Dante: Emacs mode for Interactive Haskell

Dante is a fork of Intero mode which aims exclusively at providing a convenient frontend to GHCi. It steals good ideas from Intero, but it aims for light weightedness (see below for a detailed comparison).

Features

Feature	Command	Keybinding
Flycheck type checking	flycheck-mode (†)
Company completion	company-mode
Type of selection	dante-type-at	C-c .
Info at point	dante-info	C-c ,
Apply Ghc suggestion for error at point	attrap-attrap	C-c /
Goto definition	xref-find-definitions	M-.
Find uses	xref-find-references	M-?
REPLoid (∗)	dante-eval-block	C-c ”
Restart	dante-restart
Diagnosis	dante-diagnose
Remote operation over ssh, via tramp	N/A	N/A

REPLoid (∗)

You can evaluate code by writing it in a comment of the form -- >>> and run dante-eval-block.

Example:

example :: [String]
example = ["This is an example", "of", "interactive", "evaluation"]

-- >>> intercalate " " example

In the above file, if you invoke dante-eval-block on the line containing “intercalate”, you’ll get:

-- >>> intercalate " " example
-- "This is an example of interactive evaluation"
--

Several commands in the same block will be executed in at once, so you can have local let statements.

-- >>> let foo = "foo"
--
-- >>> foo ++ "bar"
-- "foobar"
---

Any GHCi command can be put in such a block, but note however that:

There is no guarantee that state will be maintained across several calls to dante-eval-block. In fact, Dante liberally calls :r and :l, and (re)sets various GHCi options.
It is not supported to load and/or unload modules in such blocks, or set unexpected options. This may work, or may mess with Dante internals.

So if your goal is run your webserver/database/etc. within GHCi, you should not do it using dante.

Installation

Turn on Dante in your haskell-mode-hook. I recommend:

(use-package dante
  :ensure t
  :after haskell-mode
  :commands 'dante-mode
  :init
  (add-hook 'haskell-mode-hook 'dante-mode)
  
  (add-hook 'haskell-mode-hook 'flycheck-mode)
  ;; OR:
  ;; (add-hook 'haskell-mode-hook 'flymake-mode)
  )

(†) Dante can only check saved buffers. Therefore, it will save the current buffer every time that a check is triggered. To avoid any surprise (and a bug where GHCi does not see file changes), you may want to disable checks on idle after change:

(setq flymake-no-changes-timeout nil)
(setq flymake-start-syntax-check-on-newline nil)
(setq flycheck-check-syntax-automatically '(save mode-enabled))

You may additionally want to save buffer after some idle time after a change. With Emacs 26:

(auto-save-visited-mode 1)
(setq auto-save-visited-interval 1)

Using dante + hlint

You can activate the hlint checker in addition to the dante checker as follows:

(add-hook 'dante-mode-hook
   '(lambda () (flycheck-add-next-checker 'haskell-dante
                '(warning . haskell-hlint))))

Customization

Customization can be important to make sure that GHCi is properly loaded by dante, notably the variables dante-project-root and dante-repl-command-line. Use M-x customize-group dante to read the documentation. Note in particular that customization can be done on a per-file or per-project basis by using file- and directory-local variables.

Another useful variable to customize is dante-load-flags

Comparison with Intero

To the best of my knowledge, here is how Dante compares with Intero:

Dante has no dependency on “Stack”
Dante’s Emacs code is about half as long as that of Intero.
Dante does not depend on custom Haskell code, contrary to Intero. Thus, it will work if (and only if) GHCi works for your project. (Including via “Stack”.)
Dante supports xref-find-definitions and xref-find-references.
With Dante, Flychecking is optional (yet recommended), whereas Intero demands that you flycheck your code.
Dante has has a different approach to Haskell evaluation
Dante offers no support for eldoc, nor Hoogle.