do not use oneOf for single constructor data type

src/json/blueprint.gleam

@@ -184,18 +184,29 @@ pub fn union_type_decoder(
     |> result.flatten
   }
 
-  #(
-    enum_decoder,
-    list.map(decoders, fn(field_dec) {
-      let #(name, dec) = field_dec
+  let schema = case decoders {
+    [] -> jsch.Object([], Some(False), None)
+
+    [#(name, dec)] ->
       jsch.Object(
         [#("type", jsch.Enum([json.string(name)])), #("data", dec.1)],
         Some(False),
         Some(["type", "data"]),
       )
-    })
-      |> jsch.OneOf,
-  )
+
+    xs ->
+      list.map(xs, fn(field_dec) {
+        let #(name, dec) = field_dec
+        jsch.Object(
+          [#("type", jsch.Enum([json.string(name)])), #("data", dec.1)],
+          Some(False),
+          Some(["type", "data"]),
+        )
+      })
+      |> jsch.OneOf
+  }
+
+  #(enum_decoder, schema)
 }
 
 /// Function to encode an enum type (unions where constructors have no arguments) into a JSON object.

test/json_blueprint_test.gleam

@@ -659,4 +659,11 @@ pub fn drawing_test() {
   "{\"type\":\"box\",\"data\":{\"width\":15.0}}"
   |> blueprint.decode(using: drawing_decoder())
   |> should.be_error()
+
+  drawing_decoder()
+  |> blueprint.generate_json_schema
+  |> json.to_string
+  |> should.equal(
+    "{\"$schema\":\"http://json-schema.org/draft-07/schema#\",\"required\":[\"type\",\"data\"],\"additionalProperties\":false,\"type\":\"object\",\"properties\":{\"type\":{\"enum\":[\"box\"]},\"data\":{\"required\":[\"width\",\"height\",\"position\"],\"additionalProperties\":false,\"type\":\"object\",\"properties\":{\"width\":{\"type\":\"number\"},\"height\":{\"type\":\"number\"},\"position\":{\"maxItems\":2,\"minItems\":2,\"prefixItems\":[{\"type\":\"number\"},{\"type\":\"number\"}]},\"color\":{\"required\":[\"enum\"],\"additionalProperties\":false,\"type\":\"object\",\"properties\":{\"enum\":{\"enum\":[\"red\",\"green\",\"blue\"]}}}}}}}",
+  )
 }