Get return value of range function

Question

In vimscript, when I want to store the return value of a function, I do

let a:foo = Function()

But I now have a ranged function. I would like to store its return value, ideally via something like the following:

let a:foo = l:start,l:end call RangedFunction()

(this does not work, but it illustrates my objective)

Attempts/ideas

1. Using execute, this calls my function, but does not let me save its return value. There must be a tricky smart way to get this to work, but it feels pretty heavy.

execute a:start.",".a:end." call RangedFunction()"

2. I could pass the range lines as two explicit arguments directly in the function.

function RangedFunction(start,end) 

but then I couldn't call my function using :10,12call RangedFunction(), which is a flexibility I'd like to keep, and it would make my function implementation heavier.

This is probably a syntax thing I've missed while searching the doc, but it keeps escaping me !

Answer 1

I'm not sure this is possible.

I only see a workaround for now where you could define 2 functions as 12,42let someresult = RangeFunction() is not supported.

function! Function(firstline, lastline) abort
  " where the code is actually defined
  return getline(a:firstline, a:lastline)
endfunction

function RangeFunction() range abort
  " the version that is meant to be used from commands
  return Function(a:firstline, a:lastline)
endfunction

" Usage 1
command! -range RangeCommand <first>,<last>call RangeFunction()
" Usage 2
let lines = Function(12, 42)

Honestly, I find strange a function that returns important information that will get discarded when called from a command, and saved when used as an expression. It's quite likely the reason why this isn't supported: the calling syntax would be much to cumbersome. Imagine let expr = f1,l1 call('Function1', ['other', 'parameters']) + f2,l2 call('Function2', []). It's better to keep the code simple.

PS: :let a:whatever = 42 shall not be possible. Only Vim is authorized to assign values to formal parameters.