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;;; haskell-tab-indent.el --- tab-based indentation for haskell-mode
;; Copyright (C) 2015 Sean Whitton
;; Author: Sean Whitton <spwhitton@spwhitton.name>
;; URL: https://spwhitton.name/tech/code/haskell-tab-indent/
;; Version: 0.1.0
;; Keywords: indentation, haskell
;; This file is NOT part of GNU Emacs.
;;; License:
;; This file is free software; you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation; either version 3, or (at your option)
;; any later version.
;; This file is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
;; GNU General Public License for more details.
;; You should have received a copy of the GNU General Public License
;; along with this program. If not, see <http://www.gnu.org/licenses/>.
;;; Commentary:
;; This file provides `haskell-tab-indent-mode', a simple indentation
;; mode for Haskell projects which require tabs for indentation and do
;; not permit spaces (except for where clauses, as a special case).
;;
;; The user may use TAB to cycle between possible indentations.
;;
;; Installation:
;;
;; If you set `indent-tabs-mode' in the .dir-locals.el file for a
;; project requiring tabs, you can use something like this in your
;; init file to enable this mode for such projects:
;;
;; (add-hook 'haskell-mode-hook
;; (lambda ()
;; (add-hook 'hack-local-variables-hook
;; (lambda ()
;; (if indent-tabs-mode
;; (haskell-tab-indent-mode)
;; (haskell-indentation-mode)))
;; nil t))) ; local hook
;;; Code:
(defun haskell-tab-indent ()
"Auto indentation on TAB for `haskell-tab-indent-mode'."
(interactive)
(save-excursion
(back-to-indentation)
;; check for special case of where clause
(if (looking-at "where")
(haskell-tab-indent--where)
;; check for special case of being called by
;; `newline-and-indent': if the user has `electric-indent-mode'
;; on and RET bound to `newline-and-indent', we'll end up
;; indenting too far, or not enough if the previous line was a
;; top level declaration
(unless (let ((previous-line-tabs (haskell-tab-indent--previous-line-tabs))
(this-line-tabs (haskell-tab-indent--this-line-tabs)))
(or
;; avoid indenting too far
(and (equal this-command 'newline-and-indent)
(= this-line-tabs previous-line-tabs)
(not (haskell-tab-indent--previous-line-topdecl-p)))
;; avoid indenting too little
(and (haskell-tab-indent--previous-line-topdecl-p)
(= 1 this-line-tabs))))
(haskell-tab-indent--cycle))))
;; On a line with only indentation, ensure point is at the end of
;; it.
(when (save-excursion (beginning-of-line) (looking-at "[[:space:]]*$"))
(end-of-line)))
(defun haskell-tab-indent--previous-line-topdecl-p ()
"Determine whether previous line is a top-level declaration."
(save-excursion
(beginning-of-line 0) ; go up one line
(equal 'haskell-definition-face (car (cdr (text-properties-at (point)))))))
(defun haskell-tab-indent--where ()
;; `haskell-tab-indent' leaves us just after the indentation
(delete-region (line-beginning-position) (point))
(insert " "))
(defun haskell-tab-indent--cycle ()
(let ((previous-line-tabs (haskell-tab-indent--previous-line-tabs))
(this-line-tabs (haskell-tab-indent--this-line-tabs)))
(if (= (1+ previous-line-tabs) this-line-tabs)
(haskell-tab-indent--reset)
(haskell-tab-indent--indent))))
(defun haskell-tab-indent--reset ()
(save-excursion
(back-to-indentation)
(delete-region (line-beginning-position) (point))))
(defun haskell-tab-indent--indent ()
(save-excursion
(back-to-indentation)
(insert "\t")))
(defun haskell-tab-indent--previous-line-tabs ()
(save-excursion
(beginning-of-line 0) ; go up one line
;; keep going up past blank spacer lines
(while (looking-at "[[:space:]]*$") (beginning-of-line 0))
(haskell-tab-indent--this-line-tabs)))
(defun haskell-tab-indent--this-line-tabs ()
(save-excursion
(save-restriction
(back-to-indentation)
(narrow-to-region (line-beginning-position) (point))
(beginning-of-line)
(let ((count 0))
(while (re-search-forward "\t" nil t)
(setq count (1+ count)))
count))))
;;;###autoload
(define-minor-mode haskell-tab-indent-mode
"Haskell indentation mode for projects requiring that only tabs
-- with no spaces -- be used for indentation.
Binds the TAB key to cycle between possible indents."
:lighter " TabInd"
(kill-local-variable 'indent-line-function)
(when haskell-tab-indent-mode
(set (make-local-variable 'indent-line-function) 'haskell-tab-indent)
(set (make-local-variable 'indent-tabs-mode) t)))
(provide 'haskell-tab-indent)
;;; haskell-tab-indent.el ends here
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