Implement electric comma, semicolon, etc

[gracjan]

haskell-indentation reacts to a couple of characters, most notably comma and semicolonbut also other chars.

Implement function that can be used in post-command-hook that will:

• check if there is exactly one character on the line
• check if there is only one indentation position returned from haskell-indentation-find-indentations
• do the indentation if both are true
• connect this function to ,; and )}]

More info:
http://www.gnu.org/software/emacs/manual/html_node/ccmode/Electric-Keys.html

[bch29]

This function does the job, but unfortunately I'm not familiar enough with emacs to know how to hook it into electirc keys:

(defun haskell-try-auto-indent ()
  (if (and
       (= 1 (length (s-trim (current-line))))
       (= 1 (length (haskell-indentation-find-indentations))))
      (haskell-indentation-indent-line)))

[gracjan]

cc-mode knows how to do electric keys. Can you look there?

[gracjan]

This is a great find: #1175 (comment) .

[fice-t]

I don't believe that we should put this in post-command-hook (as far as I can tell, cc-mode doesn't use that approach either). I'm unsure about using electric-indent-mode but it does make the code shorter. The following should work (>= emacs 24.4 anyway):

(defun haskell-electric-keys (char)
  "Indent after typing CHAR when:
1) There is no other character on the current line.
2) There is only one possible indentation position.
3) The variable `haskell-use-electric-keys' is either t or, if it's a list, CHAR is in it."
  (and (if (listp haskell-use-electric-keys)
           (memq char haskell-use-electric-keys)
         haskell-use-electric-keys)
       (eq char (string-to-char (string-trim-left (thing-at-point 'line t))))
       (= 1 (length (save-excursion
                      (move-to-column (haskell-indentation-current-indentation))
                      (haskell-indentation-find-indentations))))))


(defcustom haskell-use-electric-keys t
  "If t, various special keys will be 'electric' when `electric-indent-mode' is turned on. 
If a list, only the special characters present in the list will be 'electric'."
  :group 'haskell-indentation
  :type '(choice boolean (repeat character)))

(setq haskell-use-electric-keys (string-to-list ",;=|)}]"))
(setq-local electric-indent-functions #'haskell-electric-keys)

We'd have to turn off electric-indent-inhibit of course.

[fice-t]

Actually, looking a bit more closely, the above won't cut it. I thought that electric-indent-functions would only be called after typing one of electric-indent-chars. Apparently electric-indent-functions gets called after every character typed, and if one of the functions returns true or the character typed was in electric-indent-chars then it indents.

So now I don't think electric-indent should be used here, as it runs the check after every self-insert, which is unnecessary. I changed the above to a version that actually works, but I don't like it.

[geraldus]

@fice-t very nice! Will you send a pull request for this?

[fice-t]

I could, but I don't think it's a good way to go about it. Wouldn't just defining the electric functions as needed be a better solution since nothing meaningful would be added to post-self-insert-hook?
(cc-mode does this, though in their case they pretty much need to do it as the electric keys have different behaviour).

I have a solution for it that I can post later, which is slightly more involved. Instead it only checks after the special characters are typed, and not after every self-insert-command like the above does.

I think the slight increase in complexity would be worth it since @gracjan was talking about performance recently. It probably doesn't really matter either way though, so it's up to you two.

[geraldus]

@fice-t maybe customizing electric-indent-chars should be sufficient if we'll implement haskell-mode-electric-indent function and add it electric-indent-functions?

[fice-t]

Unless I misunderstand, that was my previous approach -- electric-indent-functions would be called after every self-insert-command, and typing characters in electric-indent-chars would unconditionally re-indent, which is not what we want.

[geraldus]

@fice-t I can't remember have you tried to implement some function which should decide what to do in your previous approach?

Sorry, I have weak mind today, all this indentation stuff is still messy in my head. I'm more inclined in direction to provide just semantic indentation support but not aesthetic (as was recently discussed in #1178) leaving this to third party packages. For now I'm confused a bit and I can't give good advices because of lacking experience with electric indentation in other modes, and it requires some time to research all of this.

[fice-t]

Yeah, the problem is that electric-indent (to my understanding) will indent automatically if at least one of these is true:

1. A function in electric-indent-functions returns true
2. The character typed is in electric-indent-chars

So we can't just add special characters to electric-indent-chars because then when typing these characters it will pretty much always indent (it doesn't go through electric-indent-functions).

Which means that electric-indent-functions is called after every character typed, which as it's short-circuited doesn't mean the end of the world, but it's not something that I'd say is very good.