Live Markdown previews from vim

January 6, 2019

This is more of a technical post

This site is written using Markdown, which:

allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML)

It's a common technique for writing blogs, as it lets the author write text in the text editor of their choice.

I write this page in vim, which is a command line text editor. I love vim for many reasons that are outside the scope of this post. I've been using it as my primary editor for almost 5 years. I wanted a good solution for writing Markdown in vim.

Because vim is a command line editor, it makes it tougher to use it to write rich text. In a GUI program, rich text can be rendered inline and as-you-edit. A command line tool can show you the formatting you have applied to text, but it does not give you an accurate rendering of how your content will look as you edit it.

I wanted to continue to use vim to edit my blog posts (I find vim plugins for other editors to be a worse approximation than "I can't believe it's not butter"). This post is about how I solved this problem.

A Markdown preview app

On macOS, you can get apps to render Markdown in a window. One that I like a lot is Marked, which automatically refreshes its preview when the Markdown file changes. This means you can open a .md file in Marked and get live updates as you change it. This can be used with any editor, including vim. The flow is something like:

As you save to it with :w, the preview will be auto-refreshed. I like this, but wanted to open it with a single key combination.

Adding Marked to vim

We can write a vim function to help us with this. Something like this:

function OpenInMarked2()
    " Open the file in Marked here
Running a shell command from vim

We can use ! to run a command from our function. Technically, this is the filter command. From :help !:

!{motion}{filter} Filter {motion} text lines through the external program {filter}.

You can try this in your vim. Just run :!pwd, and you should be presented with your shell and your current working directory. Below it, vim helpfully tells us "Press ENTER or type command to continue".

This is great! Using this and the macOS open command, we can at least launch Marked 2 with something like this:

!open -a Marked\

But what about opening the current file? vim lets us use % from Ex commands as a stand-in for the path to the current file. So we can do something like:

!echo %

to see the current file. We can use this with open to open the current file in vim:

!open -a Marked\ %

This is the body of our OpenInMarked2 function:

function OpenInMarked2()
    !open -a Marked\ %

Once we have this function, we can call it with the Ex call command, like this:

:call OpenInMarked2()

This will open the current file in Marked, but will leave us at the shell with "Press ENTER or type command to continue" there. We still have to press enter before we can edit the file live.

Defining a mapping

We can use a vim mapping to call the function for us. A mapping lets us take a key combination and map it to a command. For the key combination, I like to use <leader>, which is a user-controlled modifier in vim. This means that <leader> _ is wide open for use in your .vimrc. For my leader key, I use , (comma), as it's near the other modifiers on a US keyboard. For leader mappings, I like a mnemonic key combination, as it makes it easier to remember.

A good choice for this mapping, which is applicable for Markdown and for opening Marked, is <leader>m. Our mapping to call the function might look like this:

nmap <silent> <leader>m :call OpenInMarked2()

Here's a decoder ring for what these commands mean: - nmap means a mapping that exists in vim's normal mode - <silent> means the mapping will not be echoed on the command line - <leader>m is the mapping we've picked for this particular command - :call will call a particular function - OpenInMarked2() is the name of the function we'd like to call

If you add this mapping and function to your .vimrc, you'll see that it works kind of. You have to hit enter to get the command to actually run, and then after doing so you're left at the shell, so you need to hit enter again before returning to the editor.

Hitting enter twice

We can add a press of the enter key to our mapping using <CR>. This acts as though the user has pressed the carriage return. We'll add two of them, as we need one to run the function and another to return to the editor. Our mapping now looks like this:

nmap <silent> <leader>m :call OpenInMarked2() <CR> <CR>

which is the mapping you'll find in my .vimrc.

I hope you found this post helpful. I have been using OpenInMarked2 to help me write Markdown for 2 years now, and I think it's a cool example of how to extend vim to fit your needs.