I’m in the process of writing a vim plugin. I think it might be Stockholm syndrome, but I’m beginning to like the language. Some of the functions, and most of the regex, however feel a little inconsistent. Testing these functions (and especially the regex) has been a pain in the arse.

I was about ready to halt writing my project and start writing a buffer based REPL when I remembered Ex mode(:h Ex-mode):

Like Command-line mode, but after entering a command
you remain in Ex mode.  Very limited editing of the
command line.  |Ex-mode|

Everyone knows Ex mode right? That annoying mode that no one remembers how to escape, and serves no obvious purpose? Well I still have no idea what this mode is actually for. However it makes for an excellent REPL. Well maybe not REPL maybe just REL as you have to type your own echo commands, but it’s close enough.

By this point you have almost certainly rebound the default Q keybind for Ex mode to an infinitely more useful function, however you probably still have access to the gQ keybind that is (practically) the same.

Quiting Ex mode is the second most important issue and also one of the simplest. Typing vi or visual will put you back into to sanity mode.

#The RE(P)L

This isn’t really a REPL because it won’t automatically print after evaluating. I was thinking about doing some autocmd voodoo to automatically prefix my Ex mode commands with the echo command but I don’t think it’s actually going to save much time.

Your basic Ex mode RE(P)L experience will probably look something like this:

Entering Ex mode.  Type "visual" to go to Normal mode.

:echo test

All hail the glory of the Ex mode RE(P)L! On the face of it this might not look that exciting however if your doing any thing with regex or functions this is pretty useful:

Entering Ex mode.  Type "visual" to go to Normal mode.
:let link='<a href="http://www.vimcasts.org">Vimcasts</a>'

:let pattern='.*\([''"]\)\zs.\+:\/\/.\+\ze\1.*'

:echo link =~ 'FUBAR'
:echo link =~ pattern
:echo matchstr(link, pattern)
:echo matchstr(link, 'FUBAR')


Super exciting no? This looks great for single liners right? What about multi-line code:

Entering Ex mode.  Type "visual" to go to Normal mode.
:let x=4

:let y=6

:if x < y

:  echo x + y
:  endif
:if x > y

:  echo x+y
:  endif

A bit unusual, but still usable, how about for loops:

Entering Ex mode.  Type "visual" to go to Normal mode.
:for animal in ['cat', 'dog', 'fish', 'llama']

:  echo animal "goes 'MMMMOOOO!'"
cat goes 'MMMMOOOO!'
:  endfor
dog goes 'MMMMOOOO!'
fish goes 'MMMMOOOO!'
llama goes 'MMMMOOOO!'

I think this is actually pretty cool, because each line of the for loop is evaluated separately so you know you are getting the result you expect before you complete the loop (think of it as a trial run). You might have to prepend some dummy data to your list if you want to get the output from your original list without the line by line output however!

Finally functions:

Entering Ex mode.  Type "visual" to go to Normal mode.
:function! AddNums(x, y)

:  return a:x + a:y
:  endfunction
:echo AddNums(5, 10)

Ex mode also respects your command line settings like wildmenu, so you get some fairly well scoped tab completion which is lacking in a lot of stock REPL’s for other languages.

I think this is a good use case for Ex mode, and it’s very helpful if you are venturing out of your vimrc for the first time. Happy RE(P)L-ing!

EDIT: it’s worth mentioning the :PP command of Tim Popes scriptease plugin. This is an actual REPL for vim. I still find my self in Ex mode more often than not though.