer::True => serializer.serialize_bool(true),
+            LeafAnswer::False => serializer.serialize_bool(false),
+            LeafAnswer::Exception(e) => {
+                let mut map = serializer.serialize_map(Some(1))?;
+                map.serialize_entry("exception", &e)?;
+                map.end()
+            }
+            LeafAnswer::LeafAnswer { bindings } => {
+                let mut map = serializer.serialize_map(Some(1))?;
+                map.serialize_entry("bindings", &bindings)?;
+                map.end()
+            }
+        }
+    }
+}
+
 impl LeafAnswer {
     /// Creates a leaf answer with no residual goals.
     pub fn from_bindings<S: Into<String>>(bindings: impl IntoIterator<Item = (S, Term)>) -> Self {

src/machine/lib_machine/tests.rs

@@ -692,3 +692,26 @@ fn term_json_serialize_disjunctions() {
 
     assert_eq!(json_value, serde_json::to_value(prolog_value).unwrap());
 }
+
+#[test]
+#[cfg_attr(miri, ignore)]
+fn leaf_answer_json_serialize() {
+    let leaf_answers = [
+        LeafAnswer::True,
+        LeafAnswer::False,
+        LeafAnswer::Exception(Term::atom("a")),
+        LeafAnswer::from_bindings([("X", Term::atom("a")), ("Y", Term::string("b"))]),
+    ];
+
+    let json_value = json!([
+        true,
+        false,
+        { "exception": { "atom": "a" } },
+        { "bindings": {
+            "X": { "atom": "a" },
+            "Y": "b",
+        }},
+    ]);
+
+    assert_eq!(json_value, serde_json::to_value(leaf_answers).unwrap());
+}