From 88fe919d2b6af8b81c25fdad291c2dd062dff7f4 Mon Sep 17 00:00:00 2001
From: Sean Whitton <spwhitton@spwhitton.name>
Date: Wed, 13 May 2020 11:41:50 -0700
Subject: rewrite algorithm, changing what we treat as a special case

Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>
---
 NEWS.md               | 26 +++++++++++++++++-------
 haskell-tab-indent.el | 55 ++++++++++++++++++++++++++-------------------------
 2 files changed, 47 insertions(+), 34 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index b4f7838..d46fcd5 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,15 +1,27 @@
 0.3 (unreleased)
 ----------------
 
-- When checking whether we're on the first line of a where clause,
-  require 'where' followed by end of line, rather than just a line
-  starting with 'where'.
+- When checking for the first line of a where clause, require 'where'
+  followed by end of line, rather than just a line starting with
+  'where'.
 
-- Recognise all declarations, not just top level declarations, in code
-  handling the case where the last command was `newline-and-indent'.
+- Recognise all declarations, not just top level declarations.  On
+  lines following a declaration, always indent to the same level as
+  the previous line, rather than indenting one further.
 
-- Attempt to line up the first line of definitions with declarations
-  on the previous line, rather than indenting further.
+  Lines following declarations should either be indented to the same
+  level as the declaration, or be blank.
+
+- Always indent the first line of the contents of a where clause to
+  one plus the level of the line containing 'where'.
+
+- When one of the above cases does not apply, default to indenting to
+  the same level of the previous line, unless the user explicitly
+  invoked `indent-for-tab-command' (e.g. by hitting the tab key), in
+  which case cycle between possible indents.
+
+  This replaces treating the case where the current command is
+  `newline-and-indent' specially.
 
 - Reset indentation when current indentation is at least one more than
   the previous line, rather than just when it is exactly one more than
diff --git a/haskell-tab-indent.el b/haskell-tab-indent.el
index e6c58a4..15686cb 100644
--- a/haskell-tab-indent.el
+++ b/haskell-tab-indent.el
@@ -59,7 +59,8 @@
                            (length (seq-filter (lambda (c) (equal c ?\t))
                                                (buffer-substring
                                                 (line-beginning-position)
-                                                (point)))))))
+                                                (point))))))
+     (tabs (n) (make-string n ?\t)))
   (defun haskell-tab-indent ()
     "Auto indentation on TAB for `haskell-tab-indent-mode'."
     (interactive)
@@ -73,34 +74,34 @@
           (prev-line-decl
            (save-excursion
              (beginning-of-line 0)
-             (looking-at "[[:space:]]*[^[:space:]]+ ::"))))
+             (looking-at "^[[:blank:]]*[^[:graph:]]+ :: ")))
+          (prev-line-where
+           (save-excursion
+             (beginning-of-line 0)
+             (looking-at "^[[:blank:]]*where$"))))
       (save-excursion
         (back-to-indentation)
-        (if (looking-at "where$")
-            (setf (buffer-substring (line-beginning-position) (point)) "  ")
-          ;; if the previous line is a declaration, then this line
-          ;; should be either empty, or at the same indent level as
-          ;; that declaration
-          (if prev-line-decl
-              (setf (buffer-substring (line-beginning-position) (point))
-                    (make-string prev-line-tabs ?\t))
-            ;; 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
-            ;; declaration
-            (unless (or
-                     ;; avoid indenting too far
-                     (and (equal this-command 'newline-and-indent)
-                          (= this-line-tabs prev-line-tabs)
-                          (not prev-line-decl))
-                     ;; avoid indenting too little
-                     (and prev-line-decl (= 1 this-line-tabs)))
-              (if (>= this-line-tabs (1+ prev-line-tabs))
-                  ;; reset
-                  (delete-region (line-beginning-position) (point))
-                ;; indent
-                (insert "\t")))))))
+        (let ((indent
+               (if (looking-at "where$")
+                   (concat (tabs prev-line-tabs) "  ")
+                 (tabs
+                  (cond
+                   ;; if the previous line is a declaration, then this
+                   ;; line should be either empty, or at the same indent
+                   ;; level as that declaration
+                   (prev-line-decl prev-line-tabs)
+                   ;; if the previous line was the beginning of a where
+                   ;; clause, indent should be exactly one more
+                   (prev-line-where (1+ prev-line-tabs))
+                   ;; if the user explicitly requested an indent
+                   ;; change, cycle through the plausible indents
+                   ((eq this-command 'indent-for-tab-command)
+                    (if (>= this-line-tabs (1+ prev-line-tabs))
+                        0
+                      (1+ this-line-tabs)))
+                   ;; otherwise, indent to same level as previous line
+                   (t prev-line-tabs))))))
+          (setf (buffer-substring (line-beginning-position) (point)) indent))))
     ;; on a line with only indentation, ensure point is at the end
     (when (save-excursion (beginning-of-line) (looking-at "[[:space:]]*$"))
       (end-of-line))))
-- 
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