functions.vim

function! ReplaceMagicWithConstant(Arg, Start, Stop)
    let constantName = input("Enter Constant Name: ")
    call append(line("'<") - 1, printf("%s = %s", l:constantName, a:Arg))
    execute printf("'<, '>s/%s/%s/gc", a:Arg, l:constantName)
endfunction

function! CandidateMagic(ArgLead, CmdLine, CursorPos)
    let allWords = uniq(sort(split(join(getline("'<", "'>")), '\W\+')))
    call filter(allWords, {idx, val -> -1 != stridx(val, a:ArgLead)})
    return allWords
endfunction

command! -range=% -complete=customlist,CandidateMagic -nargs=1 ReplaceMagicWithConstant call ReplaceMagicWithConstant("<args>", <line1>, <line2>)