added cat --ex option, so cat can display the source of an exception

lib/pry/default_commands/shell.rb

@@ -37,6 +37,7 @@ class Pry
       command "cat", "Show output of file FILE. Type `cat --help` for more information." do |*args|
         start_line = 0
         end_line = -1
+        file_name = nil
 
         opts = Slop.parse!(args) do |opt|
           opt.on :s, :start, "Start line (defaults to start of file)Line 1 is the first line.", true, :as => Integer do |line|
@@ -47,6 +48,16 @@ class Pry
             end_line = line - 1
           end
 
+          opt.on :ex, "Show a window of N lines around last exception (defaults to 5).", :optional => true, :as => Integer do |window_size|
+            window_size = window_size ? window_size : 5
+            ex = Pry.last_exception
+            next if !ex
+            start_line = (ex.line - 1) - window_size
+            start_line = start_line < 0 ? 0 : start_line
+            end_line = (ex.line - 1) + window_size
+            file_name = ex.file
+          end
+
           opt.on :l, "line-numbers", "Show line numbers."
           opt.on :t, :type, "The specific file type for syntax higlighting (e.g ruby, python)", true, :as => Symbol
           opt.on :f, :flood, "Do not use a pager to view text longer than one screen."
@@ -57,7 +68,13 @@ class Pry
 
         next if opts.help?
 
-        file_name = args.shift
+        if opts.ex?
+          next output.puts "No Exception or Exception has no associated file." if file_name.nil?
+          next output.puts "Cannot cat exceptions raised in REPL." if Pry.eval_path == file_name
+        else
+          file_name = args.shift
+        end
+
         if !file_name
           output.puts "Must provide a file name."
           next
@@ -65,6 +82,17 @@ class Pry
 
         contents, normalized_start_line, _ = read_between_the_lines(file_name, start_line, end_line)
 
+        if opts.ex?
+          contents = contents.lines.map.with_index do |line, idx|
+            l = idx + start_line
+            if l == (Pry.last_exception.line - 1)
+              "=> #{line}"
+            else
+              "   #{line}"
+            end
+          end.join
+        end
+
         if Pry.color
           contents = syntax_highlight_by_file_type_or_specified(contents, file_name, opts[:type])
         end