Function only executing first "execute" statment

Question

I'm trying to make a vmap that takes a count and then indents all the highlighted lines so that the top one is more than the line above it, while keeping all the lines' relative indentation. I have made it to this function and I'm experiencing some behaviour I don't know how to figure out. For some reason, this function will only indent the top line of the selection. I've tested it using echomsg instead of execute, and then just pasting the commands in explicitly and it works just fine. But for some reason when it's in this for loop, it only executes the first one.

function! Test()
    let above = line("'<") - 1

    for line in range(line("'<"), line("'>"))
        execute line . "left " . (indent(above) + 4 + indent(line) - indent("'<"))
    endfor
endfunction

Also, if there's a plugin that'll already do this for me, feel free to let me know.

Edit: Putting this function at the top of the file, changing execute to echomsg, highlighting the function body, and then running :cal Test() give the following ouput

2left 4
3left 0
4left 4
5left 8
6left 4

Which is expected, so I don't understand what's going wrong when I use execute.

Answer 1

The problem is with the code indent("'<"). After your first execution, that value changes, causing the indentation of everything afterwards to not change. It can be fixed by setting it's value to a variable before you start the loop.

Answer 2

By default, a function called in visual mode is effectively once for every line in your range.

To get the behavior you want you need to add "range" to the function definition:

function! Test() range
   let above = line("'<") - 1
...