官术网_书友最值得收藏!

Definitions and S-expressions

To understand Wat, let's start with the first sentence of the description taken directly from the WebAssembly Core Specification:

"The textual format for WebAssembly modules is a rendering of their abstract syntax into S-expressions."

So what are symbolic expressions (S-expressions)? S-expressions are notations for nested list (tree-structured) data. Essentially, they provide a simple and elegant way to represent list-based data in textual form. To understand how textual representations of nested lists map to a tree structure, let's extrapolate the tree structure from an HTML page. The following example contains a simple HTML page and the corresponding tree structure diagram.

A simple HTML page:

<html>
<head>
<link rel="icon" href="favicon.ico">
<title>Page Title</title>
</head>
<body>
<div>
<h1>Header</h1>
<p>This is a paragraph.</p>
</div>
<div>Some content</div>
<nav>
<ul>
<li>Item 1</li>
<li>Item 2</li>
<li>Item 3</li>
</ul>
</nav>
</body>
</html>

The corresponding tree structure is:

A tree structure diagram for an HTML page

Even if you've never seen a tree structure before, it's still clear to see how the HTML maps to the tree in terms of structure and hierarchy. Mapping HTML elements is relatively simple because it's a markup language with well-defined tags and no actual logic.

Wat represents modules that can have multiple functions with varying parameters. To demonstrate the relationship between source code, Wat, and the corresponding tree structure, let's start with a simple C function that adds 2 to the number that is passed in as a parameter:

Here is a C function that adds 2 to the num argument passed in and returns the result:

int addTwo(int num) {
return num + 2;
}

Converting the addTwo function to valid Wat produces this result:

(module
(table 0 anyfunc)
(memory $0 1)
(export "memory" (memory $0))
(export "addTwo" (func $addTwo))
(func $addTwo (; 0 ;) (param $0 i32) (result i32)
(i32.add
(get_local $0)
(i32.const 2)
)
)
)

In Chapter 1What is WebAssembly?, we talked about language concepts associated with the Core Specification (Functions, Linear Memory, Tables, and so on). Within that specification, the Structure section defines each of these concepts in the context of an abstract syntax. The Text Format section of the specification corresponds with these concepts as well, and you can see them defined by their keywords in the preceding snippet (func, memory, table).

Tree Structure:

A tree structure diagram for Wat

The entire tree would be too large to fit on a page, so this diagram is limited to the first five lines of the Wat source text. Each filled-in dot represents a list node (or the contents of a set of parentheses). As you can see, code written in s-expressions can be clearly and concisely expressed in a tree structure, which is why s-expressions were chosen for WebAssembly's text format.

主站蜘蛛池模板: 清水河县| 汝南县| 古丈县| 尖扎县| 龙泉市| 定南县| 万安县| 彰武县| 平果县| 阿克| 绍兴市| 启东市| 永城市| 高雄县| 乐东| 克东县| 石台县| 肃南| 镇安县| 湘潭市| 云和县| 宁海县| 昔阳县| 和田市| 凤冈县| 双柏县| 固镇县| 蒲城县| 泌阳县| 清镇市| 塔河县| 江孜县| 伊金霍洛旗| 北安市| 福安市| 枞阳县| 襄城县| 潼南县| 合川市| 南溪县| 麦盖提县|