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;;; haskell-tab-indent.el --- tab-based indentation for haskell-mode -*- lexical-binding: t; -*-
;; Copyright (C) 2015, 2017, 2020 Sean Whitton
;; Author: Sean Whitton <spwhitton@spwhitton.name>
;; URL: https://spwhitton.name/tech/code/haskell-tab-indent/
;; Version: 0.3
;; Package-Version: 0.3
;; 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 1)
;; (haskell-indentation-mode 1)))
;; nil t))) ; local hook
;;; Code:
(require 'cl-lib)
(require 'seq)
(cl-flet ((count-line-tabs () (save-excursion
(back-to-indentation)
(length (seq-filter (lambda (c) (equal c ?\t))
(buffer-substring
(line-beginning-position)
(point))))))
(tabs (n) (make-string n ?\t)))
(defun haskell-tab-indent ()
"Auto indentation on TAB for `haskell-tab-indent-mode'."
(interactive)
(let ((this-line-tabs (count-line-tabs))
(prev-line-tabs (save-excursion
(cl-loop do (beginning-of-line 0)
while (looking-at "[[:space:]]*$"))
(count-line-tabs)))
;; determine whether previous line is a declaration
(prev-line-decl (save-excursion
(beginning-of-line 0)
(looking-at "^[[:blank:]]*[[:graph:]]+ :: ")))
;; determine whether previous line is the first line of a where clause
(prev-line-where (save-excursion
(beginning-of-line 0)
(looking-at "^[[:blank:]]*where$"))))
(save-excursion
(back-to-indentation)
(let ((indent (cond ((looking-at "where$")
;; if user explicitly requested an indent
;; change, cycle indentation of the where
;; clause, but no deeper than the level of the
;; previous line. Otherwise, just ensure it's
;; preceded by two spaces
(if (eq this-command 'indent-for-tab-command)
(if (>= this-line-tabs prev-line-tabs)
" "
(concat (tabs (1+ this-line-tabs)) " "))
(concat (tabs this-line-tabs) " ")))
;; if the previous line is a declaration, then
;; this line should be either empty, or at the
;; same indent level as that declaration
(prev-line-decl
(tabs prev-line-tabs))
;; if the previous line was the beginning of a
;; where clause, indent should be exactly one
;; more
(prev-line-where
;; also check indentation of the 'where' itself
(save-excursion
(beginning-of-line 0)
(skip-chars-forward "\t")
(unless (looking-at " where$")
(insert " ")))
(tabs (1+ prev-line-tabs)))
;; if the user explicitly requested an indent
;; change, cycle through the plausible indents
((eq this-command 'indent-for-tab-command)
(if (>= this-line-tabs (1+ prev-line-tabs))
""
(tabs (1+ this-line-tabs))))
;; otherwise, indent to same level as previous line
(t
(tabs prev-line-tabs)))))
(setf (buffer-substring (line-beginning-position) (point)) indent))))
;; on a line with only indentation, ensure point is at the end
(when (save-excursion (beginning-of-line) (looking-at "[[:space:]]*$"))
(end-of-line))))
;;;###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
;; recent `haskell-mode' considers `haskell-indentation-mode' to
;; be the default, and unconditionally turns it on. Follow recent
;; `haskell-indent-mode' and turn it off when activating our mode
(when (and (bound-and-true-p haskell-indentation-mode)
(fboundp 'haskell-indentation-mode))
(haskell-indentation-mode 0))
(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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