3 releases

Uses old Rust 2015

new 0.0.2 May 3, 2021
0.0.1 Aug 3, 2020
0.0.0 Jul 25, 2020

#61 in WebAssembly


Used in create-cwl-app

MIT license

45KB
1K SLoC

Cwl is a front end web language implemented with Rust procedural macros.

To install, you will need:

  1. rustc/cargo
  2. node/npm
  3. wasm-pack

Then:

git clone https://github.com/thisminute/cascading-wasm-language.git
cd cascading-wasm-language
git submodule init
git submodule update
cd create-cwl-app/www
npm install
npm start

For windows users, run in the root directory before npm start:

rustup toolchain install stable-x86_64-pc-windows-gnu
rustup default stable-x86_64-pc-windows-gnu

Understanding the Code

Execution Steps

Rust folder structure

"lib.rs" and "mod.rs" are special names that are entry points for the folders they are in. The top level is ./src/lib.rs. The whole project is a library, and every subsequent directory contains a private module included from there.

src/lib.rs

Execution of the code starts in one of 3 procedural macros exported from lib.rs. cwl is the main one, cwl_document and cwl_header are helpers meant to be used for writing tests.

Data flow

This diagram is helpful for remembering the order that the steps happen in, reference it to help with this section and if you get lost while navigating the code!

cwl       -> lex     ->
tokens    -> parse*  ->
ast       -> analyze ->
semantics -> render  ->
semantics -> write   ->
compiled code!

* this library starts at the parse step (src/transform/parse.rs), because lexing is taken care of us by some libraries, so we have some tokens to start

In words: When a piece of CWL code is compiled, it is first lexed into tokens, which are then parsed into an Abstract Syntax Tree (AST). Semantic analysis is performed on the AST to generate an object representing the meaning of the code. This "Semantics" object is rendered into an abstract representation of an HTML DOM, which is finally compiled into text strings containing HTML and CSS, and executables containing Webassembly.

Data flows between data structures (the data module, in the left column of the diagram) by way of transformations between those structures (the transform module, in the right column of the diagram). All of the code sits in the src directory, and starts in lib.rs, then passes to transform/parse.rs to create an object which is described in data/ast.rs, which are then passed to transform/analyze.rs to create a semantics object defined in data/semantics.rs, and so on through the diagram. The write transformation is defined in several parts, one for each of several outputs that don't resemble each other - HTML, CSS, and Rust code that is then compiled into a Wasm binary. Each output is generated with a different trait implemented on the Semantics struct.

This is the core of CWL (And could be fairly easy to adapt to other syntaxes!). In src/misc are files outside of this core flow, such as the helper context which is used during semantic analysis.

Transformations in detail

Transformations are not entirely distinct from one another, and some of the tasks that a transformation does could have been placed in a different transformation to achieve the same result. For example, different kinds of blocks (class, element, or event) are written to different arrays during parsing, but the same thing could be achieved by writing to a single blocks array during parsing and then determining what kind of block each block is later, during analysis.

A general principle to adhere to is to place logic as early as it can happen without losing information that we need later. In our block parsing example, by determining whether a block is a class, element, or event in parsing, we lose the ability to tell whether a particular class came before or after another element. For example:

// 1
.some_rule {}
some_rule {}

// 2
some_rule {}
.some_rule {}

Both of these bits of code parse into exactly the same AST, because both create a classes array with one item and an elements array with one item, and it is impossible to tell after parsing which order they were in originally. This is in fact desired behavior! CWL syntax should not require us to place classes before elements for the rules to apply, and as there is currently no planned distinction between putting an element and a class in either order (note: the order of any element relative to other elements on the other hand is very important, and preserved in the elements array), we are okay with completely eliminating that information from the pipeline in the very first transformation, and it saves us from having to worry about that information affecting something in a later transformation.

Parse

Parsing takes tokens derived from some CWL input and provided to us by the syn crate, and it transforms them into an AST. The AST is a minimum representation of the input - it is close to 1:1 with the original code, but unlike the input code it is in a tree rather than a sequence of tokens. The AST should represent the minimum information necessary to reproduce a CWL program. For example:

.box {
     content {}
}
box {}
box {}

The root of the AST parsed from this code would contain a class block with an element block inside of it, and then 2 separate element blocks, like this:

// AST (made of blocks)
     / .box - content
page - box
     \ box

Analyze and Render

Analyzing an AST generates a different tree structure which we call "semantics", which represents the meaning of the code in a way that continues to reflect the structure of the input, but which is filled out with information that will later let us transform the tree into a different shape, as well as any other information we need to gather along the way. To illustrate why the tree must be transformed, we can look at the tree structure of the desired output for the above code:

// DOM (made of elements)
page - box - content
     \ box - content

Analysis walks the AST (which we say consists of blocks) and creates a tree modeled after it (which we say consists of groups), and then runs steps we call rendering, which gather and fill out information in the groups, creating two-way links between element and class nodes in addition to the normal tree structure. In this case, these allow us to walk from either box group to the .box group, and from there to the content element group:

// semantics (made of groups, looks like AST)
     / .box - content
page - box
     \ box

// also has paths to be walked in this order
page - box - .box - content
     \ box /

Only one group node exists for the .box class, and only one for the content elements, but these elements can be reached from two places. This semantics tree can now be used to generate a DOM during rendering.

Compile

The semantics tree compiles to an HTML document and a TokenStream of Rust code to be compiled into webassembly. We create the HTML document in compile/html.rs step describes how to recursively generate a single string containing an HTML document that can then get written to a file, leveraging compile/css.rs to complete the contents of style tags attributes. In a separate step, the TokenStream is generated by a process outlined in the several files in compile/wasm, and finally compiled by Rust where the macro invocation was. The HTML compile step is relatively simple, but the Webassembly compile step is one of the most complicated parts of the whole project (TO BE CONTINUED).

Tests

Tests currently broken from adding a global static CLASSES variable :(

./tests has a collection of CWL examples that render different features, and then checks the DOM to see that it rendered as expected! Run them with wasm-pack test --headless --firefox. --chrome works too, and you'll have to have installed the browser you choose.

Debugging

We use create-cwl-app to provide a server and some debugging tools to work on cwl. Use git submodule update to pull down the contents of the directory. cd create-cwl-app/www and you will have access to some npm commands. Run npm install first. npm start will compile the test app, and npm run debug will run some steps that generate the macro output at create-cwl-app/target/cwl_macro_output_formatted.rs and also try to compile it. This is helpful when there is a build error in a bit of cwl code, but the library compiles, because it happens in code generated by the macro and the error messages just points at the macro, to see the error message in the generated code.

Dependencies

~1.2–1.7MB
~39K SLoC