Fixing Enum with only one element being ignored (#721)

Fixes #720

outlines/fsm/json_schema.py

@@ -216,6 +216,14 @@ def to_regex(
 
         return f"({'|'.join(choices)})"
 
+    elif "const" in instance:
+        const = instance["const"]
+        if type(const) in [int, float, bool, None]:
+            const = re.escape(str(const))
+        elif type(const) == str:
+            const = f'"{re.escape(const)}"'
+        return const
+
     elif "$ref" in instance:
         path = f"{instance['$ref']}"
         instance = resolver.lookup(path).contents

tests/fsm/test_json_schema.py

@@ -163,6 +163,24 @@ def test_match_number(pattern, does_match):
                 ("0", False),
             ],
         ),
+        # Const string
+        (
+            {"title": "Foo", "const": "Marc", "type": "string"},
+            '"Marc"',
+            [('"Marc"', True), ('"Jean"', False), ('"John"', False)],
+        ),
+        # Make sure strings are escaped
+        (
+            {"title": "Foo", "const": ".*", "type": "string"},
+            r'"\.\*"',
+            [('".*"', True), (r'"\s*"', False), (r'"\.\*"', False)],
+        ),
+        # Const integer
+        (
+            {"title": "Foo", "const": 0, "type": "integer"},
+            "0",
+            [("0", True), ("1", False), ("a", False)],
+        ),
         # Enum string
         (
             {"title": "Foo", "enum": ["Marc", "Jean"], "type": "string"},