Using range as argument returns a comparison

Question

I am trying to capture the result returned from a function implementing range as an argument but apparently I get a comparison.

With the following code:

function! GetNumber() range
  return 4
endfunction

function! GetString() range
  return ""
endfunction

function! GetList() range
  echo a:firstline
  echo a:lastline
  return []
endfunction

After having selected some lines, I do the following operations:

• echo '<,'>GetNumber() or let a = <,'>GetNumber(): returns 0
• echo '<,'>GetList() or let a = <,'>GetString(): returns 1
• echo '<,'>GetList() or let a = <,'>GetList():
  • Error E691: Can only compare List with List
  • E15: Invalid expression: '<,'>GetList().

Yet, I can access to the variables a:firstline and a:lastline in the functions. I would like to know where could be my error and also where comes this comparison?

Does the same thing on Vim 7.4 and Vim 8.

Answer 1

You're comparing the string to the result of the function: echo '<,' > GetNumber().

:echo and :let accept an expression, where the only way to call a function is funcname(args); you can't specify a range there. You could use :call, but it discards the return value. Thus the only way to get something out of such a function is to set a global variable:

function! GetList() range
  echo a:firstline
  echo a:lastline
  let g:GetListResult = []
endfunction