styleguide/vimscriptguide.md

# Google Vimscript Style Guide

* Nate Soares
* Artemis Sparks
* David Barnett

<!-- mdformat on -->

## Background

This is a casual version of the vimscript style guide, because vimscript is a
casual language. When submitting vim plugin code, you must adhere to these
rules. For clarifications, justifications, and explanations about the finer
points of vimscript, please refer to
the [heavy guide](./vimscriptfull.md).

## Portability

It's hard to get vimscript right. Many commands depend upon the user's settings.
By following these guidelines, you can hope to make your scripts portable.

### Strings


Prefer single quoted strings

Double quoted strings are semantically different in vimscript, and you probably
don't want them (they break regexes).

Use double quoted strings when you need an escape sequence (such as `"\n"`) or
if you know it doesn't matter and you need to embed single quotes.


### Matching Strings


Use the `=~#` or `=~?` operator families over the `=~` family.

The matching behavior depends upon the user's ignorecase and smartcase settings
and on whether you compare them with the `=~`, `=~#`, or `=~?` family of
operators. Use the `=~#` and `=~?` operator families explicitly when comparing
strings unless you explicitly need to honor the user's case sensitivity
settings.


### Regular Expressions


Prefix all regexes with `\m\C`.

In addition to the case sensitivity settings, regex behavior depends upon the
user's nomagic setting. To make regexes act like nomagic and noignorecase are
set, prepend all regexes with `\m\C`.

You are welcome to use other magic levels (`\v`) and case sensitivities (`\c`)
so long as they are intentional and explicit.


### Dangerous commands


Avoid commands with unintended side effects.

Avoid using `:s[ubstitute]` as it moves the cursor and prints error messages.
Prefer functions (such as `search()`) better suited to scripts.

For many vim commands, functions exist that do the same thing with fewer side
effects. See `:help functions()` for a list of built-in functions.


### Fragile commands


Avoid commands that rely on user settings.

Always use `normal!` instead of `normal`. The latter depends upon the user's key
mappings and could do anything.

Avoid `:s[ubstitute]`, as its behavior depends upon a number of local settings.

The same applies to other commands not listed here.


### Catching Exceptions


Match error codes, not error text.

Error text may be locale dependant.


## General Guidelines

### Messaging


Message the user infrequently.

Loud scripts are annoying. Message the user only when:

*   A long-running process has kicked off.
*   An error has occurred.



### Type checking


Use strict and explicit checks where possible.

Vimscript has unsafe, unintuitive behavior when dealing with some types. For
instance, `0 == 'foo'` evaluates to true.

Use strict comparison operators where possible. When comparing against a string
literal, use the `is#` operator. Otherwise, prefer `maktaba#value#IsEqual` or
check `type()` explicitly.

Check variable types explicitly before using them. Use functions from
`maktaba#ensure`, or check `maktaba#value` or `type()` and throw your own
errors.

Use `:unlet` for variables that may change types, particularly those assigned
inside loops.


### Python


Use sparingly.

Use python only when it provides critical functionality, for example when
writing threaded code.


### Other Languages


Use vimscript instead.

Avoid using other scripting languages such as ruby and lua. We can not guarantee
that the end user's vim has been compiled with support for non-vimscript
languages.


### Boilerplate


Use [maktaba](https://github.com/google/maktaba).

maktaba removes boilerplate, including:

*   Plugin creation
*   Error handling
*   Dependency checking



### Plugin layout


Organize functionality into modular plugins

Group your functionality as a plugin, unified in one directory (or code
repository) which shares your plugin's name (with a "vim-" prefix or ".vim"
suffix if desired). It should be split into plugin/, autoload/, etc.
subdirectories as necessary, and it should declare metadata in the
addon-info.json format (see the [VAM documentation](https://goo.gl/CUXJZC) for
details).


### Functions


In the autoload/ directory, defined with `[!]` and `[abort]`.

Autoloading allows functions to be loaded on demand, which makes startuptime
faster and enforces function namespacing.

Script-local functions are welcome, but should also live in autoload/ and be
called by autoloaded functions.

Non-library plugins should expose commands instead of functions. Command logic
should be extracted into functions and autoloaded.

`[!]` allows developers to reload their functions without complaint.

`[abort]` forces the function to halt when it encounters an error.


### Commands


In the plugin/commands.vim or under the ftplugin/ directory, defined without `[!]`.

General commands go in `plugin/commands.vim`. Filetype-specific commands go in
`ftplugin/`.

Excluding `[!]` prevents your plugin from silently clobbering existing commands.
Command conflicts should be resolved by the user.


### Autocommands


Place them in `plugin/autocmds.vim`, within augroups.

Place all autocommands in augroups.

The augroup name should be unique. It should either be the plugin name or be
prefixed with the plugin name.

Clear the augroup with `autocmd!` before defining new autocommands in the
augroup. This makes your plugin re-entrable.


### Mappings


Place them in `plugin/mappings.vim`, using `maktaba#plugin#MapPrefix` to get a prefix.

All key mappings should be defined in `plugin/mappings.vim`.

Partial mappings (see `:help using-<Plug>`.) should be defined in
`plugin/plugs.vim`.


### Settings


Change settings locally

Use `:setlocal` and `&l:` instead of `:set` and `&` unless you have explicit
reason to do otherwise.


## Style

Follow google style conventions. When in doubt, treat vimscript style like
python style.

### Whitespace


Similar to python.

*   Use two spaces for indents
*   Do not use tabs
*   Use spaces around operators

    This does not apply to arguments to commands.

    ```
    let s:variable = 'concatenated ' . 'strings'
    command -range=% MyCommand
    ```

*   Do not introduce trailing whitespace

    You need not go out of your way to remove it.

    Trailing whitespace is allowed in mappings which prep commands for user
    input, such as `noremap <leader>gf :grep -f `.

*   Restrict lines to 80 columns wide

*   Indent continued lines by four spaces

    ```
    let l:something = SomeReallyLongFunctionName(
        \ 'arg1', 0, ['XXXXX'], {'somekey': 9999999999})
    ```

*   Do not align arguments of commands

    ```
    command -bang MyCommand  call myplugin#foo()
    command       MyCommand2 call myplugin#bar()
    ```

    ```
    command -bang MyCommand call myplugin#foo()
    command MyCommand2 call myplugin#bar()
    ```



### Naming


In general, use `plugin-names-like-this`, `FunctionNamesLikeThis`,
`CommandNamesLikeThis`, `augroup_names_like_this`, `variable_names_like_this`.
Always prefix variables with their scope.

#### plugin-names-like-this

Keep them short and sweet.

#### FunctionNamesLikeThis

Prefix script-local functions with `s:`

Autoloaded functions may not have a scope prefix.

Do not create global functions. Use autoloaded functions instead.

#### CommandNamesLikeThis

Prefer succinct command names over common command prefixes.

#### variable_names_like_this

Augroup names count as variables for naming purposes.

#### Prefix all variables with their scope.

*   Global variables with `g:`
*   Script-local variables with `s:`
*   Function arguments with `a:`
*   Function-local variables with `l:`
*   Vim-predefined variables with `v:`
*   Buffer-local or window-local variables with `b:` or `w:`

`g:`, `s:`, and `a:` must always be used.

`b:` and `w:` change the variable semantics; use them when you want buffer- or
window-local semantics.

`l:` and `v:` should be used for consistency, future proofing, and to avoid
subtle bugs. They are not strictly required. Add them in new code but don't go
out of your way to add them elsewhere.