Add prompt stack to fix bug where prompt is not restored after returning from shell-mode

When returning from shell-mode, the prompt is not properly restored.
The problem is that it gets clobbered by the new prompt, so we need somewhere to store it.
This adds a prompt stack so you can push and pop prompts in order to temporarily change
the current prompt and then later restore the previous prompt.

prompt and prompt= are also overridden to have meaning in the context of the prompt stack.

lib/pry/commands.rb

@@ -270,10 +270,10 @@ e.g: gist -d my_method
     command "shell-mode", "Toggle shell mode. Bring in pwd prompt and file completion." do
       case Pry.active_instance.prompt
       when Pry::SHELL_PROMPT
-        Pry.active_instance.prompt = Pry::DEFAULT_PROMPT
+        Pry.active_instance.pop_prompt
         Pry.active_instance.custom_completions = Pry::DEFAULT_CUSTOM_COMPLETIONS
       else
-        Pry.active_instance.prompt = Pry::SHELL_PROMPT
+        Pry.active_instance.push_prompt Pry::SHELL_PROMPT
         Pry.active_instance.custom_completions = Pry::FILE_COMPLETIONS
         Readline.completion_proc = Pry::InputCompleter.build_completion_proc target,
         Pry.active_instance.instance_eval(&Pry::FILE_COMPLETIONS)

lib/pry/pry_instance.rb

@@ -30,7 +30,7 @@ class Pry
     default_options.merge!(options)
 
     CONFIG_OPTIONS.each do |key|
-      instance_variable_set("@#{key}", default_options[key])
+      send "#{key}=", default_options[key]
     end
 
     @command_processor = CommandProcessor.new(self)
@@ -320,6 +320,63 @@ class Pry
     end
   end
 
+  # the array that the prompt stack is stored in
+  def prompt_stack
+    @prompt_stack ||= Array.new
+  end
+  private :prompt_stack
+
+  # The current prompt, this is the prompt at the top of the prompt stack.
+  # @return [Array<Proc>] Current prompt.
+  # @example
+  #    push_prompt(Pry::SIMPLE_PROMPT)
+  #    prompt # => Pry::SIMPLE_PROMPT
+  def prompt
+    prompt_stack.last
+  end
+
+  # Replaces the current prompt with the new prompt.
+  # Does not change the rest of the prompt stack.
+  # @param [Array<Proc>] new_prompt
+  # @return [Array<Proc>] new_prompt
+  # @example
+  #    pry.prompt = Pry::SIMPLE_PROMPT # => Pry::SIMPLE_PROMPT
+  #    pry.prompt # => Pry::SIMPLE_PROMPT
+  def prompt=(new_prompt)
+    if prompt_stack.empty?
+      push_prompt new_prompt
+    else
+      prompt_stack[-1] = new_prompt
+    end
+  end
+
+  # Pushes the current prompt onto a stack that it can be restored from later.
+  # Use this if you wish to temporarily change the prompt.
+  # @param [Array<Proc>] new_prompt
+  # @return [Array<Proc>] new_prompt
+  # @example
+  #    new_prompt = [ proc { '>' }, proc { '>>' } ]
+  #    push_prompt(new_prompt) # => new_prompt
+  def push_prompt(new_prompt)
+    prompt_stack.push new_prompt
+  end
+
+  # Pops the current prompt off of the prompt stack.
+  # If the prompt you are popping is the last prompt, it will not be popped.
+  # Use this to restore the previous prompt.
+  # @return [Array<Proc>] Prompt being popped.
+  # @example
+  #    prompt1 = [ proc { '>' }, proc { '>>' } ]
+  #    prompt2 = [ proc { '$' }, proc { '>' } ]
+  #    pry = Pry.new :prompt => prompt1
+  #    pry.push_prompt(prompt2)
+  #    pry.pop_prompt # => prompt2
+  #    pry.pop_prompt # => prompt1
+  #    pry.pop_prompt # => prompt1
+  def pop_prompt
+    if prompt_stack.size > 1 then prompt_stack.pop else prompt end
+  end
+
   if RUBY_VERSION =~ /1.9/ && RUBY_ENGINE == "ruby"
     require 'ripper'
 

test/test.rb

@@ -737,6 +737,67 @@ describe Pry do
             Pry.new.select_prompt(true, 0).should == "test prompt> "
             Pry.new.select_prompt(false, 0).should == "test prompt* "
           end
+
+          describe 'storing and restoring the prompt' do
+            before do
+              make = lambda do |name,i|
+                prompt = [ proc { "#{i}>" } , proc { "#{i+1}>" } ]
+                (class << prompt; self; end).send(:define_method, :inspect) { "<Prompt-#{name}>" }
+                prompt
+              end
+              @a , @b , @c = make[:a,0] , make[:b,1] , make[:c,2]
+              @pry = Pry.new :prompt => @a
+            end
+            it 'should have a prompt stack' do
+              @pry.push_prompt @b
+              @pry.push_prompt @c
+              @pry.prompt.should == @c
+              @pry.pop_prompt
+              @pry.prompt.should == @b
+              @pry.pop_prompt
+              @pry.prompt.should == @a
+            end
+
+            it 'should restore overridden prompts when returning from file-mode' do
+              pry = Pry.new :input => InputTester.new('shell-mode', 'shell-mode'),
+                            :prompt => [ proc { 'P>' } ] * 2
+              pry.select_prompt(true, 0).should == "P>"
+              pry.re
+              pry.select_prompt(true, 0).should =~ /\Apry .* \$ \z/
+              pry.re
+              pry.select_prompt(true, 0).should == "P>"
+            end
+
+            it '#pop_prompt should return the popped prompt' do
+              @pry.push_prompt @b
+              @pry.push_prompt @c
+              @pry.pop_prompt.should == @c
+              @pry.pop_prompt.should == @b
+            end
+
+            it 'should not pop the last prompt' do
+              @pry.push_prompt @b
+              @pry.pop_prompt.should == @b
+              @pry.pop_prompt.should == @a
+              @pry.pop_prompt.should == @a
+              @pry.prompt.should == @a
+            end
+
+            describe '#prompt= should replace the current prompt with the new prompt' do
+              it 'when only one prompt on the stack' do
+                @pry.prompt = @b
+                @pry.prompt.should == @b
+                @pry.pop_prompt.should == @b
+                @pry.pop_prompt.should == @b
+              end
+              it 'when several prompts on the stack' do
+                @pry.push_prompt @b
+                @pry.prompt = @c
+                @pry.pop_prompt.should == @c
+                @pry.pop_prompt.should == @a
+              end
+            end
+          end
         end
 
         it 'should set the hooks default, and the default should be overridable' do