Vim eval feature

Using your .vimrc file on many machines with different versions and feature sets for Vim is generally not too much of a problem if you apply a little care in making a gracefully degrading .vimrc. In most cases, using the output of vim --version and checking the help files will tell you what features to check in order to determine which parts of your .vimrc configuration to load, and which to ignore.

There’s one particular feature that’s less obvious, though, and that’s eval. Vim’s help describes it like this in :help +eval:

N  *+eval*      expression evaluation |eval.txt|

The eval.txt document, in turn, describes the syntax for various features fundamental to Vimscript, including variables, functions, lists, and dictionaries.

All this makes eval perhaps the most central feature of Vim. Without it, Vim doesn’t have much of its programmable nature, and remains not much more than classic vi. If your particular build of Vim doesn’t include it, then Vimscript essentially does not work as a programming language, and elementary calls like function and let will throw errors:

E319: Sorry, the command is not available in this version: function
E319: Sorry, the command is not available in this version: let

If you’re getting this kind of error, you’re probably using a very stripped-down version of Vim that doesn’t include eval. If you just want to prevent the error by ignoring a block of code if the feature is unavailable, you can do this with has():

if has("eval")
    ...
endif

Vim will still be perfectly functional as a vi-style editor without +eval, but if you’re going to need any Vimscript at all, you should recompile Vim with a --with-features value for the ./configure line that does include it, such as normal, big, or huge, or install a more fully-featured packaged version. For example, on Debian-like systems, the vim-tiny package that is included in the netinst system does not include eval, but the vim and vim-runtime packages do.

Inspecting Vim’s source, in particular the ex_docmd.c file, gives some indication of how fundamental this feature is, applying the C function ex_ni which simply prints the “not available” error shown above to a large number of control structures and statements if the FEAT_EVAL constant is not defined:

#ifndef FEAT_EVAL
# define ex_scriptnames     ex_ni
# define ex_finish      ex_ni
# define ex_echo        ex_ni
# define ex_echohl      ex_ni
# define ex_execute     ex_ni
# define ex_call        ex_ni
# define ex_if          ex_ni
# define ex_endif       ex_ni
# define ex_else        ex_ni
# define ex_while       ex_ni
# define ex_continue        ex_ni
# define ex_break       ex_ni
# define ex_endwhile        ex_ni
# define ex_throw       ex_ni
# define ex_try         ex_ni
# define ex_catch       ex_ni
# define ex_finally     ex_ni
# define ex_endtry      ex_ni
# define ex_endfunction     ex_ni
# define ex_let         ex_ni
# define ex_unlet       ex_ni
# define ex_lockvar     ex_ni
# define ex_unlockvar       ex_ni
# define ex_function        ex_ni
# define ex_delfunction     ex_ni
# define ex_return      ex_ni
# define ex_oldfiles        ex_ni
#endif

Gracefully degrading .vimrc

If you work on many machines with varying operating systems (Windows, Linux, MacOS X, BSD) and in various upgrade states, particularly if some of the machines are a lot older or have more minimal or custom installations of Vim, you might not be using your .vimrc file on all of them because it includes features that aren’t available on some other machines, meaning that Vim spits a lot of errors at you on startup.

This might have prompted you to perhaps keep a simpler .vimrc file, a “lesser” .vimrc, that you put onto your remote machines, and you keep the “real” .vimrc on your own machine to include lines that use all of the features only available to you on that machine. If you like to version your configuration files, maintaining and testing both of the .vimrc files on all the machines gets old fast; it would be much better to have a single .vimrc file that simply ignored configuration it didn’t understand. There are several ways to approach this.

Check features

Perhaps the best way to manage this is to group all of your configuration items by Vim feature, and to check for their presence in your .vimrc file before attempting to set any of the relevant options. You can do this with the has() function. As an example, here’s a stanza from my .vimrc:

if has("spell")
    set spelllang=en_nz
    nnoremap <leader>s :set spell!<CR>
endif

I set the spellang option and perform the remapping only if the +spell feature is actually available.

If an option is dependent on a feature having been compiled into Vim, you can usually tell by calling :help on it and looking for a phrase like “not available when compiled without the +xyz feature.” You can also view a list all the features available with :help feature-list, and see which features are compiled into a given vim binary with the --version parameter:

$ vim --version
VIM - Vi IMproved 7.3 (2010 Aug 15, compiled Feb 11 2012 03:54:05) Included patches: 1-429
Modified by pkg-vim-maintainers@lists.alioth.debian.org
Compiled by jamessan@debian.org
Huge version with GTK2 GUI.  Features included (+) or not (-):
+arabic +autocmd +balloon_eval +browse ++builtin_terms +byte_offset +cindent
+clientserver +clipboard +cmdline_compl +cmdline_hist +cmdline_info +comments
+conceal +cryptv +cscope +cursorbind +cursorshape +dialog_con_gui +diff ...

There are certain special features, like +unix, that you can use to check whether the Vim instance running is on a platform suitable for an option or not. I find this is handy for choosing the correct syntax for specifying fonts on Windows:

if has("unix")
    set guifont=Inconsolata\ 14
else
    set guifont=Consolas:h12
endif

Check options

You can check whether an option itself exists rather than a feature with exists():

if exists("&foldenable")
    set foldenable
endif

Check version number

Another way of filtering out options for older versions of Vim is by version number, which you can perform by checking the value of v:version. For example, to only set folding options if you’re working with at least Vim 7, you could do:

if v:version >= 700
    set foldenable
endif

In this particular case, though, it’s a little clearer and more robust to check the condition with if has("folding"), because the version number being recent enough does not necessarily mean the feature exists. However, one good application of using version numbers is fixing bugs or unexpected behaviour in older instances of Vim, to make it behave consistently with newer versions, or even vice-versa if necessary.

Silent calls

If you can’t find a way to filter by feature or version number, a simple way to suppress any error messages from a configuration line is to preface it with silent!. I find this is a nice compact way to call plugin-dependent functions like pathogen#infect(), or custom colorschemes that you can’t be sure actually exist on the machine:

silent! call pathogen#infect()
silent! colorscheme zellner

Try/Catch/If

If you’re not dealing with versions of Vim older than 7.0, another possibility is the try/catch/endtry block. This is handy for setting a default or fallback option if a call fails, such as selecting colorschemes:

try
    colorscheme zenburn
catch
    colorscheme torte
endtry

This is my least-favoured method of making .vimrc files degrade gracefully, as it breaks completely on older instances of Vim.

Thanks to Reddit user bouncingsoul for suggesting the second method which I initially missed.