[ruby/prism] Provide Parameters#signature for mirroring Method#parameters

https://github.com/ruby/prism/commit/90b3245528

2 files changed, 140 insertions, 3 deletions

diff --git a/lib/prism/node_ext.rb b/lib/prism/node_ext.rb
index 8d8bee074d..4febc615c1 100644
--- a/lib/prism/node_ext.rb
+++ b/lib/prism/node_ext.rb
@@ -61,7 +61,8 @@ module Prism
   end
 
   class ConstantReadNode < Node
-    # Returns the list of parts for the full name of this constant. For example: [:Foo]
+    # Returns the list of parts for the full name of this constant.
+    # For example: [:Foo]
     def full_name_parts
       [name]
     end
@@ -73,7 +74,8 @@ module Prism
   end
 
   class ConstantPathNode < Node
-    # Returns the list of parts for the full name of this constant path. For example: [:Foo, :Bar]
+    # Returns the list of parts for the full name of this constant path.
+    # For example: [:Foo, :Bar]
     def full_name_parts
       parts = [child.name]
       current = parent
@@ -93,7 +95,8 @@ module Prism
   end
 
   class ConstantPathTargetNode < Node
-    # Returns the list of parts for the full name of this constant path. For example: [:Foo, :Bar]
+    # Returns the list of parts for the full name of this constant path.
+    # For example: [:Foo, :Bar]
     def full_name_parts
       (parent&.full_name_parts || [:""]).push(child.name)
     end
@@ -103,4 +106,47 @@ module Prism
       full_name_parts.join("::")
     end
   end
+
+  class ParametersNode < Node
+    # Mirrors the Method#parameters method.
+    def signature
+      names = []
+
+      requireds.each do |param|
+        names << (param.is_a?(MultiTargetNode) ? [:req] : [:req, param.name])
+      end
+
+      optionals.each { |param| names << [:opt, param.name] }
+      names << [:rest, rest.name || :*] if rest
+
+      posts.each do |param|
+        names << (param.is_a?(MultiTargetNode) ? [:req] : [:req, param.name])
+      end
+
+      # Regardless of the order in which the keywords were defined, the required
+      # keywords always come first followed by the optional keywords.
+      keyopt = []
+      keywords.each do |param|
+        if param.is_a?(OptionalKeywordParameterNode)
+          keyopt << param
+        else
+          names << [:keyreq, param.name]
+        end
+      end
+
+      keyopt.each { |param| names << [:key, param.name] }
+
+      case keyword_rest
+      when ForwardingParameterNode
+        names.concat([[:rest, :*], [:keyrest, :**], [:block, :&]])
+      when KeywordRestParameterNode
+        names << [:keyrest, keyword_rest.name || :**]
+      when NoKeywordsParameterNode
+        names << [:nokey]
+      end
+
+      names << [:block, block.name || :&] if block
+      names
+    end
+  end
 end
diff --git a/test/prism/parameters_signature_test.rb b/test/prism/parameters_signature_test.rb
new file mode 100644
index 0000000000..788ce7b907
--- /dev/null
+++ b/test/prism/parameters_signature_test.rb
@@ -0,0 +1,91 @@
+# frozen_string_literal: true
+
+require_relative "test_helper"
+
+return if RUBY_VERSION < "3.2" || RUBY_ENGINE == "truffleruby"
+
+module Prism
+  class ParametersSignatureTest < TestCase
+    def test_req
+      assert_parameters([[:req, :a]], "a")
+    end
+
+    def test_req_destructure
+      assert_parameters([[:req]], "(a, b)")
+    end
+
+    def test_opt
+      assert_parameters([[:opt, :a]], "a = 1")
+    end
+
+    def test_rest
+      assert_parameters([[:rest, :a]], "*a")
+    end
+
+    def test_rest_anonymous
+      assert_parameters([[:rest, :*]], "*")
+    end
+
+    def test_post
+      assert_parameters([[:rest, :a], [:req, :b]], "*a, b")
+    end
+
+    def test_post_destructure
+      assert_parameters([[:rest, :a], [:req]], "*a, (b, c)")
+    end
+
+    def test_keyreq
+      assert_parameters([[:keyreq, :a]], "a:")
+    end
+
+    def test_key
+      assert_parameters([[:key, :a]], "a: 1")
+    end
+
+    def test_keyrest
+      assert_parameters([[:keyrest, :a]], "**a")
+    end
+
+    def test_nokey
+      assert_parameters([[:nokey]], "**nil")
+    end
+
+    def test_keyrest_anonymous
+      assert_parameters([[:keyrest, :**]], "**")
+    end
+
+    def test_key_ordering
+      assert_parameters([[:keyreq, :a], [:keyreq, :b], [:key, :c], [:key, :d]], "a:, c: 1, b:, d: 2")
+    end
+
+    def test_block
+      assert_parameters([[:block, :a]], "&a")
+    end
+
+    def test_block_anonymous
+      assert_parameters([[:block, :&]], "&")
+    end
+
+    def test_forwarding
+      assert_parameters([[:rest, :*], [:keyrest, :**], [:block, :&]], "...")
+    end
+
+    private
+
+    def assert_parameters(expected, source)
+      eval("def self.m(#{source}); end")
+
+      begin
+        assert_equal(expected, method(:m).parameters)
+        assert_equal(expected, signature(source))
+      ensure
+        singleton_class.undef_method(:m)
+      end
+    end
+
+    def signature(source)
+      program = Prism.parse("def m(#{source}); end").value
+      program.statements.body.first.parameters.signature
+    end
+  end
+end