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01/04 — 2020
79.12 cm   5.4 min

Rapid Refactoring With Vim

Last weekend, I was tasked with refactoring the 96 unit tests on ruma-events to use strictly typed json objects using serde_json::json! instead of raw strings. It was rather painless thanks to vim :)

Here’s a small sample of what had to be done (note the lines prefixed with the arrow):

use serde_json::{from_str};
  fn deserialize() {
from_str::<Action>(r#"{"set_tweak": "highlight"}"#),
        Action::SetTweak(Tweak::Highlight { value: true })

had to be converted to:

use serde_json::{from_value};
  fn deserialize() {
from_value::<Action>(json!({"set_tweak": "highlight"})),
        Action::SetTweak(Tweak::Highlight { value: true })

The arglist

For the initial pass, I decided to handle imports, this was a simple find and replace operation, done to all the files containing tests. Luckily, modules (and therefore files) containing tests in Rust are annotated with the #[cfg(test)] attribute. I opened all such files:

# `grep -l pattern files` lists all the files
#  matching the pattern

vim $(grep -l 'cfg\(test\)' ./**/*.rs)

# expands to something like:
vim push_rules.rs room/member.rs key/verification/lib.rs

Starting vim with more than one file at the shell prompt populates the arglist. Hit :args to see the list of files currently ready to edit. The square [brackets] indicate the current file. Navigate through the arglist with :next and :prev. I use tpope’s vim-unimpaired 1, which adds ]a and [a, mapped to :next and :prev.

All that’s left to do is the find and replace, for which we will be using vim’s argdo, applying a substitution to every file in the arglist:

:argdo s/from_str/from_value/g

The quickfix list

Next up, replacing r#" ... "# with json!( ... ). I couldn’t search and replace that trivially, so I went with a macro call 2 instead, starting with the cursor on ‘r’, represented by the caret, in my attempt to breakdown the process:

BUFFER:    r#" ... "#;

ACTION:    vllsjson!(

BUFFER     json!( ... "#;

ACTION:    <esc>$F#

BUFFER:    json!( ... "#;

ACTION:    vhs)<esc>

BUFFER:    json!( ... );

Here’s the recorded 3 macro in all its glory: vllsjson!(<esc>$F#vhs)<esc>.

Great! So now we just go ahead, find every occurrence of r# and apply the macro right? Unfortunately, there were more than a few occurrences of raw strings that had to stay raw strings. Enter, the quickfix list.

The idea behind the quickfix list is to jump from one position in a file to another (maybe in a different file), much like how the arglist lets you jump from one file to another.

One of the easiest ways to populate this list with a bunch of positions is to use vimgrep:

# basic usage
:vimgrep pattern files

# search for raw strings
:vimgrep 'r#' ./**/*.rs

Like :next and :prev, you can navigate the quickfix list with :cnext and :cprev. Every time you move up or down the list, vim indicates your index:

(1 of 131): r#"{"set_tweak": "highlight"}"#;

And just like argdo, you can cdo to apply commands to every match in the quickfix list:

:cdo norm! @q

But, I had to manually pick out matches, and it involved some button mashing.

External Filtering

Some code reviews later, I was asked to format all the json inside the json! macro. All you have to do is pass a visual selection through a pretty json printer. Select the range to be formatted in visual mode, and hit :, you will notice the command line displaying what seems to be gibberish:


'< and '> are marks 4. More specifically, they are marks that vim sets automatically every time you make a visual selection, denoting the start and end of the selection.

A range is one or more line specifiers separated by a ,:

:1,7       lines 1 through 7
:32        just line 32
:.         the current line
:.,$       the current line to the last line
:'a,'b     mark 'a' to mark 'b'

Most : commands can be prefixed by ranges. :help usr_10.txt for more on that.

Alright, lets pass json through python -m json.tool, a json formatter that accepts stdin (note the use of ! to make use of an external program):

:'<,'>!python -m json.tool

Unfortunately that didn’t quite work for me because the range included some non-json text as well, a mix of regex and macros helped fix that. I think you get the drift.

Another fun filter I use from time to time is :!sort, to sort css attributes, or :!uniq to remove repeated imports.

  1. https://github.com/tpope/vim-unimpaired It also handles various other mappings, ]q and [q to navigate the quickfix list for example↩︎

  2. :help recording↩︎

  3. When I’m recording a macro, I prefer starting out by storing it in register q, and then copying it over to another register if it works as intended. I think of qq as ‘quick record’.↩︎

  4. :help mark-motions↩︎


I'm Akshay, I go by nerd or nerdypepper on the internet.

I am a compsci undergrad, Rust programmer and an enthusiastic Vimmer. I write open-source stuff to pass time. I also design fonts: scientifica, curie.

Send me a mail at [email protected] or a message at [email protected]

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