Make it possible to forget the last used session (pull request #33)

Pull request #33 on GitHub by Ingo Karkat:
#33

README.md

@@ -2,11 +2,11 @@
 
 The `session.vim` plug-in improves upon [Vim](http://www.vim.org/)'s built-in [:mksession][mksession] command by enabling you to easily and (if you want) automatically persist and restore your Vim editing sessions. It works by generating a [Vim script](http://vimdoc.sourceforge.net/htmldoc/usr_41.html#script) that restores your current settings and the arrangement of tab pages and/or split windows and the files they contain.
 
-To persist your current editing session you can execute the `:SaveSession` command. If you don't provide a name for the session 'default' is used. You're free to use whatever characters you like in session names. When you want to restore your session simply execute `:OpenSession`. Again the name 'default' is used if you don't provide one. When a session is active, has been changed and you quit Vim you'll be prompted whether you want to save the open session before quitting Vim:
+To persist your current editing session you can execute the `:SaveSession` command. If you don't provide a name for the session 'default' is used (you can change this name with an option). You're free to use whatever characters you like in session names. When you want to restore your session simply execute `:OpenSession`. Again the name 'default' is used if you don't provide one. When a session is active, has been changed and you quit Vim you'll be prompted whether you want to save the open session before quitting Vim:
 
 ![Screenshot of auto-save prompt](http://peterodding.com/code/vim/session/autosave.png)
 
-When you start Vim without editing any files and the 'default' session exists, you'll be prompted whether you want to restore the default session:
+When you start Vim without editing any files and the default session exists, you'll be prompted whether you want to restore the default session:
 
 ![Screenshot of auto-open prompt](http://peterodding.com/code/vim/session/autoopen.png)
 
@@ -28,7 +28,7 @@ Note that environment variables inside command arguments are expanded by the plu
 
 ### The `:SaveSession` command
 
-This command saves your current editing session just like Vim's built-in [:mksession][mksession] command does. The difference is that you don't pass a full pathname as argument but just a name, any name really. Press `<Tab>` to get completion of existing session names. If you don't provide an argument the name 'default' is used, unless an existing session is open in which case the name of that session will be used.
+This command saves your current editing session just like Vim's built-in [:mksession][mksession] command does. The difference is that you don't pass a full pathname as argument but just a name, any name really. Press `<Tab>` to get completion of existing session names. If you don't provide an argument the default session name is used, unless an existing session is open in which case the name of that session will be used.
 
 If the session you're trying to save is already active in another Vim instance you'll get a warning and nothing happens. You can use a bang (!) as in `:SaveSession! ...` to ignore the warning and save the session anyway.
 
@@ -103,7 +103,7 @@ The filename extension of session scripts. This should include the dot that sepa
 
 ### The `g:session_autoload` option
 
-By default this option is set to `'prompt'`. This means that when you start Vim without opening any files and the `default` session script exists, the session plug-in will ask whether you want to restore your default session. When you set this option to `'yes'` and you start Vim without opening any files the default session will be restored without a prompt. To completely disable automatic loading you can set this option to `'no'`.
+By default this option is set to `'prompt'`. This means that when you start Vim without opening any files and the default session script exists, the session plug-in will ask whether you want to restore your default session. When you set this option to `'yes'` and you start Vim without opening any files the default session will be restored without a prompt. To completely disable automatic loading you can set this option to `'no'`.
 
 ### The `g:session_autosave` option
 

autoload/xolox/session.vim

@@ -3,7 +3,7 @@
 " Last Change: May 2, 2013
 " URL: http://peterodding.com/code/vim/session/
 
-let g:xolox#session#version = '1.6.4'
+let g:xolox#session#version = '1.7'
 
 call xolox#misc#compat#check('session', 2)
 
@@ -261,13 +261,25 @@ function! xolox#session#auto_load() " {{{2
       endfor
     endif
     " Default to the last used session or the default session?
-    let session = s:last_session_recall()
+    let [has_last_session, session] = s:get_last_or_default_session()
     let path = xolox#session#name_to_path(session)
-    if filereadable(path) && !s:session_is_locked(path)
-      let msg = "Do you want to restore your %s editing session?"
-      let label = session != g:session_default_name ? 'last used' : 'default'
-      if s:prompt(printf(msg, label), 'g:session_autoload')
+    if (g:session_default_to_last == 0 || has_last_session) && filereadable(path) && !s:session_is_locked(path)
+      " Compose the message for the prompt.
+      let is_default_session = (session == g:session_default_name)
+      let msg = printf("Do you want to restore your %s editing session%s?",
+            \ is_default_session ? 'default' : 'last used',
+            \ is_default_session ? '' : printf(' (%s)', session))
+      " Prepare the list of choices.
+      let choices = ['&Yes', '&No']
+      if !is_default_session
+        call add(choices, '&Forget')
+      endif
+      " Prompt the user (if not configured otherwise).
+      let choice = s:prompt(msg, choices, 'g:session_autoload')
+      if choice == 1
         call xolox#session#open_cmd(session, '')
+      elseif choice == 3
+        call s:last_session_forget()
       endif
     endif
   endif
@@ -278,7 +290,7 @@ function! xolox#session#auto_save() " {{{2
     let name = s:get_name('', 0)
     if name != ''
       let msg = "Do you want to save your editing session before quitting Vim?"
-      if s:prompt(msg, 'g:session_autosave')
+      if s:prompt(msg, ['&Yes', '&No'], 'g:session_autosave') == 1
         call xolox#session#save_cmd(name, '')
       endif
     endif
@@ -299,18 +311,20 @@ endfunction
 
 " Commands that enable users to manage multiple sessions. {{{1
 
-function! s:prompt(msg, var)
-  let value = eval(a:var)
-  if value == 'yes' || (type(value) != type('') && value)
+function! s:prompt(msg, choices, option_name)
+  let option_value = eval(a:option_name)
+  if option_value == 'yes'
     return 1
+  elseif option_value == 'no'
+    return 0
   else
     if g:session_verbose_messages
       let format = "%s Note that you can permanently disable this dialog by adding the following line to your %s script:\n\n\t:let %s = 'no'"
       let prompt = printf(format, a:msg, xolox#misc#os#is_win() ? '~\_vimrc' : '~/.vimrc', a:var)
     else
       let prompt = a:msg
     endif
-    return confirm(prompt, "&Yes\n&No", 1, 'Question') == 1
+    return confirm(prompt, join(a:choices, "\n"), 1, 'Question')
   endif
 endfunction
 
@@ -398,7 +412,7 @@ function! xolox#session#close_cmd(bang, silent, save_allowed) abort " {{{2
   if name != ''
     if a:save_allowed
       let msg = "Do you want to save your current editing session before closing it?"
-      if s:prompt(msg, 'g:session_autosave')
+      if s:prompt(msg, ['&Yes', '&No'], 'g:session_autosave') == 1
         call xolox#session#save_cmd(name, a:bang)
       endif
     else
@@ -573,14 +587,22 @@ function! s:last_session_persist(name)
   endif
 endfunction
 
-function! s:last_session_recall()
-  if g:session_default_to_last
-    let fname = s:last_session_file()
-    if filereadable(fname)
-      return readfile(fname)[0]
-    endif
+function! s:last_session_forget()
+  let last_session_file = s:last_session_file()
+  if filereadable(last_session_file) && delete(last_session_file) != 0
+    call xolox#misc#msg#warn("session.vim %s: Failed to delete name of last used session!", g:xolox#session#version)
+  endif
+endfunction
+
+function! s:get_last_or_default_session()
+  let last_session_file = s:last_session_file()
+  let has_last_session = filereadable(last_session_file)
+  if g:session_default_to_last && has_last_session
+    let lines = readfile(last_session_file)
+    return [has_last_session, lines[0]]
+  else
+    return [has_last_session, g:session_default_name]
   endif
-  return g:session_default_name
 endfunction
 
 " Lock file management: {{{2

doc/session.txt

@@ -43,16 +43,16 @@ your current settings and the arrangement of tab pages and/or split windows
 and the files they contain.
 
 To persist your current editing session you can execute the |:SaveSession|
-command. If you don't provide a name for the session 'default' is used. You're
-free to use whatever characters you like in session names. When you want to
-restore your session simply execute |:OpenSession|. Again the name 'default'
-is used if you don't provide one. When a session is active, has been changed
-and you quit Vim you'll be prompted whether you want to save the open session
-before quitting Vim:
+command. If you don't provide a name for the session 'default' is used (you
+can change this name with an option). You're free to use whatever characters
+you like in session names. When you want to restore your session simply
+execute |:OpenSession|. Again the name 'default' is used if you don't provide
+one. When a session is active, has been changed and you quit Vim you'll be
+prompted whether you want to save the open session before quitting Vim:
 
     Screenshot of auto-save prompt, see reference [1]
 
-When you start Vim without editing any files and the 'default' session exists,
+When you start Vim without editing any files and the default session exists,
 you'll be prompted whether you want to restore the default session:
 
     Screenshot of auto-open prompt, see reference [2]
@@ -95,9 +95,9 @@ The *:SaveSession* command
 This command saves your current editing session just like Vim's built-in
 |:mksession| command does. The difference is that you don't pass a full pathname
 as argument but just a name, any name really. Press '<Tab>' to get completion
-of existing session names. If you don't provide an argument the name 'default'
-is used, unless an existing session is open in which case the name of that
-session will be used.
+of existing session names. If you don't provide an argument the default
+session name is used, unless an existing session is open in which case the
+name of that session will be used.
 
 If the session you're trying to save is already active in another Vim instance
 you'll get a warning and nothing happens. You can use a bang (!) as in
@@ -221,7 +221,7 @@ separates the basename from the extension. Defaults to '.vim'.
 The *g:session_autoload* option
 
 By default this option is set to 'prompt'. This means that when you start Vim
-without opening any files and the 'default' session script exists, the session
+without opening any files and the default session script exists, the session
 plug-in will ask whether you want to restore your default session. When you
 set this option to 'yes' and you start Vim without opening any files the
 default session will be restored without a prompt. To completely disable