WEBVTT
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Hello, everyone! My name is Tuấn-Anh.
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I've been using Emacs for about 10 years.
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Today, I'm going to talk about tree-sitter,
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a new Emacs package that allows Emacs
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to parse multiple programming languages
in real-time.
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So what is the problem statement?
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In order to support programming
functionalities
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for a particular language,
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a text editor needs to have some degree
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of language understanding.
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Traditionally, text editors have relied
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very heavily on regular expressions for
this.
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Emacs is no different.
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Most language major modes use regular
expressions
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for syntax-highlighting, code navigation,
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folding, indexing, and so on.
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Regular expressions are problematic for
a couple of reasons.
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They're slow and inaccurate.
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They also make the code hard to read and
write.
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Sometimes it's because the regular
expressions themselves are very hairy,
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and sometimes because they are just not
powerful enough.
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Some helper code is usually needed
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to parse more intricate language
features.
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That also illustrates the core problem
with regular expressions,
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in that they are not powerful enough to
parse programming languages.
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An example feature that regular
expressions cannot handle very well
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is string interpolation, which is a very
common feature
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in many modern programming languages.
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It would be much nicer if Emacs somehow
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had structural understanding of source
code, like IDEs do.
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There have been multiple efforts
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to bring this kind of programming
language understanding into Emacs.
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There are language-specific parsers
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written in Elisp
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that can be thought of
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as the next logical step
of the glue code
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on top of regular expressions,
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moving from partial local pattern
recognition
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into a full-fledged parser.
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The most prominent example of this
approach
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is probably the famous js2-mode.
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However, this approach has several issues.
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Parsing is computationally expensive,
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and Emacs Lisp is not good at that kind
of stuff.
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Furthermore, maintenance is very
troublesome.
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In order to work on these parsers,
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first, you have to know Elisp
well enough,
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and then you have to be comfortable with
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writing a recursive descending parser,
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while constantly keeping up with changes
to the language itself,
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which can be evolving very quickly,
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like Javascript, for example.
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Together, these constraints
significantly reduce
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the pool of potential maintainers.
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The biggest issue, though, in my opinion,
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is lack of the set of generic and
reusable APIs.
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This makes them very hard to use
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for minor modes that want to deal with
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cross-cutting concerns across multiple
languages.
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The other approach which has been
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gaining a lot of momentum
in recent years
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is externalizing language understanding
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to another process,
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also known as language server protocol.
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This second approach is actually a very
interesting one.
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By decoupling language understanding
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from the editing facility itself,
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the LSP servers can attract a lot more
contributors,
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which makes maintenance easier.
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However, they also have several issues
of their own.
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Being a separate process,
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they are usually more
resource-intensive,
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and depending on the language,
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the LSP server itself can bring with it
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a host of additional dependencies
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external to Emacs, which may be messy to
install and manage.
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Furthermore, JSON over RPC has pretty
high latency.
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For one-off tasks like jumping to source
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or on-demand completion, it's great.
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But for things like code highlighting,
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the latency is just too much.
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I was using Rust and I was following the
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community effort to improve its
IDE support,
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hoping to integrate some of that into
Emacs itself.
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Then I heard someone from the community
mention tree-sitter,
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and I decided to check it out.
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Basically, tree-sitter is an incremental
parsing library and a parser generator.
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It was introduced by the Atom editor in
2018.
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Besides Atom, it is also being
integrated
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into the NeoVim editor,
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and Github is using it to power
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their source code analysis
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and navigation features.
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It is written in C and can be compiled
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for all major platforms.
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It can even be compiled
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to web assembly to run on the web.
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That's how Github is using it
on their website.
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So why is tree-sitter an interesting
solution to this problem?
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There are multiple features that make it
an attractive option.
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It is designed to be fast.
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By being incremental,
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the initial parse of a typical big file
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can take tens of milliseconds,
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while subsequent incremental processes
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are sub-millisecond.
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It achieves this by using
structural sharing,
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meaning replacing only affected nodes
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in the old tree when it needs to.
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Also, unlike LSP, being in
the same process,
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it has much lower latency.
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Secondly, it provides a uniform
programming interface.
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The same data structures and functions
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work on parse trees of different
languages.
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Syntax nodes of different languages
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differ only by their types
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and their possible child nodes.
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This is a big advantage over
language-specific parsers.
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Thirdly, it's written in self-contained
embeddable C.
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As I mentioned previously, it can even
be compiled to webassembly.
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This makes integrating it into various
editors quite easy
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without having to install any external
dependencies.
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One thing that is not mentioned here
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is that being a parser generator,
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its grammars are declarative.
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Together with being editor-independent,
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this makes the pool of potential
contributors much larger.
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So I was convinced that tree-sitter is a
good fit for Emacs.
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Last year, I started writing the bindings
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using dynamic module support introduced
in Emacs 25.
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Dynamic module means there is
platform-specific native code involved,
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but since there are pre-compiled binaries
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for the three major platforms,
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it should work in most places.
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Currently, the core functionalities are
in a pretty good shape.
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Syntax highlighting is working nicely.
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The whole thing is split into three
packages.
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tree-sitter is the main package that
other packages should depend on.
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tree-sitter-langs is the language bundle
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that includes support
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for most common languages.
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And finally, the core APIs are in the
package tsc,
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which stands for tree-sitter-core.
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It is the implicit dependency of the
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tree-sitter package.
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The main package includes the minor mode
tree-sitter-mode.
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This provides the base for other major
or minor modes to build on.
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Using Emacs's change tracking hooks,
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it enables incremental parsing
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and provides a syntax tree that is
always up to date
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after any edits in a buffer.
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There is also a basic debug mode
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that shows the parse tree in
another buffer.
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Here is a quick demo.
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Here I'm in an empty Python buffer
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with tree-sitter enabled.
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I'm going to turn on the debug mode to
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see the parse tree.
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Since the buffer is empty,
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there is only one node in the
syntax tree:
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the top-level module node.
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Let's try typing some code.
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As you can see, as I type into the
Python buffer,
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the syntax tree updates in real time.
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The other minor mode included in the
main package
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is tree-sitter-hl-mode.
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It overrides font-lock mode
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and provides its own set of phases
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and customization options
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It is query-driven.
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That means instead of regular
expressions,
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it uses a Lisp-like query language
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to map syntax nodes
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to highlighting phrases.
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I'm going to open a python file with
small snippets
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that showcase syntax highlighting.
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So this is the default highlighting
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provided by python-mode.
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This is the highlighting enabled
by tree-sitter.
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as you can see string interpolation
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and decorators are highlighted correctly
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function calls are also highlighted
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you can also note that property
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assessors
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and property assignments are highlighted
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differently
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what I like the most about this is that
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new bindings are consistently
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highlighted
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this included local variable
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function parameters and property
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mutations
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before going through the three queries
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and the syntax highlighting
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customization options
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let's take a brief look at the core data
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structures and functions
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that tree sitter provides
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so parsing is done with the help of a
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generic parser object
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a single parser object can be used to
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pass different languages
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by sending different language objects to
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it
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the language objects themselves are
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loaded from shared libraries
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since three seater mode already handles
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the parsing part
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we will instead focus on the functions
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that inspect nodes
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and in the resulting path tree
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we can ask tree sitter what is the
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syntax node at point
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uh is it an opaque object so this is not
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very useful
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we can instead ask what is its type
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so his type is the symbol comparison
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operator
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trees there are two kinds of nodes
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anonymous nodes and named nodes
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anonymous nodes correspond to simple
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grammar elements
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like keywords operators punctuations and
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so on
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name nodes on the other hand grammar
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elements that are interesting enough for
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their own
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to have a name like an identifier an
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expression
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or a function definition
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name node types are symbols while
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anonymous node types are strings
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for example if we are on this
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comparison operator
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the node type should be a string
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we can also get other information about
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the node
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for example what is this text
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or where it is in the buffer
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or what is its parent
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there are many other apis to query or
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not
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properties
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tree sitter allows searching for
00:13:54.399 --> 00:13:58.240
structural patterns within a parse tree
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it does so through a list like language
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this language supports by the matching
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by node types
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field names and predicates
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it also allows capturing nodes for
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further processing
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let's try to see some examples
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so in this very simple query we just
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try to highlight all the identifiers in
00:14:43.839 --> 00:14:49.040
the buffer
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this s side tells trisito to capture a
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node
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in the context of the query builder it's
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not very important
00:14:57.360 --> 00:15:00.320
but in normal highlighting query this
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will determine
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the face used to highlight the note
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suppose we want to capture all the
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function names
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instead of just any identifier
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you can improve the query like this
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uh this will highlight the whole
00:15:31.600 --> 00:15:32.639
definition
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but we only want to capture the function
00:15:35.519 --> 00:15:36.399
name
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which means the identifier
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here so we
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move the capture to after the identifier
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node
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if we want to capture the class names as
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well
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we just add another pattern
00:16:10.079 --> 00:16:20.320
let's look at a more practical example
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here we can see that single quotes
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strings and
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double quotes screens are highlighted
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the same
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but in some places
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because of some coding conventions
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it may be desirable to highlight them
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differently for example if
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the string is single quoted we may want
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to highlight it
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as a constant
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let's try to see whether we can
00:16:46.160 --> 00:16:47.600
distinguish these
00:16:47.600 --> 00:16:56.240
two cases
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so here we get all the strings
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if we want to see if it's single quotes
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or
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double quote strings
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we can try looking at the first
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character
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of the string I mean the first character
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of the note
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to check whether it's a single quote or
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a double quote
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yeah so for that we use the three
00:17:36.080 --> 00:17:36.799
setters
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support for predicate in this case
00:17:40.160 --> 00:17:43.360
we use a match predicate
00:17:43.360 --> 00:17:46.080
to check whether the string where the
00:17:46.080 --> 00:17:46.799
note
00:17:46.799 --> 00:17:50.320
starts with a single quote and with this
00:17:50.320 --> 00:17:51.280
pattern
00:17:51.280 --> 00:17:58.840
we only capture the single quotes
00:17:58.840 --> 00:18:00.400
strings
00:18:00.400 --> 00:18:03.760
let's try to give it a different face
00:18:03.760 --> 00:18:13.039
so we copy the pattern
00:18:13.039 --> 00:18:18.640
and we add this pattern
00:18:18.640 --> 00:18:25.120
pop item only
00:18:25.120 --> 00:18:28.400
but we also want to give the
00:18:28.400 --> 00:18:31.440
capture a different name
00:18:31.440 --> 00:18:40.840
let's say we want to highlight it as a
00:18:40.840 --> 00:18:46.559
keyword
00:18:46.559 --> 00:19:06.320
and now if we refresh the buffer
00:19:06.320 --> 00:19:08.799
we see that single quote strings are
00:19:08.799 --> 00:19:10.320
highlighted as
00:19:10.320 --> 00:19:14.400
keywords
00:19:14.400 --> 00:19:16.400
the highlighting patterns can also be
00:19:16.400 --> 00:19:19.200
set for a single project
00:19:19.200 --> 00:19:23.440
using directory local variable
00:19:23.440 --> 00:19:26.880
for example let's take a look at
00:19:26.880 --> 00:19:35.760
ems source code
00:19:35.760 --> 00:19:40.400
so in image c source there are a lot of
00:19:40.400 --> 00:19:43.760
uses of these different macros
00:19:43.760 --> 00:19:47.679
to define functions
00:19:47.679 --> 00:19:51.200
and you can see
00:19:51.200 --> 00:19:53.520
this is actually the function name but
00:19:53.520 --> 00:19:55.760
it's highlighted as the
00:19:55.760 --> 00:19:59.120
string so what we want
00:19:59.120 --> 00:20:03.679
is to somehow recognize this pattern
00:20:03.679 --> 00:20:07.600
and highlight it
00:20:07.600 --> 00:20:11.280
as highlight this part
00:20:11.280 --> 00:20:14.559
with the function phase instead
00:20:14.559 --> 00:20:17.679
in order to do that
00:20:17.679 --> 00:20:20.240
we put a pattern in this project
00:20:20.240 --> 00:20:21.760
directory local
00:20:21.760 --> 00:20:31.760
settings file
00:20:31.760 --> 00:20:34.799
so we can put this button in the c
00:20:34.799 --> 00:20:40.159
mode section
00:20:40.159 --> 00:20:48.000
and now if we enable tree sitter
00:20:48.000 --> 00:20:50.480
you can see that this is the highlighted
00:20:50.480 --> 00:20:53.200
uh
00:20:53.200 --> 00:20:55.520
as a normal function definition so this
00:20:55.520 --> 00:20:56.559
is the function
00:20:56.559 --> 00:21:01.200
face like we wanted
00:21:01.200 --> 00:21:03.760
the pattern for this is actually pretty
00:21:03.760 --> 00:21:07.200
simple
00:21:07.200 --> 00:21:10.720
it's only
00:21:10.720 --> 00:21:14.720
only this part so
00:21:14.720 --> 00:21:17.440
if it's a function call where the name
00:21:17.440 --> 00:21:19.679
of the function is different
00:21:19.679 --> 00:21:21.600
then we highlight the different as a
00:21:21.600 --> 00:21:24.240
keyword
00:21:24.240 --> 00:21:27.360
and then the first string element we
00:21:27.360 --> 00:21:28.159
highlighted
00:21:28.159 --> 00:21:35.360
as a function name
00:21:35.360 --> 00:21:37.679
since the language objects are actually
00:21:37.679 --> 00:21:39.280
native code
00:21:39.280 --> 00:21:40.799
they have to be compiled for each
00:21:40.799 --> 00:21:43.440
platform that we want to support
00:21:43.440 --> 00:21:45.600
this will become a big obstacle for
00:21:45.600 --> 00:21:48.159
3-seater adoption
00:21:48.159 --> 00:21:50.240
therefore I've created a language window
00:21:50.240 --> 00:21:52.960
package 3-seater length
00:21:52.960 --> 00:21:54.960
that takes care of pre-compiling the
00:21:54.960 --> 00:21:56.320
grammars the
00:21:56.320 --> 00:21:59.679
most common grammars for all three major
00:21:59.679 --> 00:22:01.600
platforms
00:22:01.600 --> 00:22:04.080
it also takes care of distributing these
00:22:04.080 --> 00:22:05.360
binaries
00:22:05.360 --> 00:22:08.080
and provides some highlighting queries
00:22:08.080 --> 00:22:11.440
for some of the languages
00:22:11.440 --> 00:22:13.760
it should be noted that this package
00:22:13.760 --> 00:22:15.919
should be treated as a temporary
00:22:15.919 --> 00:22:19.919
distribution mechanism only
00:22:19.919 --> 00:22:22.240
to help with bootstrapping three-seaters
00:22:22.240 --> 00:22:24.720
adoption
00:22:24.720 --> 00:22:27.760
the plan is that eventually these files
00:22:27.760 --> 00:22:29.760
should be provided by the language major
00:22:29.760 --> 00:22:32.480
modes themselves
00:22:32.480 --> 00:22:35.120
but in order to do that we need better
00:22:35.120 --> 00:22:36.320
tooling
00:22:36.320 --> 00:22:40.240
so we're not there yet
00:22:40.240 --> 00:22:42.559
since the call already works reasonably
00:22:42.559 --> 00:22:43.280
well
00:22:43.280 --> 00:22:44.640
there are several areas that would
00:22:44.640 --> 00:22:46.320
benefit from the community's
00:22:46.320 --> 00:22:49.120
contribution
00:22:49.120 --> 00:22:51.520
so three seaters upstream language
00:22:51.520 --> 00:22:52.640
prepositories
00:22:52.640 --> 00:22:54.400
already contain highlighting queries on
00:22:54.400 --> 00:22:55.679
their own
00:22:55.679 --> 00:22:58.480
however they are pretty basic and they
00:22:58.480 --> 00:23:00.480
may not fit well with existing emax
00:23:00.480 --> 00:23:02.559
conventions
00:23:02.559 --> 00:23:04.320
therefore the language bundle has its
00:23:04.320 --> 00:23:07.120
own set of highlighting queries
00:23:07.120 --> 00:23:10.559
this requires maintenance until language
00:23:10.559 --> 00:23:11.600
measurements adopt
00:23:11.600 --> 00:23:13.760
three sitter and maintain the queries on
00:23:13.760 --> 00:23:16.640
their own
00:23:16.640 --> 00:23:18.480
the queries are actually quite easy to
00:23:18.480 --> 00:23:22.000
write as you've already seen
00:23:22.000 --> 00:23:24.240
you just need to be familiar with the
00:23:24.240 --> 00:23:25.360
language
00:23:25.360 --> 00:23:30.000
familiar enough to come up with sensible
00:23:30.000 --> 00:23:35.200
highlighting patterns
00:23:35.200 --> 00:23:37.600
and if you are a maintainer of a
00:23:37.600 --> 00:23:39.679
language major mode
00:23:39.679 --> 00:23:42.320
you may want to consider integrating
00:23:42.320 --> 00:23:43.360
tree sitter into
00:23:43.360 --> 00:23:46.960
your mode initially maybe as an
00:23:46.960 --> 00:23:50.080
optional feature the integration is
00:23:50.080 --> 00:23:53.279
actually pretty straightforward
00:23:53.279 --> 00:23:56.640
especially for syntax highlighting
00:23:56.640 --> 00:24:01.520
or alternatively
00:24:01.520 --> 00:24:03.760
you can also try writing a new major
00:24:03.760 --> 00:24:04.640
mode
00:24:04.640 --> 00:24:08.000
from scratch that relies on tree sitter
00:24:08.000 --> 00:24:12.559
from the very beginning
00:24:12.559 --> 00:24:16.320
the code for such a major mode is
00:24:16.320 --> 00:24:19.679
quite simple for example
00:24:19.679 --> 00:24:23.200
this is the proposed
00:24:23.200 --> 00:24:26.240
what mode for web assembly
00:24:26.240 --> 00:24:31.039
the code is just
00:24:31.039 --> 00:24:34.559
like one page of code not
00:24:34.559 --> 00:24:39.520
not a lot
00:24:39.520 --> 00:24:42.720
you can also try writing new minor modes
00:24:42.720 --> 00:24:46.559
or writing integration packages
00:24:46.559 --> 00:24:50.080
for example a lot of package a lot of
00:24:50.080 --> 00:24:50.880
packages
00:24:50.880 --> 00:24:54.559
may benefit from tree sitter integration
00:24:54.559 --> 00:24:58.840
but no one has written the integration
00:24:58.840 --> 00:25:02.960
yet
00:25:02.960 --> 00:25:05.039
if you are interested in 3-seater you
00:25:05.039 --> 00:25:06.720
can use these links to
00:25:06.720 --> 00:25:10.320
learn more about it I think that's it
00:25:10.320 --> 00:25:11.440
for me today
00:25:11.440 --> 00:25:18.159
I'm happy to answer any questions