[ruby/irb] Introduce cd command

(https://github.com/ruby/irb/pull/971)

It's essentially a combination of pushws and popws commands that are
easier to use.

Help message:

```
Usage: cd ([target]|..)

IRB uses a stack of workspaces to keep track of context(s), with `pushws` and `popws` commands to manipulate the stack.
The `cd` command is an attempt to simplify the operation and will be subject to change.

When given:
- an object, cd will use that object as the new context by pushing it onto the workspace stack.
- "..", cd will leave the current context by popping the top workspace off the stack.
- no arguments, cd will move to the top workspace on the stack by popping off all workspaces.

Examples:

  cd Foo
  cd Foo.new
  cd @ivar
  cd ..
  cd
```

https://github.com/ruby/irb/commit/4a0e0e89b7

2 files changed, 54 insertions, 0 deletions

diff --git a/lib/irb/command/cd.rb b/lib/irb/command/cd.rb
new file mode 100644
index 0000000000..b83c8689ae
--- /dev/null
+++ b/lib/irb/command/cd.rb
@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+module IRB
+  module Command
+    class CD < Base
+      category "Workspace"
+      description "Move into the given object or leave the current context."
+
+      help_message(<<~HELP)
+        Usage: cd ([target]|..)
+
+        IRB uses a stack of workspaces to keep track of context(s), with `pushws` and `popws` commands to manipulate the stack.
+        The `cd` command is an attempt to simplify the operation and will be subject to change.
+
+        When given:
+        - an object, cd will use that object as the new context by pushing it onto the workspace stack.
+        - "..", cd will leave the current context by popping the top workspace off the stack.
+        - no arguments, cd will move to the top workspace on the stack by popping off all workspaces.
+
+        Examples:
+
+          cd Foo
+          cd Foo.new
+          cd @ivar
+          cd ..
+          cd
+      HELP
+
+      def execute(arg)
+        case arg
+        when ".."
+          irb_context.pop_workspace
+        when ""
+          # TODO: decide what workspace commands should be kept, and underlying APIs should look like,
+          # and perhaps add a new API to clear the workspace stack.
+          prev_workspace = irb_context.pop_workspace
+          while prev_workspace
+            prev_workspace = irb_context.pop_workspace
+          end
+        else
+          begin
+            obj = eval(arg, irb_context.workspace.binding)
+            irb_context.push_workspace(obj)
+          rescue StandardError => e
+            warn "Error: #{e}"
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/irb/default_commands.rb b/lib/irb/default_commands.rb
index 91c6b2d041..e27a3d4e00 100644
--- a/lib/irb/default_commands.rb
+++ b/lib/irb/default_commands.rb
@@ -5,6 +5,7 @@ require_relative "command/internal_helpers"
 require_relative "command/backtrace"
 require_relative "command/break"
 require_relative "command/catch"
+require_relative "command/cd"
 require_relative "command/chws"
 require_relative "command/context"
 require_relative "command/continue"
@@ -240,6 +241,8 @@ module IRB
     _register_with_aliases(:irb_disable_irb, Command::DisableIrb,
       [:disable_irb, NO_OVERRIDE]
     )
+
+    register(:cd, Command::CD)
   end
 
   ExtendCommand = Command