vimscriptfull.md

# Google Vimscript Guide

*   Nate Soares
*   Artemis Sparks
*   David Barnett

<!-- mdformat on -->

## Background

This is the in-depth vimscript guide. If you're just a casual user looking to
write a plugin, the [abbreviated style guide](./vimscriptguide.md) is for you.

This rather rotund guide dives into justifications and clarifications. It
provides an idealized set of rules that are rather too draconian to push on
casual scripters.

It's for users who want to know why certain decisions were made in the
abbreviated guide and who want to learn a thing or two about using vimscript
safely.

Fair warning: Vimscript is a maddening abyss. When you gaze into it, it gazes
also into you. Proceed with caution.

<!-- mdformat off -->
* TOC
{:toc}
<!-- mdformat on -->

## Portability

Vim is highly configurable. Users can change many of the default settings,
including the case sensitivity, the regular expression rules, the substitution
rules, and more. In order for your vimscript to work for all users, follow these
guidelines:

*   Always prefix regular expressions with one of `\m`, `\v`, `\M`, or `\V`
    (prefer tersity)

    *   Users can change the global "magic level" of regular expressions. This
        changes how atoms are parsed in regular expressions, including `.`, `*`,
        and `{`.
    *   Even if your regular expression does not contain characters which are
        affected by the `magic` setting you must prefix it with one of the magic
        control atoms. This future-proofs your regular expression against other
        devs modifying it and forgetting to add the control atom.
    *   This is recommended, but not required, when using regexes in a context
        where the "magic level" is not used (e.g., calls to `substitute()`)
        since it makes the pattern self-contained and future-proofs against the
        same pattern being used in other contexts.
    *   If you have no opinion about what type of regular expression to use,
        prefer the one which makes your regular expression most concise.

*   Avoid using `:s[ubstitute]` in scripts.

    *   `:substitute` moves the cursor.
    *   `:substitute` outputs an error message when the match does not exist.
    *   The meaning of the `g` flag depends upon the `gdefault` setting. If you
        do use `:substitute` you must save `gdefault`, set it to `0` or `1`,
        perform the substitution, and then restore it.
    *   Script authors who want a safe way to replace text in the buffer are
        encouraged to use `maktaba#buffer#Replace`.

*   Always use case-explicit operators for strings (`=~#` and `=~?`, never
    `=~`).

    *   This also applies to `!~ == != > >= < <= is` and `isnot`.
    *   This only applies for strings. `==` and `>=` are fine for numbers, but
        `==#` and `>=#` must be used for comparing values that may be strings.
    *   The behavior of `=~` and friends is dependant upon the `ignorecase`
        setting.
    *   You may break this rule when you explicitly want to obey the user's
        `ignorecase` setting. Be prepared to justify your reasoning.

*   When using regular expressions as arguments to functions, prepend them with
    `\c` or `\C`.

    *   This forces case to be either explicitly matched or ignored.
    *   This is recommended, but not required, when comparing regexes with
        operators that specify case sensitivity (`=~#`, etc.).
    *   This rule applies when your regexes are matching syntax, external APIs,
        external messages, and most other cases.
    *   It does not apply when matching text in the buffer. When matching text
        in the buffer you should honor the `ignorecase` setting.
    *   You may also ignore this rule any time that you explicitly want to honor
        the `ignorecase` setting. Be prepared to justify your reasoning.

*   Always use `normal!` instead of `normal`.

    *   If you forgo the `!` the command will use the user's key mappings and
        you have literally no idea what your macro will do.

*   Always use the `noremap` family of commands.

    *   Your plugins generally shouldn't introduce mappings, but if they do, the
        `map` command respects the users existing mappings and could do
        anything.

*   When using `catch`, match the error code rather than the error text.

    *   The error text may be locale-dependant.
    *   See `:help error-messages`.

In general, guard all commands and functions against user settings.

## Language Guide

*   Line continuations: **Yes**

    *   Plugins that support vi compatibility mode must save and restore
        compatibility options as described in the **Errata section** so line
        continuations work properly.

*   Exceptions: **Yes, with caution**

    *   Always use an error code in thrown exception messages.
    *   Prefer the `maktaba#error` codes found in `maktaba`.
    *   Fall back to the vim error codes. See `:help error-messages`.
    *   Generate custom error messages using `maktaba#error#Message`.
    *   Public user commands and functions should catch and echo known errors
        (via echomsg or `maktaba#error#Shout`) before they reach the user.

        <div class='goodcode'></div>

        ```
        try
          call somelibrary#FuncThatCanFail()
        catch /ERROR(NotFound):/
          call maktaba#error#Shout('Failed to do thing: %s', v:exception)
        endtry
        ```

    *   Library functions are encouraged to throw errors to callers instead of
        shouting them so the caller can manage error conditions. Always document
        thrown errors in vimdoc doc comments with the `@throws` directive.

*   Global Variables: **As configuration only**

    *   See the plugin guide.

*   Messaging: **As little as possible.**

    *   Loud scripts are annoying.
    *   Message the user when an error has occured.
    *   Message the user when an operation which takes a long time has begun
        work.
    *   Avoid messaging otherwise.

*   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.

*   FuncRefs: **No in most cases.**

    *   FuncRefs have inconsistently enforced naming restrictions. (Functions
        can have names that FuncRefs can not.)
    *   FuncRefs have inconsistent ability to be reassigned (in Vim 7.2 and
        before you must unlet a FuncRef before assigning it).
    *   In most instances where a FuncRef is needed a string works just as well:
        just pass the string that you would use to make the FuncRef.
    *   Consider using `maktaba#function` instead to create and manipulate
        handles to functions.

*   Python: **Sparingly**

    *   Hurts code reuse since python code embedded in python plugins is awkward
        to share between plugins.
    *   Using python introduces python language version dependencies, which are
        likely to get stale.
    *   Exception: It's reasonable to use python for plugin functionality that
        needs to do work in the background, as vimscript can not do this.

*   Ruby: **No**

    *   We can not assume ruby interoperability.
    *   You shouldn't depend upon the version of the ruby language that the user
        has installed.

*   Lua: **No**

    *   For the same reasons an Ruby.

*   Dict Functions: **Encouraged**

    *   Vimscript can attach functions to dictionaries. Such functions have
        access to the `self` parameter which access the dict state.
    *   Use these where you would use a class in python.
    *   Do not over-use this feature; it is not necessary for helper functions
        or API functions, only for encapsulated objects.

All other language features are fair game.

## Structure

*   Provided functionality should be packed into modular plugins.

    *   Every function in your plugin should be specific to your plugin.
    *   General utility functions should be abstracted into library plugins.
    *   Manage dependencies with `maktaba`.

*   `plugin-names-like-this`

    *   Plugin names should be descriptive and concise.

*   Each plugin must consist of one directory (or code repository), sharing a
    name with the plugin (with a "vim-" prefix or ".vim" suffix if desired).

*   Plugin metadata should be declared in the addon-info.json format (see the
    [VAM documentation](https://goo.gl/CUXJZC) for details).

*   Functions should go in the `autoload/` subdirectory of your plugin.

    *   This allows them to be late-loaded, which speeds up startup time.
    *   This helps vim enforce namespacing conventions.

*   Each file in the `plugin/` or `instant/` directory should begin with the
    boilerplate

    <div class='goodcode'></div>

    ```
    let [s:plugin, s:enter] = maktaba#plugin#Enter(expand('<sfile>:p'))
    if !s:enter
      finish
    endif
    ```

    (This prevents re-entry and allows users to selectively disable
    functionality.)

*   User configuration should be via plugin flags defined in
    `instant/flags.vim`.

    *   Define flags with `call s:plugin.Flag('FLAGNAME', DEFAULT_VALUE)`.
    *   Users can configure these flags using the `:Glaive` command (see
        [glaive](https://github.com/google/glaive)).

*   Commands, autocommands, mappings, and settings changes should occur either
    in the `plugin/` or the `ftplugin/` subdirectories.

    *   All commands should be defined in `plugin/commands.vim` or `ftplugin/`
        files.
    *   Autocommands should be defined in `plugin/autocmds.vim`, inside an
        augroup.
    *   Mappings should be defined in `plugin/mappings.vim` and will be disabled
        unless explicitly enabled by users.
    *   If the plugin configures any standard vim settings, those should be
        configured in `plugin/settings.vim` or `instant/settings.vim`.

*   Avoid using the `after/` subdirectory.

    *   `after/` should be reserved for the user.
    *   It is difficult for the user to add their own overrides when plugins use
        `after/`.

### Libraries vs. Functionality

Separate library-providing plugins from command-providing plugins.

Many plugins provide either user functionality (commands, autocommands, etc) or
an API (of autoloaded functions) but not both. This separation is encouraged, as
it allows other plugins to pull in a library without also pulling in commands,
setting changes, and other plugin functionality that affects the end user.

### Configuration

Don't clobber user settings. Provide as much configurability as possible: that's
what Vim's all about.

*   Use maktaba flags for plugin configuration. Users can configure them using
    the `:Glaive` command.

    <!-- TODO(dbarnett): Add links and/or more detail for flags. -->

*   Check if configuration variables exist before setting them.

    <div class='goodcode'></div>

    ```
    if !exists('g:myplugin_option')
      let g:myplugin_option = 1
    endif
    ```

## Style Guide

Follow google-wide style conventions. Mimic google python style when in doubt.

### Documentation

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

Provide help files generated by [vimdoc](https://github.com/google/vimdoc).
Write documentation in .vim files in conformance with the vimdoc standards and
include fields like "description" and "author" in the addon-info.json file (see
the [VAM documentation](https://goo.gl/CUXJZC)).

### Whitespace

Follow google-wide conventions.

*   Use two spaces for indents.
*   Do not use tabs.
*   Use spaces around operators except for arguments to commands.

    *   Using spaces around operators for commands is often invalid syntax. This
        is inconsistently enforced by vimscript. To be safe, always omit
        whitespace around arguments to commands.

        <div class='goodcode'></div>

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

        <div class='badcode'></div>

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

*   Do not introduce trailing whitespace.

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

*   Restrict lines to 80 columns wide.

*   Indent continued lines by four spaces.

    <div class='goodcode'></div>

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

*   Do not waste whitespace aligning common segments of similar commands. It is
    both difficult and expensive to maintain.

    <div class='goodcode'></div>

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

    <div class='badcode'></div>

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

#### Line Continuations

*   Prefer line continuations on semantic boundaries.

    <div class='goodcode'></div>

    ```
    command SomeLongCommand
        \ call some#function()
    ```

    <div class='badcode'></div>

    ```
    command SomeLongCommand call
        \ some#function()
    ```

    Use your best judgement.

*   Place one space after the backslash denoting a line continuation.

    *   When continuing a multi-line command a pipe can be substituted for this
        space as necessary, as follows:

        <div class='goodcode'></div>

        ```
        autocommand BufEnter <buffer>
            \ if !empty(s:var)
            \|  call some#function()
            \|else
            \|  call some#function(s:var)
            \|endif
        ```

*   Do not continue multi-line commands when you can avoid it. Prefer function
    calls.

#### Comments

*   Place a space after the `"` before the comment text.

    <div class='goodcode'></div>

    ```
    " I am a line comment.
    call call(s:my_function)
    ```

*   Do not use inline comments.

    *   Some commands treat them as comments and others as unclosed quotes.
        There are many edge cases. It's difficult to get right and difficult to
        maintain.
    *   Where you would use an inline comment, put a line comment on the line
        above.
    *   When leaving blank lines in comments, include the quote in the blank
        line.

        <div class='goodcode'></div>

        ```
        " I am one continuous
        "
        " comment block
        ```

### Variables

`plugin-names-like-this`, `FunctionNamesLikeThis`, `CommandNamesLikeThis`,
`augroup_names_like_this`, `variable_names_like_this`. Prefix all variables with
their scope.

*   `variable_names_like_this`

    *   FuncRef variables count as functions and should be named like functions.
    *   This (pathological) convention is enforced by vim itself.

*   Prefix global variables with `g:`

    *   Vimscript allows you to create global variables without prefixing them.
    *   It is very bad practice to introduce non-prefixed global variables into
        scope.
    *   Global variables should only be used for plugin configuration.
    *   This does not apply to functions defined in `autoload` directories.

*   Prefix script-local variables with `s:`

    *   This prevents namespace collisions between plugins.
    *   This also applies to script-local functions.

*   Prefix function arguments with `a:`

    *   This is enforced by vim itself.

*   Prefix function-local variables with `l:`

    *   This is not enforced by vimscript but is good practice.
    *   It helps you remember that all other variables must be prefixed with
        scope.
    *   `l:` disambiguates between function-local and vim-predefined variables.
        For example, `count` refers to `v:count`, not `l:count`.
    *   It future proofs your scripts against the introduction of new
        vim-predefined variables.

*   Prefix pre-defined vim variables with `v:`

    *   This is not enforced by vimscript but is good practice.
    *   It provides context as to where the (undeclared) variable is coming
        from.
    *   It reminds you that the variable can not be assigned to.

*   Prefix buffer-local or window-local variables with `b:` or `w:`

    *   This is useful for plugins that keep per-buffer or per-window state.

### Strings

Prefer single quotes.

Prefer single quoted strings. Specifically, in order of precedence:

*   Always use single quotes for regular expressions.

    *   `'\s*'` is not the same as `"\s*"`
    *   Single quotes will prevent the need for excessive backslashes.
    *   Double single quotes escape to one single quote in single quoted
        strings: `'example ('')'` represents the string `example (')`

*   If your string requires escape characters (`\n`, `\t`, etc.) use double
    quotes.

    *   Escapes can not be expressed in single quoted strings.
    *   Remember that `'\n'` in a regex does not represent a newline, but rather
        "\n". You only need to use double quotes when you want to embed the
        represented character itself (e.g. a newline) in the string.

*   If your string contains no escapes nor single quotes, use single quoted
    strings.

    *   Most strings in vimscript are regexes, so this provides maximum
        consistency.

*   If your non-regex string contains single quotes but no double quotes, use
    double quotes.

    *   Don't bother escaping strings if you don't have to.
    *   This is similar to the python string rules.

*   If your string contains both single and double quotes, use whichever quoting
    style requires less escaping.

    *   Break ties in favor of single quotes.

### Settings

Prefer long names. Set settings locally.

*   Prefer long names of built in settings (i.e. `tabstop` over `ts`).
*   Set local settings unless you explicitly want to set global settings.

    *   Use `setlocal` and `&l:` instead of `set` and `&`.

## Usage Guide

Vim plugins should provide any or all of the following: **Commands**,
**Autocommands**, **Functions**, **Statusline Flags**, and **Mappings**.

### Commands

<div>

*   Define in `plugin/commands.vim`.
*   CommandNamesLikeThis.
*   Prefer semantic names to a unified prefix.
*   Do not use `[!]`
*   Extract logic into functions.

</div>

*   `CommandNamesLikeThis`

*   Commands should be defined in one block with no whitespace between them.

    *   Name commands semantically at the expense of a common prefix.

        <div class='badcode'></div>

        ```
        command WhitespaceFixTrailing
        command WhitespaceFixIndentation
        ```

        <div class='goodcode'></div>

        ```
        command FixTrailingWhitespace
        command FixIndentation
        ```

*   Use `command` without a bang.

    *   This notifies users to command name conflicts immediately at startup.
    *   Command name collisions are an error and should not fail silently.
    *   Plugins are guarded against re-entry, so a single vim session should
        never attempt to re-define defined commands.

*   Do not put logic in commands.

    *   Delegate to functions instead.
    *   Pass non-argument command parameters (`<bang>`, `<register>`, etc.)
        before argument parameters (`<f-args>`, etc.).
    *   Otherwise variable-length argument functions are difficult to implement.

*   Do not autoload commands.

    *   Autoloaded commands will not be available until after a function in the
        same file is called.
    *   Commands intended to be used in the .vimrc should be defined in a
        `instant/commands.vim` file in plugins using maktaba, or explicitly
        installed via an autoload function in non-maktaba plugins.

<a id="commands-conventions"></a>

#### Conventions

*   Pass `<bang>` to functions with `'<bang>' == '!'`.

    *   The function should receive a boolean parameter, not a string.

### Autocommands

<div>

*   Define in `plugin/autocmds.vim`.
*   Use augroups.
*   `augroup_names_like_this`.
*   Clear the augroup first.
*   Extract logic into functions.

</div>

*   All autocommands should be defined in the `plugin/autocmds.vim` file.

    *   This allows users to disable your autocommands with
        `Glaive myplugin !plugin[autocmds]`.

*   Declare all autocommands in an `augroup` block.

    *   This allows your autocommands to be cleared with `autocmd!`.

    *   If your plugin only has one `augroup`, the `augroup` name should be the
        same as your plugin name, with underscores in place of any hyphens.

    *   Otherwise `augroup` names should start with your plugin name followed by
        an underscore.

*   Do not put logic in autocommands.

    *   Delegate to functions instead.

*   When creating a new `augroup`, clear it with `autocmd!`

    *   This allows your plugins to be re-enterable.

### Functions

<div>

*   FunctionNamesLikeThis.
*   Autoload all functions.
*   Prefix script-local functions with `s:`
*   Use `[!]`.
*   Use `[abort]`.

</div>

*   `FunctionNamesLikeThis`

    *   Prefix all script-local functions with `s:`

*   Do not provide global functions. Use autoloaded functions instead.

*   Place two blank lines between top-level functions.

*   Declare all functions with `abort`.

    *   If you do not do this, the function's behavior depends upon whether it
        is called within a `try..endtry` block somewhere on the stack.
    *   The `abort` keyword forces the function to act consistently.
    *   Without it, the function may (or may not) attempt to continue execution
        after an error occurs.

*   Use `function!` with a bang.

    *   This allows developers to re-source their scripts and have the functions
        reloaded without complaint.
    *   Function names should never collide because functions should always be
        either script-local or defined in an `autoload` directory.
    *   Failing to use a bang in any function in an autoload file will lead to
        cryptic errors if vim tries to re-source the file (e.g., if you refer to
        an nonexistent autoload function).

*   Use `...` for optional arguments, not for lists of arguments.

    *   Vimscript functions take at most 20 arguments.
    *   Lists have no such length restriction.
    *   Your function is likely to break when given too many arguments if you
        use `...` for a list of arguments.

*   Throw exceptions rather than printing errors.

    *   Printed errors can not be caught.
    *   Top-level functions expecting errors may catch them and print error
        messages, but even those should throw their own errors when they choke.

### Mappings

<div>

*   Provide opt-in key mappings in `plugin/mappings.vim`.
*   `<Plug>` mappings can be defined in `plugin/plugs.vim` (unlike mappings.vim,
    plugs.vim is opt-out).

</div>

*   Define key mappings in `plugin/mappings.vim`, using
    `maktaba#plugin#MapPrefix` to get a prefix.

    *   Mappings defined in the special `plugin/mappings.vim` file will be
        disabled by default (by the standard `maktaba#plugin#Enter()`
        boilerplate).
    *   Users can enable key mappings with `Glaive myplugin plugin[mappings]`.

*   Make all mappings with `<unique>`.

    *   This will inform the user when they have a mapping conflict instead of
        silently clobbering their existing mappings.

*   You may provide pseudo-mappings using `<Plug>` and your plugin's name in
    `plugin/plugs.vim` (separate from standard key mappings).

    *   `<Plug>` is a sequence which can not be typed.
    *   You can do something like
        `noremap <Plug>namespace#MappingName some_key_sequence` and then users
        can do `noremap <leader>x <Plug>namespace#MappingName` to take advantage
        of your pseudo-mapping.
    *   Pseudo-mappings should **not** be in `plugin/mappings.vim` or they will
        be disabled by default.
    *   Such pseudo-mappings should be named `<Plug>` followed by your plugin
        name, a pound sign, and a unique mapping name (CamelCased like a
        function).

*   Always use the `noremap` family of commands. Never use the `map` family.

    *   `map` depends upon the user's existing mappings, and could do anything.

*   Only use `noremap` for commands that both make a motion and take a range.

    *   `noremap` makes mappings in normal, visual, and operator-pending modes.
    *   If you don't want all these use `nnoremap` `onoremap` or `vnoremap`
        explicitly.

*   Always use `<SID>` in place of `s:` when accessing script locals in
    mappings.

    *   Using `s:` will often fail as the mapping attempts to type a literal s
        and colon.

## Conventions

### Dependency Checking

Declare dependencies in addon-info.json and use `maktaba`.

Declaring dependencies in addon-info.json allows conformant plugin managers
(like VAM) to ensure dependencies are installed. See the
[VAM documentation](https://goo.gl/CUXJZC) for details.

Calling `maktaba#library#Require` from dependent code at runtime ensures that
dependencies have been installed and that they don't include unsafe non-library
files.

### Statusline Flags

Use `<plugin-name>#status#Status()` or its finer-grained variants to provide
statusline flags.

Following is a convention for exposing statusline flags to the user. A plugin
should never modify the user's statusline except for when that is the only
purpose of the plugin (powerline, etc.).

*   Provide the `Info`, `Alert`, `Warning`, and `Error` functions under the
    `<plugin-name>#status` namespace.
*   `Info`should provide information about the state of the buffer.
    *   Example: The current git branch.
*   `Alert` should provide a quiet reminder that the buffer is non-standard.
    *   Example: The readonly setting is on.
*   `Warning`should provide a warning about the current state of the buffer.
    *   Example: The file has been edited elsewhere.
*   `Error` should bring to attention a loud issue with the buffer.
    *   Example: The file does not pass the syntax checker.
*   By following these conventions, users can easily build up their own
    statusline customizing the verbosity and colors to their tastes.
*   All functions should take no arguments and should return either empty
    strings or strings enclosed by square brackets, e.g. `[Google]`. For
    example:
    *   A trailing whitespace plugin might return `[$]` if the file contains
        trailing whitespace
    *   A prose writing plugin might return `[write]` if vim is in writing mode.
*   Consider providing the `<plugin-name>#status#Status` function.
    *   It should return the first non-empty of `Error`, `Warning`, `Alert`, or
        `Info`.
    *   This is useful for users who want only the most relevant flag and do not
        have a colored statusline.

## Forbidden Commands

These are commands which can only be used by a limited number of plugins, and
should not in general be used by yours.

*   Do not use `:match :2match` or `:3match`
    *   These are reserved for the user and for vim itself.
    *   Use `matchadd()` to create a matchlevel unique to your plugin.
*   Do not use `echoerr`.
    *   `echoerr` does not print the red error message that you might think it
        does.
    *   `echoerr` prints an error message as well as context about the code
        where `echoerr` was called.
    *   `echoerr` is best suited for debugging.
    *   Use `echohl` in tandem with `echomsg` if you want the red error bar.
*   Use `echomsg` instead of `echo`.
    *   `echomsg` messages can be reviewed with the `:messages` command.
    *   `echo` messages disappear permanently on redraw, which can be very
        annoying to users who failed to read the message in time.

## Layout

Lay out `plugin/` files in the following sections, if applicable, separated by
two blank lines:

*   Declaration of script constants
*   Declaration of configuration variables
*   Other declarations (commands in `commands.vim` file, autocommands in
    `autocmds.vim` file, etc.)

Lay out `autoload/` files in the following sections, if applicable, separated by
two blank lines:

*   `maktaba#library#Require` calls
*   Script-local variables
*   Script-local functions
*   Private autoloaded functions
*   Public autoloaded functions

This is recommended convention and is not enforced.

## Recommended Shortcuts

Use the following shortcuts:

*   `catch` over `catch /.*/`
*   `return` over `return 0` when the return value has no semantic purpose.

## Errata

This section plumbs some of the darker corners of vimscript, explaining the
language pathologies that you wish you didn't have to know.

### Compatibility Mode

If you don't support vi-compatibility mode, fail gracefully.

When `compatible` is set, many vim features are not available. The vim feature
which most commonly affects vimscript authors is line continuations.

If you want your plugin to work in vim with vi compatibility on, you will need
to save the compatibility options at the beginning of each plugin file, clear
them, and restore them at the end of each plugin file. See `:help use-cpo-save`
for details.

Plugins that depend on maktaba generally don't need to worry about compatible
mode since maktaba currently just disables it, printing a warning.