# Day 3 Nand2Tetris: Another Chapter and mode improvements

Just after finishing the nand2tetris hdl file mode I think I made a mistake naming it nand2tetris mode because after Week 3 the use of hdl files drops dramatically. And the use of assembler files that are in some Hack Assembly language are the main focus. Makes sense, but I'm kinda sad that the mode was just useful for a couple of "weeks". Still I hope it is of use for other people.

Anyway, I made some other improvements on the nand2tetris hdl mode (I think I might rename the mode later on, you'll see why)

## Company

I added support for the popup from company, it just looks like this:

(defvar nand2tetris-builtin-chips
"16-bit bitwise And:

IN a[16], b[16];
OUT out[16];

for i = 0..15: out[i] = (a[i] and b[i])")
("spec" . "Add16(a= ,b= ,out= )")))
;; ...
))

(defun company-nand2tetris--doc-buffer (candidate)
"Return documentation buffer for chosen CANDIDATE."
(let ((doc (cdr (assoc "description"
(assoc candidate nand2tetris-builtin-chips)))))
(and doc (nand2tetris-doc-buffer doc))))

(defun company-nand2tetris (command &optional arg &rest ignored)
"Company backend for nand2tetris-mode'."
(interactive (list 'interactive))
(cl-case command
(interactive (company-begin-backend 'company-nand2tetris))
(prefix (company-nand2tetris--prefix))
(candidates (company-nand2tetris--candidates arg))
(doc-buffer (company-nand2tetris--doc-buffer arg))
(annotation (company-nand2tetris--annotation arg))))


So I just put one example of the many alists... I had to add another description entry and was mostly done by hand. I found it very useful when doing the PC, ALU and RAM chips :)

## ElDoc (It works better now!)

SO after lots of reading on how regular expressions work in Emacs I think I got it now :) at least I feel way more comfortable than before, even though this is my first go at emacs lisp package development I feel confident that there are bearable rough edges.

;;; ElDoc
(require 'eldoc)

(defun get-chip-at-line ()
"Gets the chip currently used, so that placing the cursor at
any point in the line:
Not16 (in=a, out=out)
Will return Not16"
(save-excursion
(beginning-of-line)
(search-forward-regexp
(rx (group (* word))
(? space ) "("))
(match-string 1)))

(defun nand2tetris-eldoc-function ()
"Get help on SYMBOL using help'.
Interactively, prompt for symbol."
(let ((symbol (get-chip-at-line))
(enable-recursive-minibuffers t))
(message (cdr (assoc "spec" (assoc symbol nand2tetris-builtin-chips))))))


So now ElDoc information shows even if you don't have the cursor exactly at the end of the CHIP.

## nand2tetris material coming up

After completing week 5 I see that a custom assembly mode would be awesome for nand2tetris and also maybe..syntax highlighting for the bit lines in the machine code? so one could actually see what it means :) it would be awesome to have but that might be very difficult, I'm gonna attempt to make the HackAssembly mode with Keybindings to also Test the current assembly, just like C-c C-c does currently in nand2tetris-mode.