STARTER GUIDE ON WRITTING MAJOR MODE WITH TREE-SITTER -*- org -*-

This document guides you on adding tree-sitter support to a major
mode.

TOC:

- Building Emacs with tree-sitter
- Install language definitions
- Setup
- Naming convention
- Font-lock
- Indent
- Imenu
- Navigation
- Which-func
- More features?
- Common tasks (code snippets)
- Manual

* Building Emacs with tree-sitter

You can either install tree-sitter by your package manager, or from
source:

    git clone https://github.com/tree-sitter/tree-sitter.git
    cd tree-sitter
    make
    make install

Then pull the tree-sitter branch (or the master branch, if it has
merged) and rebuild Emacs.

* Install language definitions

Tree-sitter by itself doesn’t know how to parse any particular
language. We need to install language definitions (or “grammars”) for
a language to be able to parse it. There are a couple of ways to get
them.

You can use this script that I put together here:

    https://github.com/casouri/tree-sitter-module

You can also find them under this directory in /build-modules.

This script automatically pulls and builds language definitions for C,
C++, Rust, JSON, Go, HTML, Javascript, CSS, Python, Typescript,
and C#. Better yet, I pre-built these language definitions for
GNU/Linux and macOS, they can be downloaded here:

    https://github.com/casouri/tree-sitter-module/releases/tag/v2.1

To build them yourself, run

    git clone git@github.com:casouri/tree-sitter-module.git
    cd tree-sitter-module
    ./batch.sh

and language definitions will be in the /dist directory. You can
either copy them to standard dynamic library locations of your system,
eg, /usr/local/lib, or leave them in /dist and later tell Emacs where
to find language definitions by setting ‘treesit-extra-load-path’.

Language definition sources can be found on GitHub under
tree-sitter/xxx, like tree-sitter/tree-sitter-python. The tree-sitter
organization has all the "official" language definitions:

   https://github.com/tree-sitter

* Setting up for adding major mode features

Start Emacs and load tree-sitter with

    (require 'treesit)

Now check if Emacs is built with tree-sitter library

    (treesit-available-p)

* Tree-sitter major modes

Tree-sitter modes should be separate major modes, so other modes
inheriting from the original mode don't break if tree-sitter is
enabled.  For example js2-mode inherits js-mode, we can't enable
tree-sitter in js-mode, lest js-mode would not setup things that
js2-mode expects to inherit from. So it's best to use separate major
modes.

If the tree-sitter variant and the "native" variant could share some
setup, you can create a "base mode", which only contains the common
setup.  For example, there is python-base-mode (shared), python-mode
(native), and python-ts-mode (tree-sitter).

In the tree-sitter mode, check if we can use tree-sitter with
treesit-ready-p, it will error out if tree-sitter is not ready.

* Naming convention

Use tree-sitter for text (documentation, comment), use treesit for
symbol (variable, function).

* Font-lock

Tree-sitter works like this: You provide a query made of patterns and
capture names, tree-sitter finds the nodes that match these patterns,
tag the corresponding capture names onto the nodes and return them to
you. The query function returns a list of (capture-name . node). For
font-lock, we use face names as capture names. And the captured node
will be fontified in their capture name.

The capture name could also be a function, in which case (NODE
OVERRIDE START END) is passed to the function for fontification. START
and END is the start and end of the region to be fontified.  The
function should only fontify within that region.  The function should
also allow more optional arguments with (&rest _), for future
extensibility.  For OVERRIDE check out the docstring of
treesit-font-lock-rules.

** Query syntax

There are two types of nodes, named, like (identifier),
(function_definition), and anonymous, like "return", "def", "(",
"}". Parent-child relationship is expressed as

   (parent (child) (child) (child (grand_child)))

Eg, an argument list (1, "3", 1) could be:

   (argument_list "(" (number) (string) (number) ")")

Children could have field names in its parent:

   (function_definition name: (identifier) type: (identifier))

Match any of the list:

    ["true" "false" "none"]

Capture names can come after any node in the pattern:

    (parent (child) @child) @parent

The query above captures both parent and child.

   ["return" "continue" "break"] @keyword

The query above captures all the keywords with capture name
"keyword".

These are the common syntax, see all of them in the manual
("Parsing Program Source" section).

** Query references

But how do one come up with the queries? Take python for an example,
open any python source file, type M-x treesit-explore-mode RET.  Now
you should see the parse-tree in a separate window, automatically
updated as you select text or edit the buffer.  Besides this, you can
consult the grammar of the language definition. For example, Python’s
grammar file is at

    https://github.com/tree-sitter/tree-sitter-python/blob/master/grammar.js

Neovim also has a bunch of queries to reference:

    https://github.com/nvim-treesitter/nvim-treesitter/tree/master/queries

The manual explains how to read grammar files in the bottom of section
"Tree-sitter Language Definitions".

** Debugging queires

If your query has problems, use ‘treesit-query-validate’ to debug the
query. It will pop a buffer containing the query (in text format) and
mark the offending part in red.

** Code

To enable tree-sitter font-lock, set ‘treesit-font-lock-settings’ and
‘treesit-font-lock-feature-list’ buffer-locally and call
‘treesit-major-mode-setup’. For example, see
‘python--treesit-settings’ in python.el. Below I paste a snippet of
it.

Note that like the current font-lock, if the to-be-fontified region
already has a face (ie, an earlier match fontified part/all of the
region), the new face is discarded rather than applied. If you want
later matches always override earlier matches, use the :override
keyword.

Each rule should have a :feature, like function-name,
string-interpolation, builtin, etc. Users can then enable/disable each
feature individually.

#+begin_src elisp
(defvar python--treesit-settings
  (treesit-font-lock-rules
   :feature 'comment
   :language 'python
   '((comment) @font-lock-comment-face)

   :feature 'string
   :language 'python
   '((string) @font-lock-string-face
     (string) @contextual) ; Contextual special treatment.

   :feature 'function-name
   :language 'python
   '((function_definition
      name: (identifier) @font-lock-function-name-face))

   :feature 'class-name
   :language 'python
   '((class_definition
      name: (identifier) @font-lock-type-face))

   ...))
#+end_src

Then in ‘python-mode’, enable tree-sitter font-lock:

#+begin_src elisp
(treesit-parser-create 'python)
(setq-local treesit-font-lock-settings python--treesit-settings)
(setq-local treesit-font-lock-feature-list
            '((comment string function-name)
              (class-name keyword builtin)
              (string-interpolation decorator)))
...
(treesit-major-mode-setup)
#+end_src

Concretely, something like this:

#+begin_src elisp
(define-derived-mode python-mode prog-mode "Python"
  ...
  (cond
   ;; Tree-sitter.
   ((treesit-ready-p 'python-mode 'python)
    (treesit-parser-create 'python)
    (setq-local treesit-font-lock-settings python--treesit-settings)
    (setq-local treesit-font-lock-feature-list
                '((comment string function-name)
                  (class-name keyword builtin)
                  (string-interpolation decorator)))
    (treesit-major-mode-setup))
   (t
    ;; No tree-sitter
    (setq-local font-lock-defaults ...)
    ...)))
#+end_src

* Indent

Indent works like this: We have a bunch of rules that look like

   (MATCHER ANCHOR OFFSET)

When the indentation process starts, point is at the BOL of a line, we
want to know which column to indent this line to. Let NODE be the node
at point, we pass this node to the MATCHER of each rule, one of them
will match the node (eg, "this node is a closing bracket!"). Then we pass
the node to the ANCHOR, which returns a point, eg, the BOL of the
previous line. We find the column number of that point (eg, 4), add
OFFSET to it (eg, 0), and that is the column we want to indent the
current line to (4 + 0 = 4).

Matchers and anchors are functions that takes (NODE PARENT BOL &rest
_). Matches return nil/non-nil for no match/match, and anchors return
the anchor point. Below are some convenient builtin matchers and anchors.

For MATHCER we have

    (parent-is TYPE) => matches if PARENT’s type matches TYPE as regexp
    (node-is TYPE) => mathces NODE’s type
    (query QUERY) => matches if querying PARENT with QUERY
                     captures NODE.

    (match NODE-TYPE PARENT-TYPE NODE-FIELD
           NODE-INDEX-MIN NODE-INDEX-MAX)

    => checks everything. If an argument is nil, don’t match that. Eg,
    (match nil nil TYPE) is the same as (parent-is TYPE)

For ANCHOR we have

    first-sibling => start of the first sibling
    parent => start of parent
    parent-bol => BOL of the line parent is on.
    prev-sibling => start of previous sibling
    no-indent => current position (don’t indent)
    prev-line => start of previous line

There is also a manual section for indent: "Parser-based Indentation".

When writing indent rules, you can use ‘treesit-check-indent’ to
check if your indentation is correct. To debug what went wrong, set
‘treesit--indent-verboase’ to non-nil. Then when you indent, Emacs
tells you which rule is applied in the echo area.

#+begin_src elisp
(defvar typescript-mode-indent-rules
  (let ((offset typescript-indent-offset))
    `((typescript
       ;; This rule matches if node at point is "}", ANCHOR is the
       ;; parent node’s BOL, and offset is 0.
       ((node-is "}") parent-bol 0)
       ((node-is ")") parent-bol 0)
       ((node-is "]") parent-bol 0)
       ((node-is ">") parent-bol 0)
       ((node-is "\\.") parent-bol ,offset)
       ((parent-is "ternary_expression") parent-bol ,offset)
       ((parent-is "named_imports") parent-bol ,offset)
       ((parent-is "statement_block") parent-bol ,offset)
       ((parent-is "type_arguments") parent-bol ,offset)
       ((parent-is "variable_declarator") parent-bol ,offset)
       ((parent-is "arguments") parent-bol ,offset)
       ((parent-is "array") parent-bol ,offset)
       ((parent-is "formal_parameters") parent-bol ,offset)
       ((parent-is "template_substitution") parent-bol ,offset)
       ((parent-is "object_pattern") parent-bol ,offset)
       ((parent-is "object") parent-bol ,offset)
       ((parent-is "object_type") parent-bol ,offset)
       ((parent-is "enum_body") parent-bol ,offset)
       ((parent-is "arrow_function") parent-bol ,offset)
       ((parent-is "parenthesized_expression") parent-bol ,offset)
       ...))))
#+end_src

Then you set ‘treesit-simple-indent-rules’ to your rules, and call
‘treesit-major-mode-setup’:

#+begin_src elisp
(setq-local treesit-simple-indent-rules typescript-mode-indent-rules)
(treesit-major-mode-setup)
#+end_src

* Imenu

Not much to say except for utilizing ‘treesit-induce-sparse-tree’ (and
explicitly pass a LIMIT argument: most of the time you don't need more
than 10).  See ‘js--treesit-imenu-1’ in js.el for an example.

Once you have the index builder, set ‘imenu-create-index-function’ to
it.

* Navigation

Mainly ‘beginning-of-defun-function’ and ‘end-of-defun-function’.
You can find the end of a defun with something like

(treesit-search-forward-goto "function_definition" 'end)

where "function_definition" matches the node type of a function
definition node, and ’end means we want to go to the end of that node.

Tree-sitter has default implementations for
‘beginning-of-defun-function’ and ‘end-of-defun-function’.  So for
ordinary languages, it is suffice to set ‘treesit-defun-type-regexp’
to something that matches all the defun struct types in the language,
and call ‘treesit-major-mode-setup’.  For example,

#+begin_src emacs-lisp
(setq-local treesit-defun-type-regexp (rx bol
                                          (or "function" "class")
                                          "_definition"
                                          eol))
(treesit-major-mode-setup)
#+end_src>

* Which-func

If you have an imenu implementation, set ‘which-func-functions’ to
nil, and which-func will automatically use imenu’s data.

If you want independent implementation for which-func, you can find
the current function by going up the tree and looking for the
function_definition node. See the function below for an example.
Since Python allows nested function definitions, that function keeps
going until it reaches the root node, and records all the function
names along the way.

#+begin_src elisp
(defun python-info-treesit-current-defun (&optional include-type)
  "Identical to `python-info-current-defun' but use tree-sitter.
For INCLUDE-TYPE see `python-info-current-defun'."
  (let ((node (treesit-node-at (point)))
        (name-list ())
        (type nil))
    (cl-loop while node
             if (pcase (treesit-node-type node)
                  ("function_definition"
                   (setq type 'def))
                  ("class_definition"
                   (setq type 'class))
                  (_ nil))
             do (push (treesit-node-text
                       (treesit-node-child-by-field-name node "name")
                       t)
                      name-list)
             do (setq node (treesit-node-parent node))
             finally return (concat (if include-type
                                        (format "%s " type)
                                      "")
                                    (string-join name-list ".")))))
#+end_src

* More features?

Obviously this list is just a starting point, if there are features in
the major mode that would benefit a parse tree, adding tree-sitter
support for that would be great. But in the minimal case, just adding
font-lock is awesome.

* Common tasks

How to...

** Get the buffer text corresponding to a node?

(treesit-node-text node)

BTW ‘treesit-node-string’ does different things.

** Scan the whole tree for stuff?

(treesit-search-subtree)
(treesit-search-forward)
(treesit-induce-sparse-tree)

** Move to next node that...?

(treesit-search-forward-goto)

** Get the root node?

(treesit-buffer-root-node)

** Get the node at point?

(treesit-node-at (point))

* Manual

I suggest you read the manual section for tree-sitter in Info. The
section is Parsing Program Source. Typing

    C-h i d m elisp RET g Parsing Program Source RET

will bring you to that section. You can also read the HTML version
under /html-manual in this directory. I find the HTML version easier
to read. You don’t need to read through every sentence, just read the
text paragraphs and glance over function names.