Pass variable in vimscript function to split

Question

I am trying to create a command which compares the current file with another by calling :DiffWith(foo.rb). See the following code:

function DiffWith(file)
  botright vsplit a:file
  windo diffthis
endfunction
command -nargs=1 -complete=file Diffwith call DiffWith(<f-args>)

However, it opens a new file named a:file instead of the file I state. What do I do wrong?

Answer 1

Many commands in Vim, including :botright and :vsplit, expect plain text, no variables or expressions ( see :help 41.3 and :help expression to know what a expression means). But there are commands that do expect variables and/or expressions, such as :echo and :execute.

String interpolation is common in many programming languages such as Groovy, Kotlin, Perl, PHP, Python, Ruby, Scala, and Swift, and most Unix shells, but not Vimscript. However, when a Vim command expects an expression, it can evaluate variables and use the concatenation operator (the dot .) to obtain the same effect.

So it would be great if we could use the content of a variable, rather than just plain text, to issue commands such as :botright vsplit a:file. For that purpose, Vim gives us the :execute command, from :help :execute:

:exe[cute] {expr1} ..    Executes the string that results from the
                         evaluation of {expr1} as an Ex command.  Multiple
                         arguments are concatenated, with a space in
                         between.  To avoid the extra space use the "."
                         operator to concatenate strings into one 
                         argument.

In that way, you can take advantage of the :execute command to get the intended result, as follows:

function DiffWith(file)
  execute "botright vsplit" a:file
  windo diffthis
endfunction
command -nargs=1 -complete=file Diffwith call DiffWith(<f-args>)

and then you can call it by typing in:

:Diffwith foo.rb

Bonus

As Peter Rincker pointed out, Vim includes a similar functionality, see below:

:botright vertical diffsplit foo.rb

and you can rewrite your command in this way:

:command! -nargs=1 -complete=file Diffwith botright vertical diffsplit <args>