How do I create a mapping that calls a script-local function in Vim without namespace collisions?

Explanation

<SID> (Script ID) is a Vimscript token that expands to a unique, per-script prefix at runtime. It lets you safely call s: (script-local) functions from key mappings, even when multiple plugins define internal functions with the same name.

How it works

In Vimscript, the s: prefix makes a function local to the script file — it cannot be called from outside. But key mappings are executed globally, so you cannot write :call s:MyFunc() in a mapping directly. <SID> solves this:

function! s:MyHelper()
  echo 'Running from: ' . expand('%')
endfunction

" <SID> expands to a unique SNR prefix at runtime
nnoremap <silent> <leader>h :<C-u>call <SID>MyHelper()<CR>

When Vim loads the script, <SID> expands to <SNR>{N}_ where N is the script's internal number. So <SID>MyHelper might become <SNR>42_MyHelper. Each script gets a different N, so there are no conflicts.

Example

" WRONG: s: prefix doesn't work in a mapping (evaluated globally)
nnoremap <leader>x :call s:Process()<CR>

" RIGHT: <SID> is translated correctly to the script's SNR prefix
nnoremap <leader>x :<C-u>call <SID>Process()<CR>

You can verify the expansion with :map <leader>x — you will see the <SNR>{N}_ prefix in the output.

Tips

• <SID> only works inside the script file where the s: function is defined
• Always combine with :<C-u> before call to clear any visual range prefix that may be auto-inserted
• Use :scriptnames to see each script's assigned number and correlate it with the <SNR> prefix
• For functions meant to be called from outside the script, use autoload style (myplugin#funcname) instead of s:
• In Neovim Lua, use local functions and closures in vim.keymap.set — <SID> is a Vimscript-only concept