Function-argument a:var not recognized as variable?

Question

I'm struggling with strange case. I've read many help pages and search through net but found nothing.

In my vimrc:

function! s:rename(name)
    try
        echom a:name
        saveas a:name
        " bd #
        " silent !rm #
    endtry
endfunction
command! -nargs=1 Ren call s:rename(<f-args>)

In a file:

:Ren newname
:messages

newname
"a:name" 4L, 9C written

Why a:name is not recognized as a variable!?

Answer 1

:saveas expects a filename as an argument (see :h :saveas).

So, inside your function, when you pass a:name to :saveas, it treats it as a filename.
If you want to pass the contents of the variable a:name, you can write this:

execute 'saveas ' . a:name

The difference is that :execute expects an expression (see :h :execute).
'saveas ' is a string, which is a type of expression.
a:name is a variable, which here is replaced with its contents.
It's the same kind of evaluation you see when you type:

:let myvar = 'hello'
:echo myvar

myvar is automatically evaluated into its value after :echo, just like after :execute, but not after :saveas. Because, again, the first 2 commands expect expressions as arguments, whereas the latter expects a filename.

The evaluation of a:name is a string which contains your filename, and the dot between saveas and a:name is the concatenation operator (see :h expr-.). The result of the concatenation is again a string which is still a kind of expression.

So, :execute receives an expression as an argument, which is what it expects, and executes it as an Ex command.