Add special local _ to store previously evaluated value

lib/debug/session.rb

@@ -286,7 +286,10 @@ def process_event evt
         end
 
         wait_command_loop tc
-
+      when :sync_prev_evaled
+        @th_clients.each_value do |tc|
+          tc << [:store_prev_evaled, ev_args[0]]
+        end
       when :dap_result
         dap_event ev_args # server.rb
         wait_command_loop tc

lib/debug/thread_client.rb

@@ -87,6 +87,7 @@ def initialize id, q_evt, q_cmd, thr = Thread.current
       @obj_map = {} # { object_id => obj } for CDP
       @recorder = nil
       @mode = :waiting
+      @prev_evaled_value = nil
       set_mode :running
       thr.instance_variable_set(:@__thread_client_id, id)
 
@@ -358,6 +359,11 @@ def frame_eval src, re_raise: false
         b.local_variable_set(name, var) if /\%/ !~ name
       end
 
+      # this check needs to be performed on the original binding
+      if !current_frame.binding&.local_variable_defined?(:_)
+        b.local_variable_set(:_, @prev_evaled_value)
+      end
+
       result = if b
                   f, _l = b.source_location
                   b.eval(src, "(rdbg)/#{f}")
@@ -366,6 +372,8 @@ def frame_eval src, re_raise: false
                   instance_eval_for_cmethod(frame_self, src)
                 end
       @success_last_eval = true
+
+      event! :sync_prev_evaled, result
       result
 
     rescue Exception => e
@@ -824,6 +832,8 @@ def wait_next_action_
           end
 
           event! :result, result_type, result
+        when :store_prev_evaled
+          @prev_evaled_value = args[0]
         when :frame
           type, arg = *args
           case type

test/debug/debugger_local_test.rb

@@ -161,5 +161,77 @@ def test_raised_doesnt_leak_to_program_binding
         end
       end
     end
+
+    class UnderscoreTest < TestCase
+      def program
+        <<~RUBY
+     1| a = 10
+     2| b = 100
+     3| binding.b
+        RUBY
+      end
+
+      def test_underscore_returns_nil_by_default
+        debug_code(program) do
+          type "_"
+          assert_line_text(/nil/)
+          type "q!"
+        end
+      end
+
+      def test_underscore_returns_the_previous_value
+        debug_code(program) do
+          type "c"
+          type "a"
+          assert_line_text(/10/)
+          type "_ + 40"
+          assert_line_text(/50/)
+          type "_ + b"
+          assert_line_text(/150/)
+          type "c"
+        end
+      end
+
+      def test_underscore_ignores_exceptions
+        debug_code(program) do
+          type "c"
+          type "a"
+          assert_line_text(/10/)
+          type "a / 0"
+          assert_line_text(/divided by 0/)
+          type "_ + 40"
+          assert_line_text(/50/)
+          type "c"
+        end
+      end
+
+      def test_underscore_keeps_value_to_next_bp
+        debug_code(program) do
+          type "x = 100"
+          type "c"
+          type "_ + 40"
+          assert_line_text(/140/)
+          type "c"
+        end
+      end
+
+      def test_underscore_doesnt_override_program_value
+        program = <<~RUBY
+         1| def foo(_)
+         2|   binding.b
+         3| end
+         4|
+         5| foo(100)
+        RUBY
+
+        debug_code(program) do
+          type "a = 50"
+          type "c"
+          type "_"
+          assert_line_text(/100/)
+          type "c"
+        end
+      end
+    end
   end
 end