This repo provides syntax highlighting files for Alex and Happy, the lexer generator and parser generator for Haskell.
This is a plain Vimscript repo, so it should be compatible with both Vim and Neovim.
The easiest way to install is by using a package manager, such as vim-plug. However, you may also install manually if wished.
Using vim-plug, it's as simple as adding the following to your list of plugins:
Plug 'heitor-lassarote/alex-happy-vim'
The proceedure should be similar for other package managers.
Alternatively, you may copy and paste the ftdetect and syntax directories into ~/.vim/.