deoplete.nvim

deoplete.nvim is a complete package for NeoVim.
On the condition, you already install dein.vim.
Please, add bellow code in init.vim

call dein#add('Shougo/deoplete.nvim')
" Use deoplete 
let g:deoplete#enable_at_startup = 1

The deoplete dependents on Python3.
You can enable Python3 interface with pip
First one, please install python-pip then, install pynvim.

$ sudo pacman -S python-pip
$ pip3 install --user pynvim

The completion source plugin for deoplete

call dein#add('Shougo/neco-syntax')
call dein#add('ujihisa/neco-look')

neco-syntax found each file type, so it added complementary candidate.

neco-look can show completion result with look command.
For example, you use look command on the command line.

$ look ab

For result

Ab
abc
Avvvvv
Abddddd
abbbbbb
abaaaa