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Alloy

Alloy is a framework for code generation that borrows concepts from JavaScript front-end frameworks like React and Solid to provide highly ergonomic and productive source code generation. Use JSX syntax or string templates to define source files and the language elements within them. Alloy helps handle a lot of the hard bits like building source text from strings, linking declarations and importing necessary dependencies, applying naming conventions, formatting, and generating syntax for a number of languages.

import * as ay from "@alloy-js/core";
import * as ts from "@alloy-js/typescript";

const helloWorldRef = ay.refkey();

const res = ay.render(
  <ay.Output>
    <ay.SourceFile path="readme.md" filetype="markdown">
      This is a sample output project.
    </ay.SourceFile>

    <ts.SourceFile path="test1.ts">
      <ts.VarDeclaration export name="foo" refkey={helloWorldRef}>
        "Hello world"
      </ts.VarDeclaration>
    </ts.SourceFile>

    <ts.SourceFile path="test2.ts">
      <ts.VarDeclaration name="v">
        {helloWorldRef}
      </ts.VarDeclaration>
      console.log(v);
    </ts.SourceFile>
  </ay.Output>
);

console.log(res[2].contents);
// import { foo } from "./test2.ts";
//
// const v = foo;

Or, if you prefer, you can use string templates:

import { refkey, render, code } from "@alloy-js/core";
import * as ay from "@alloy-js/core/stc";
import * as ts from "@alloy-js/typescript/stc";

const helloWorldRef = refkey();

const res = render(
  ay.Output().children(
    ay.SourceFile({ path: "readme.md", filetype: "markdown" }).code`
      This is a sample output project.
    `,

    ts.SourceFile({ path: "test1.ts" }).children(
      ts.VarDeclaration({ export: true, name: "foo", refkey: helloWorldRef }).code`
        "Hello world"
      `
    ),

    ts.SourceFile({ path: "test2.ts" }).code`
      ${ts.VarDeclaration({ name: "v" }).children(helloWorldRef)}
      
      console.log(v);
    `
  )
);

console.log(res[2].contents);
// import { foo } from "./test2.ts";
//
// const v = foo;

This project is in pre-beta. Expect everything to change. Feedback is especially appreciated. The docs are not great but are being worked on.

Dependencies

  • pnpm
  • node 20+

Installation

Use this template to get started.

Alternatively, you can find the latest packages on GitHub by clicking the green checkbox next to the latest commit and clicking "details" next to Continuous Releases. Alloy will be published to NPM in the coming weeks.

Building

Clone the repo, then:

> pnpm install
> pnpm build

Supported Languages

  • TypeScript: @alloy-js/typescript

More are coming soon.

Main APIs

  • @alloy-js/core
    • Components:
      • Declaration: Create a declaration. May either pass symbol with the symbol of the declaration, or else can pass name and refkey and a generic symbol is created. Provides DeclarationContext.
      • Indent: Indents its contents one level. Provides IndentContext.
      • Output: The top-level Alloy component. Pass namePolicy to provide the name policy for all your declared names. Pass externals to provide external symbols. Provides NamePolicyContext and BinderContext.
      • Scope: Create a scope which symbols can be declared in. May either pass value to provide the scope object, or else pass name and type and a scope will be created. Provides ScopeContext.
      • SourceDirectory: A directory in your output. Pass path to set the relative path of the directory. Provides SourceDirectoryContext.
      • SourceFile: A file in your output. Pass path to set the relative path of the file. Pass filetype to set the file type to any string. Provides SourceFileContext.
    • APIs:
      • code: A template literal tag for output source text.
      • createContext: Create a context object.
      • createNamePolicy: Create a name policy. Provide to the namePolicy prop of Output.
      • render: Renders a component, fragment, or string template into a list of directories, files, and source text.
      • useContext: Get the value of the provided context object.
      • useBinder: Get the current binder.
      • useScope: Get the current scope.
      • useNamePolicy: Get the current name policy.
      • stc(component: ComponentDefinition): Wrap a functional component for use in string templates.
      • Reactive utilities: ref, shallowRef, reactive, shallowReactive, memo, computed, effect, untrack.
  • @alloy-js/core/stc: String template components for all the core components.
  • @alloy-js/typescript
    • APIs:
      • createPackage(descriptor): Create symbols for an external package, such as a dependency from npm. Pass the result to the externals prop of the Output component.
      • node: Symbol definitions for node built-ins. Pass any you use to the externals prop of the Output component.
    • Components
      • Structure
        • BarrelFile: Create a file which exports all the files contained within the current directory.
        • PackageDirectory: A directory for a package. Generates a package.json and tsconfig.json that are updated depending on the package contents.
        • SourceFile: A TypeScript module source file. Pass export to export the source file from the current package (sets package.json exports). The source file will import anything referenced within it.
      • Declarations
        • Declaration: Declares a symbol in the current scope. Pass export or default to control how this symbol is exported from the module. children are the syntax for the declaration. All declaration forms take these props.
        • FunctionDeclaration: Declares a function. Pass parameters or returnType to set those. children is the function body.
        • InterfaceDeclaration: Declares an interface. Define members by putting InterfaceMember in the children.
        • TypeDeclaration: A TypeScript type declaration. children is the initializer of the declaration.
        • VarDeclaration: Declares a const, let, or var. children is the initializer.
      • Expressions
        • ArrayExpression: A JavaScript array literal. Pass jsValue to populate it with data.
        • ObjectExpression: A JavaScript object literal. Pass jsValue to populate it with data, or pass children with ObjectPropertys.
        • Reference: Create a reference to a declaration. Pass the refkey of the declaration.
        • ValueExpression: A JavaScript value. Pass jsValue to populate it with data. Handles any JSON value.
  • @alloy-js/typescript/stc: String template components for all the TypeScript components.

Basic Concepts

Components

Components are JavaScript functions that receive parameters (called props) that are provided by the user of the component. The component then does computation based on these props and the component's context, and returns children which represent the component's contents. Components may also receive a prop named children which allows component users to provide children that should be placed somewhere within the component.

interface SayHelloProps {
  name: string;
}

function SayHello(props: SayHelloProps) {
  return (
    <>
      Hello {props.name}!
    <>
  );
}

const text = <SayHello name="Brian">);
renderString(text) // "Hello Brian"

The example above uses JSX syntax to define and reference components, but you can use string template literals as well. However, when using string templates, you must use the code string template tag, and you cannot call the component directly. Instead, the component must be wrappe into a string template component. You can import string template components from e.g. @alloy-js/core/stc or @alloy-js/typescript/stc. You can also import the stc function to create this wrapper for your own components:

interface SayHelloProps {
  name: string;
}

function SayHello(props: SayHelloProps) {
  return code`
    Hello ${props.name}!
  `
}

const SayHelloStc = stc(SayHello);

const text = SayHelloStc({ name: "Brian" });
renderString(text) // "Hello Brian"

Context

It is often useful for a component to expose data to components that are nested underneath. This can be done by passing props, but props are burdensome when the component which needs the data is deeply nested. Instead, your component can create and provide context via createContext(), and any nested components can retrieve that context via useContext().

export interface NameContext {
  name: string
}

export const NameContext = createContext<NameContext>();

function Person(props) {
  return <NameContext.Provider value="Brian">
    {props.children}
  </NameContext.Provider>
}

// in some other component
function Greeting(props) {
  const name = useContext(NameContext);

  return <>Hello {name}</>;
}

Many of the built-in components provide context. For example, Output provides the binder, SourceDirectory provides directory metadata, Declaration provides the current declaration.

Declarations and references

When you create a declarations, you need to provide the declaration name, and optionally a refkey. Some declarations may have additional props to control things like whether its exported or private. The name you provide may look different in the output depending on your situation, for example a name policy may turn camelCase names into snake_case, or a conflict with another like-named declaration might disambiguate one or the other with a number.

One of the more challenging bits of doing codegen is generating references to things you've declared. Alloy makes this painless with refkeys. A refkey is a unique identifier for a symbol you declare in your output code. When you reference a refkey, Alloy calculates the necessary reference syntax along with any imports, package dependencies, or other such things needed for the reference to work.

You can create a refkey by calling refkey(). Every time this function is called, you get a new refkey.

Often you want to create a declaration for a JavaScript value you're holding, like a schema you're converting to source text. In these cases, you don't need to pass along the refkey separately. Instead, you can call refkey(schemaObj), and you'll get the same refkey whenever you pass the same schema.

Sometimes you might want to emit multiple declarations for the same schema, and in these cases you can pass multiple params to refkey, for example refkey(schemaObj, "deserialize") and refkey(schemaObj, "serialize").

Reactivity

Alloy uses fine-grained reactivity to update the rendered source code as the tree is being built. This is achieved by depending on the emerging standard for Signals as implemented in the @vue/reactivity library. With Signals, you can read and update data normally, and anything that depends on that data will also update. This is how things like TypeScript's barrel file works - the barrel file maps the current files in the directory to a list of import statements, and because the list of current files is reactive, whenever it changes, the list of imports change.

There are two primary ways to create a reactive data - ref and reactive. ref creates a single data cell whose value you can get and update by via ref.value. reactive turns an entire object into a reactive object but does not work on non-objects and you cannot reassign the object.

In order for reactive data to propagate, you must observe reactive data in a reactive context. Generally speaking, this is within a JSX template, or inside the callback of a memo or computed. Note that the non-JSX code within a component function is not reactive, so it is best to avoid reading reactive data there.

interface ChildCounterContext {
  increment(): void;
}
const ChildCounterContext = createContext<ChildCounterContext>();

function ChildCounter() {
  const numChildren = ref(0);

  // this will always be 0 and never update because this is not
  // a reactive context.
  const initialValue = numChildren.value;

  // however its fine to make changes
  const context: ChildCounterContext = {
    increment() {
      numChildren.value++;
    }
  }

  return <ChildCounterContext.Provider value={context}>
    Number of children: {numChildren}
    <Child />
    <Child />
  </ChildCounterContext.Provider>
}

function Child() {
  const childContext = useContext(ChildCounterContext)!;
  childContext.increment();
  return "child";
}

renderString(<ChildCounter />);
// Number of children: 2
// child
// child

The two primary ways to compute values that depend on other reactive values are memo and computed. memo returns a function which when called gives the current value of the computation. Computations which depend on the memo are only updated when the memoized value changes. computed returns a ref, and the ref is updated whenever the computation is run. Generally, memo is preferred. Whichever you use, when embedded within JSX or within code-tagged string templates, the value will be unwrapped, so no need to call the memo or access .value on a ref.

function MemoExample() {
  const v = ref(2);
  const twiceValue = memo(() => v.value * 2);
  const fourTimesValue = computed(() => twiceValue() * 2);

  return <>
    value: {v}
    twiceValue: {twiceValue}
    fourTimesValue: {squaredValue}
  </>
}

renderString(<MemoExample />);
// value: 1
// twiceValue: 2
// fourTimesValue: 4

Whitespace Rules

Whether using JSX or string templates, whitespace is processed according to the following rules:

Any leading and trailing line breaks are ignored

renderString(<>
x
</>); // "x"

The first significant line sets the base indent

renderString(<>
  x
  y
</>); // "x\ny"

Lines which increase indent set indent level for any contents on that line

renderString(<>
  base
    {"a\nb"}
</>);
// base
//   a
//   b