Duplicate selected lines programmatically

Question

I'm creating a Vim script and a critical part of it is duplicating the selected lines. To duplicate lines, I have this mapping which works as intended:

vnoremap <Leader>d :copy '><CR>

But, when inside a function/script, the behavior of copy seems different:

function Foo()
    copy '>
endfunction

Sample Selected Lines

x = 'alpha'
y = 'bravo'
z = 'charlie'

Expected Result

x = 'alpha'
y = 'bravo'
z = 'charlie'
x = 'alpha'
y = 'bravo'
z = 'charlie'

Actual Result (lines seem to be copied in reverse order)

x = 'alpha'
y = 'bravo'
z = 'charlie'
z = 'charlie'
y = 'bravo'
x = 'alpha'

Likewise, running the copy command manually against the selected lines works as expected: :'<,'>copy '>

Am I missing something? Are there better ways to duplicate selected lines programmatically? Thanks

Answer 1

I figured out the culprit. Thanks to Tim Pope's comment on my posted issue and the related answer here.

copy here is executed once per line selection which is not the intended behavior (i.e. the reverse order in the duplicated lines):

function Foo()
    copy '>
endfunction

We want to execute the '<,'>copy '> inside the script instead. But, this will duplicate the N selected lines N times:

function Foo()
    '<,'>copy '>
endfunction

Effectively, we want to duplicate the N selected lines once only. There are 2 options:

• append range at the function definition:

  function Foo() range
      '<,'>copy '>
  endfunction
  

• prepend the command with <C-U> to clear the selection before calling the function:

  function Foo()
      '<,'>copy '>
  endfunction
  
  xnoremap <Leader>r :<C-U>call Foo()<CR>
  

Both work as intended!