Recs.
Updated
Joe Nelson has collected a decent set of plugins and configuration for vim called haskell-vim-now that work together well and make Vim a decent option for programming Haskell.
SpecsUpdate
Pros
Pro Useful undo features
Vim does not only offer unlimited undo levels, later releases support an undo tree. It eventually gives the editor VCS-like features. You can undo the current file to any point in the past, even if a change was already undone again. Another neat feature is persistent undo, which enables to undo changes after the file was closed and reopened again.
Pro Asynchronous I/O support
Since Vim 8, Vim can exchange characters with background processes asynchronously. This avoids the problem of the text editor getting stuck when a plugin that had to communicate with a server was running. Now plugins can send and receive data from external scripts without forcing Vim to freeze.
Pro Productivity enhancing modal paradigm
As with all vi-like editors, Vim provides a modal paradigm for text editing and processing that provides a rich syntax and semantic model for composing succinct, powerful commands. While this requires some initial investment in learning how it works in order to take full advantage of its capabilities, it rewards the user well in the long run. This modal interface paradigm also lends itself surprisingly well to many other types of applications that can be controlled by vi-like keybindings, such as browsers, image viewers, media players, network clients (for email and other communication media), and window managers. Even shells (including zsh, tcsh, mksh, and bash, among others) come with vi-like keybinding features that can greatly enhance user comfort and efficiency when the user is familiar with the vi modal editing paradigm.
Pro Lightweight and fast
When compared to modern graphical editors like Atom and Brackets (which have underlying HTML5 engines, browsers, Node, etc.), Vim uses a sliver of the system's memory and it loads instantly, all the while delivering the same features. Vim is also faster than Emacs.