Question

I opened vim without loading anything:

vim -V -u NONE

And typed one line:

<?xml?>

After running:

:unlet did_load_filetypes
:filetype on

I check the file type of current buffer:

:set ft?

It print filetype=

---

If I type

:unlet did_load_filetypes
:filetype detect

then :set ft? will print filetype=xml.

---

In the vim doc:

Detail: The ":filetype on" command will load one of these files:                                    
                Amiga       $VIMRUNTIME/filetype.vim                                                
                Mac         $VIMRUNTIME:filetype.vim                                                
                MS-DOS      $VIMRUNTIME\filetype.vim                                                
                RiscOS      Vim:Filetype                                                            
                Unix        $VIMRUNTIME/filetype.vim                                                
                VMS         $VIMRUNTIME/filetype.vim                                                
        This file is a Vim script that defines autocommands for the                                 
        BufNewFile and BufRead events.  If the file type is not found by the                        
        name, the file $VIMRUNTIME/scripts.vim is used to detect it from the                        
        contents of the file.   

---

What’s differences between :filetype on and :filetype detect?
Thanks for your help.

Answer 1

From the manual:

To detect the file type again:
    :filetype detect
Use this if you started with an empty file and typed text that makes it
possible to detect the file type.