Support trailing commas

README.md

@@ -37,6 +37,8 @@ It is built with only standard library dependencies. It additionally ships with
   - [textDocument/inlayHints](#textdocumentinlayhints)
   - [syntaxTree/visualizing](#syntaxtreevisualizing)
 - [Plugins](#plugins)
+  - [Configuration](#configuration)
+  - [Languages](#languages)
 - [Integration](#integration)
   - [Rake](#rake)
   - [RuboCop](#rubocop)
@@ -409,9 +411,12 @@ You can register additional configuration and additional languages that can flow
 
 ### Configuration
 
-To register additional configuration, define a file somewhere in your load path named `syntax_tree/my_plugin` directory. Then when invoking the CLI, you will pass `--plugins=my_plugin`. That will get required. In this way, you can modify Syntax Tree however you would like. Some plugins ship with Syntax Tree itself. They are:
+To register additional configuration, define a file somewhere in your load path named `syntax_tree/my_plugin`. Then when invoking the CLI, you will pass `--plugins=my_plugin`. To require multiple, separate them by a comma. In this way, you can modify Syntax Tree however you would like. Some plugins ship with Syntax Tree itself. They are:
 
 * `plugin/single_quotes` - This will change all of your string literals to use single quotes instead of the default double quotes.
+* `plugin/trailing_comma` - This will put trailing commas into multiline array literals, hash literals, and method calls that can support trailing commas.
+
+If you're using Syntax Tree as a library, you should require those files directly.
 
 ### Languages
 

lib/syntax_tree/formatter.rb

@@ -7,14 +7,20 @@ class Formatter < PrettierPrint
     COMMENT_PRIORITY = 1
     HEREDOC_PRIORITY = 2
 
-    attr_reader :source, :stack, :quote
+    attr_reader :source, :stack
+
+    # These options are overridden in plugins to we need to make sure they are
+    # available here.
+    attr_reader :quote, :trailing_comma
+    alias trailing_comma? trailing_comma
 
     def initialize(source, ...)
       super(...)
 
       @source = source
       @stack = []
       @quote = "\""
+      @trailing_comma = false
     end
 
     def self.format(source, node)

lib/syntax_tree/formatter/trailing_comma.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module SyntaxTree
+  class Formatter
+    # This module overrides the trailing_comma? method on the formatter to
+    # return true.
+    module TrailingComma
+      def trailing_comma?
+        true
+      end
+    end
+  end
+end

lib/syntax_tree/node.rb

@@ -597,10 +597,30 @@ def format(q)
         q.indent do
           q.breakable("")
           q.format(arguments)
+          q.if_break { q.text(",") } if q.trailing_comma? && trailing_comma?
         end
         q.breakable("")
       end
     end
+
+    private
+
+    def trailing_comma?
+      case arguments
+      in Args[parts: [*, ArgBlock]]
+        # If the last argument is a block, then we can't put a trailing comma
+        # after it without resulting in a syntax error.
+        false
+      in Args[parts: [Command | CommandCall]]
+        # If the only argument is a command or command call, then a trailing
+        # comma would be parsed as part of that expression instead of on this
+        # one, so we don't want to add a trailing comma.
+        false
+      else
+        # Otherwise, we should be okay to add a trailing comma.
+        true
+      end
+    end
   end
 
   # Args represents a list of arguments being passed to a method call or array
@@ -859,6 +879,7 @@ def format(q)
             end
 
             q.seplist(contents.parts, separator) { |part| q.format(part) }
+            q.if_break { q.text(",") } if q.trailing_comma?
           end
           q.breakable("")
         end
@@ -954,6 +975,7 @@ def format(q)
           q.indent do
             q.breakable("")
             q.format(contents)
+            q.if_break { q.text(",") } if q.trailing_comma?
           end
         end
 
@@ -4751,6 +4773,7 @@ def format_contents(q)
         q.indent do
           q.breakable
           q.seplist(assocs) { |assoc| q.format(assoc) }
+          q.if_break { q.text(",") } if q.trailing_comma?
         end
         q.breakable
       end

lib/syntax_tree/plugin/trailing_comma.rb

@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+require "syntax_tree/formatter/trailing_comma"
+SyntaxTree::Formatter.prepend(SyntaxTree::Formatter::TrailingComma)

test/formatter/single_quotes_test.rb

@@ -5,41 +5,43 @@
 
 module SyntaxTree
   class Formatter
-    class TestFormatter < Formatter
-      prepend Formatter::SingleQuotes
-    end
-
-    def test_empty_string_literal
-      assert_format("''\n", "\"\"")
-    end
-
-    def test_string_literal
-      assert_format("'string'\n", "\"string\"")
-    end
-
-    def test_string_literal_with_interpolation
-      assert_format("\"\#{foo}\"\n")
-    end
-
-    def test_dyna_symbol
-      assert_format(":'symbol'\n", ":\"symbol\"")
-    end
-
-    def test_label
-      assert_format(
-        "{ foo => foo, :'bar' => bar }\n",
-        "{ foo => foo, \"bar\": bar }"
-      )
-    end
-
-    private
-
-    def assert_format(expected, source = expected)
-      formatter = TestFormatter.new(source, [])
-      SyntaxTree.parse(source).format(formatter)
-
-      formatter.flush
-      assert_equal(expected, formatter.output.join)
+    class SingleQuotesTest < Minitest::Test
+      class TestFormatter < Formatter
+        prepend Formatter::SingleQuotes
+      end
+
+      def test_empty_string_literal
+        assert_format("''\n", "\"\"")
+      end
+
+      def test_string_literal
+        assert_format("'string'\n", "\"string\"")
+      end
+
+      def test_string_literal_with_interpolation
+        assert_format("\"\#{foo}\"\n")
+      end
+
+      def test_dyna_symbol
+        assert_format(":'symbol'\n", ":\"symbol\"")
+      end
+
+      def test_label
+        assert_format(
+          "{ foo => foo, :'bar' => bar }\n",
+          "{ foo => foo, \"bar\": bar }"
+        )
+      end
+
+      private
+
+      def assert_format(expected, source = expected)
+        formatter = TestFormatter.new(source, [])
+        SyntaxTree.parse(source).format(formatter)
+
+        formatter.flush
+        assert_equal(expected, formatter.output.join)
+      end
     end
   end
 end

test/formatter/trailing_comma_test.rb

@@ -0,0 +1,97 @@
+# frozen_string_literal: true
+
+require_relative "../test_helper"
+require "syntax_tree/formatter/trailing_comma"
+
+module SyntaxTree
+  class Formatter
+    class TrailingCommaTest < Minitest::Test
+      class TestFormatter < Formatter
+        prepend Formatter::TrailingComma
+      end
+
+      def test_arg_paren_flat
+        assert_format("foo(a)\n")
+      end
+
+      def test_arg_paren_break
+        assert_format(<<~EXPECTED, <<~SOURCE)
+          foo(
+            #{"a" * 80},
+          )
+        EXPECTED
+          foo(#{"a" * 80})
+        SOURCE
+      end
+
+      def test_arg_paren_block
+        assert_format(<<~EXPECTED, <<~SOURCE)
+          foo(
+            &#{"a" * 80}
+          )
+        EXPECTED
+          foo(&#{"a" * 80})
+        SOURCE
+      end
+
+      def test_arg_paren_command
+        assert_format(<<~EXPECTED, <<~SOURCE)
+          foo(
+            bar #{"a" * 80}
+          )
+        EXPECTED
+          foo(bar #{"a" * 80})
+        SOURCE
+      end
+
+      def test_arg_paren_command_call
+        assert_format(<<~EXPECTED, <<~SOURCE)
+          foo(
+            bar.baz #{"a" * 80}
+          )
+        EXPECTED
+          foo(bar.baz #{"a" * 80})
+        SOURCE
+      end
+
+      def test_array_literal_flat
+        assert_format("[a]\n")
+      end
+
+      def test_array_literal_break
+        assert_format(<<~EXPECTED, <<~SOURCE)
+          [
+            #{"a" * 80},
+          ]
+        EXPECTED
+          [#{"a" * 80}]
+        SOURCE
+      end
+
+      def test_hash_literal_flat
+        assert_format("{ a: a }\n")
+      end
+
+      def test_hash_literal_break
+        assert_format(<<~EXPECTED, <<~SOURCE)
+          {
+            a:
+              #{"a" * 80},
+          }
+        EXPECTED
+          { a: #{"a" * 80} }
+        SOURCE
+      end
+
+      private
+
+      def assert_format(expected, source = expected)
+        formatter = TestFormatter.new(source, [])
+        SyntaxTree.parse(source).format(formatter)
+
+        formatter.flush
+        assert_equal(expected, formatter.output.join)
+      end
+    end
+  end
+end