Add qldoc comment and isOptional predicate to KeywordParameter

ql/src/codeql_ruby/ast/Parameter.qll

@@ -108,7 +108,15 @@ class HashSplatParameter extends @hash_splat_parameter, NamedParameter {
 }
 
 /**
- * TODO
+ * A keyword parameter, including a default value if the parameter is optional.
+ * For example, in the following example, `foo` is a keyword parameter with a
+ * default value of `0`, and `bar` is a mandatory keyword parameter with no
+ * default value mandatory parameter).
+ * ```rb
+ * def f(foo: 0, bar:)
+ *   foo * 10 + bar
+ * end
+ * ```
  */
 class KeywordParameter extends @keyword_parameter, NamedParameter {
   override Generated::KeywordParameter generated;
@@ -127,6 +135,12 @@ class KeywordParameter extends @keyword_parameter, NamedParameter {
    */
   AstNode getDefaultValue() { result = generated.getValue() }
 
+  /**
+   * Holds if the parameter is optional. That is, there is a default value that
+   * is used when the caller omits this parameter.
+   */
+  predicate isOptional() { exists(this.getDefaultValue()) }
+
   override string toString() { result = this.getName() }
 }
 

ql/test/library-tests/ast/params/params.ql

@@ -16,7 +16,7 @@ query predicate hashSplatParams(HashSplatParameter hsp, string name) { name = hs
 
 query predicate keywordParams(KeywordParameter kp, string name, string defaultValueStr) {
   name = kp.getName() and
-  if exists(kp.getDefaultValue())
+  if kp.isOptional()
   then defaultValueStr = kp.getDefaultValue().toString()
   else defaultValueStr = "(none)"
 }