Implement a basic show command

lib/gel/command.rb

@@ -84,3 +84,4 @@ def self.extract_word(arguments)
 require_relative "command/stub"
 require_relative "command/config"
 require_relative "command/shell_setup"
+require_relative "command/show"

lib/gel/command/show.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+class Gel::Command::Show < Gel::Command
+  def run(command_line)
+    mode = :human
+
+    until command_line.empty?
+      case argument = command_line.shift
+      when "--list"; mode = :list
+      when "-l"; mode = :list
+      when "--paths"; mode = :paths
+      when "-p"; mode = :paths
+      else
+        raise "Unknown argument #{argument.inspect}"
+      end
+    end
+
+    Gel::Environment.activate(output: $stderr)
+
+    case mode
+    when :human
+      puts "Gems included by the bundle:"
+      gems.each { |gem|
+        puts "  * #{gem.name} (#{gem.version})"
+      }
+    when :list
+      puts gems.map(&:name)
+    when :paths
+      puts gems.map(&:root)
+    end
+  end
+
+  private
+  def gems
+    gems_for(Gel::Environment.gemfile.gems)
+  end
+
+  def gems_for(gems)
+    # Special handling for bundler since it just returns nil if we try to resolve it
+    gems.map(&:first).reject { |g| g == 'bundler' }.map { |gem|
+      current_gem = Gel::Environment.find_gem(gem)
+      raise "Could not find gem #{gem}, have you successfully run gel install?" unless current_gem
+
+      [current_gem, gems_for(current_gem.dependencies)]
+    }.flatten.uniq(&:name).sort_by(&:name)
+  end
+end