Set command-line local variable

Question

How can I set a variable that only the current command line sees?

So that

:let s:one = 1 | echo s:one prints 1

but running two commands

:let s:one = 1
:echo s:one

errors E121: Undefined variable: s:one.

In the end I would like to remap to a command line that uses a temporary variable, without overriding or polluting any potential other user variables.

Answer 1

I am guessing that a simple "echo" is not the extent of what you want to do with the assigned value.

But addressing only what is in your question... why not go to the shell for this?

:!one=1; echo $one

If your larger plan can still work in the shell, then you simply capture it right below the current line with a leading :r :

:r !one1; echo $one

If on windows...

OS limitations here require delayed variable expansion
(which is the /v:on option to cmd.exe):

:r !cmd /v:on /c "set one=1 & echo \!one\!"

(escaping to prevent previous-command-substitution, see :h :!)

If you add the cmd option to your 'shellcmdflag' setting, then it's just:

:r !set one=1 & echo \!one\!

Answer 2

Here is what I could propose:

function! Variable(...)
  if a:0
    let x = a:1
  else
    let x = 0
  endif
  function! Set(...) closure
    if a:0
      let x = a:1
    endif
    return x
  endfunction
  return funcref('Set')
endfunction

With this Variable function you can create a variable that don't clutter the global workspace:

let X=Variable(4) " Initialize X to 4
call X(7) " Change X to 7
echo X() " Get the value of X