Don't update .last_input on empty lines [Fixes #414]

lib/pry/pry_instance.rb

@@ -225,7 +225,9 @@ class Pry
 
     code = r(target)
 
-    result = set_last_result(target.eval(code, Pry.eval_path, Pry.current_line), target)
+    result = target.eval(code, Pry.eval_path, Pry.current_line)
+    set_last_result(result, target, code)
+
     result
   rescue RescuableException => e
     result = set_last_exception(e, target)
@@ -403,11 +405,12 @@ class Pry
   # This method should not need to be invoked directly.
   # @param [Object] result The result.
   # @param [Binding] target The binding to set `_` on.
-  def set_last_result(result, target)
+  # @param [String] code The code that was run.
+  def set_last_result(result, target, code="")
     @last_result_is_exception = false
     @output_array << result
 
-    self.last_result = result
+    self.last_result = result unless code =~ /\A\s*\z/
   end
 
   # Set the last exception for a session.

test/test_pry.rb

@@ -254,6 +254,24 @@ describe Pry do
         end
       end
 
+      describe "last_result" do
+        it "should be set to the most recent value" do
+          mock_pry("2", "_ + 82").should =~ /84/
+        end
+
+        it "should be set to the result of a command with :keep_retval" do
+          mock_pry("Pry::Commands.block_command '++', '', {:keep_retval => true} do |a| a.to_i + 1; end", '++ 86', '++ #{_}').should =~ /88/
+        end
+
+        it "should be preserved over an empty line" do
+          mock_pry("2 + 2", " ", "\t",  " ", "_ + 92").should =~ /96/
+        end
+
+        it "should be preserved when evalling a  command without :keep_retval" do
+          mock_pry("2 + 2", "ls -l", "_ + 96").should =~ /100/
+        end
+      end
+
       describe "test loading rc files" do
 
         before do