The Emacs front-end is available from stable MELPA. The stable version should always be compatible with the latest version of ghc-mod from hackage which is not true for plain MELPA!

You can also use a git checkout of the ghc-mod source repository as your source for the Elisp code if you prefer.

To use stable MELPA add this to your .emacs:

(require 'package)
(add-to-list 'package-archives
	     '("melpa" . "https://stable.melpa.org/packages/"))
(package-initialize)

;; ... install the "ghc" package using `M-x package-list-packages' ...

(add-hook 'haskell-mode-hook #'ghc-init)

(See also http:https://www.emacswiki.org/emacs/ELPA)

With this configuration you can install the Emacs front using:

M-x package-refresh-contents RET
M-x package-install RET ghc RET

Note: the ELPA package is called ghc, not ghc-mod

To install the ghc-mod Emacs frontend without ELPA you need to obtain the ghc-mod source code either through git or via a tarball and then add this to your .emacs:

(add-to-list 'load-path "<wherever>/ghc-mod/elisp")
(setq ghc-debug t) ; enable debug logging
(autoload 'ghc-init "ghc" nil t)
(autoload 'ghc-debug "ghc" nil t)
(add-hook 'haskell-mode-hook #'ghc-init)

You'll have to replace <wherever> by the location you chose to put the ghc-mod source directory in.

Make sure the source matches the version of the ghc-mod executables you installed otherwise you will run into all sorts of trouble.

You can check this by looking at the ghc-version variable:

M-x describe-variable RET ghc-version RET

This value should match the output of the following command:

$ ghc-mod --version

Since Vim didn't have a built-in package manager until version 8, there are numerous external ones written by members of the community. One of the newer ones is vim-plug, which the rest of this section will use. The most relevant features for this guide are that it can install packages straight from Git repositories, and can run a shell command such as make whenever a certain package is updated. Since vim-plug is more convenient, it is suggested to use it even with Vim 8.

Install vim-plug by putting a single file, plug.vim, into Vim's auto-load directory (~/.vim/autoload for normal Vim, or ~/.config/nvim/autoload for NeoVim).

Most features of ghc-mod are provided by the plugin ghcmod-vim. Name completion is handled by a separate package, neco-ghc, which provides support for Vim's built-in omni completion, and also Just Works with neocomplete or deoplete (deoplete is faster, but only works with NeoVim). If you decide to use neocomplete, then it depends on vimproc to load completions asynchronously.

Keep in mind that at the moment, ghcmod-vim doesn't keep ghc-mod loaded in the background, so it can be quite slow at times.

An example .vimrc using vim-plug and neocomplete:

call plug#begin('~/.vim/plugged')

Plug 'eagletmt/ghcmod-vim', {'for': 'haskell'}
Plug 'Shougo/vimproc.vim', {'do': 'make'}
Plug 'Shougo/neocomplete.vim', {'for': 'haskell'}
Plug 'eagletmt/neco-ghc', {'for': 'haskell'}

call plug#end()

" Use neco-ghc for completion
autocmd FileType haskell setlocal omnifunc=necoghc#omnifunc

After editing, make Vim reread the file with :source %, and then install the plugins with :PlugInstall. You can update the plugins periodically with :PlugUpdate.

To use ghc-mod with Atom you should install ide-haskell and the companion haskell-ghc-mod. Installation instructions can be found here: https://atom-haskell.github.io/installation/installing-binary-dependencies/.