`eval-dante-block` on first line of file causes infinite loop

[eppolito]

Description

When the first line of a file is a -- comment, an infinite loop is caused when dante-eval-block is evoked from anywhere within the corresponding comment block.

Steps to Reproduce

1. Write a simple config file for dante and haskell packages, saving to PATH/TO/SIMPLE/CONFIG:

(let ((default-directory "PATH/TO/ELISP/DIRECTORY")) (normal-top-level-add-subdirs-to-load-path))
(require 'haskell)
(require 'dante)
(add-hook 'haskell-mode-hook 'dante-mode)
(provide 'init)

2. Launch emacs with emacs -Q -l "PATH/TO/SIMPLE/CONFIG" "tmp.hs" (this also visits a haskell file).
3. Add a dante block or comment to the first line of the file and position the point anywhere on the line.

-- >>> :t [1,2,3]

4. Run dante-eval-block on that line (or don't---it will freeze emacs and chew up a ton of CPU).

Cause and Fix

The while condition at line 858 of dante.el always evaluates to non-nil in the case that the first line is a -- comment (and is reached by the loop).

You can verify this issue is caused by the while as follows: paste the snippet below into a new elisp buffer and run eval-last-sexp from the end of either progn expression.

--
--
--
--

(progn
  ;; place point on the last `--` above
  (beginning-of-buffer)
  (forward-line 3)
  ;; current: will enter an infinite loop
  (while (looking-at "[ \t]*--") (forward-line -1)))

(progn
  ;; place point on the last `--` above
  (beginning-of-buffer)
  (forward-line 3)
  ;; corrected: stops when `forward-line` can no longer make progress
  (while (and (looking-at "[ \t]*--") (eq 0 (forward-line -1)))))

Thus, this issue is fixed if line 858 of dante.el is replaced by the following:

(while (and (looking-at "[ \t]*--") (eq 0 (forward-line -1))))

I will submit a pull request shortly.

Related Note

The same trick used to fix this infinite loop can also simplify block-end's while loop a few lines prior. It currently works and this is not my code base, so I won't submit a pull request for that (unless you think it would be worthwhile to do so).