Add fix

Gemfile

@@ -1,3 +1,4 @@
 source 'https://rubygems.org'
 
 gemspec
+gem 'pry'

lib/puppet-lint/plugins/check_trailing_comma.rb

@@ -13,4 +13,16 @@ def check
       end
     end
   end
+
+  def fix(problem)
+    comma = PuppetLint::Lexer::Token.new(
+      :COMMA,
+      ',',
+      problem[:token].line,
+      problem[:token].column
+    )
+
+    idx = tokens.index(problem[:token])
+    tokens.insert(idx, comma)
+  end
 end

spec/puppet-lint/plugins/check_trailing_comma/check_trailing_comma_spec.rb

@@ -49,4 +49,79 @@ class { '::apache':
       end
     end
   end
+
+  context 'with fix enabled' do
+    before do
+      PuppetLint.configuration.fix = true
+    end
+
+    after do
+      PuppetLint.configuration.fix = false
+    end
+
+    context 'trailing comma present' do
+      let (:code) {
+        <<-EOS
+        class { '::apache':
+          timeout => '100',
+          docroot => '/var/www',
+        }
+
+        file { '/etc/fstab':
+          ensure  => 'file',
+          content => 'foo',
+        }
+        EOS
+      }
+
+      it 'should not detect any problems' do
+        expect(problems).to have(0).problems
+      end
+
+      it 'should not modify the manifest' do
+        expect(manifest).to eq(code)
+      end
+    end
+
+    context 'trailing comma absent' do
+      let (:code) {
+        <<-EOS
+        class { '::apache':
+          timeout => '100',
+          docroot => '/var/www'
+        }
+
+        file { '/etc/fstab':
+          ensure  => 'file',
+          content => 'foo'
+        }
+        EOS
+      }
+
+      it 'should detect a single problem' do
+        expect(problems).to have(2).problems
+      end
+
+      it 'should create a warning' do
+        expect(problems).to contain_fixed(msg).on_line(3).in_column(32)
+        expect(problems).to contain_fixed(msg).on_line(8).in_column(27)
+      end
+
+      it 'should add trailing commas' do
+        expect(manifest).to eq(
+          <<-EOS
+        class { '::apache':
+          timeout => '100',
+          docroot => '/var/www',
+        }
+
+        file { '/etc/fstab':
+          ensure  => 'file',
+          content => 'foo',
+        }
+          EOS
+        )
+      end
+    end
+  end
 end