## Overview
`wac` is a tool for composing [WebAssembly Components](https://github.com/WebAssembly/component-model)
together.
The tool uses the WAC (pronounced "whack") language to define how components
composed together.
## The `wac` Language
The `wac` language is a declarative superset of [`wit`](https://component-model.bytecodealliance.org/design/wit.html)
for describing how components are composed together.
As an example, imagine two components name.wasm and greeter.wasm.
The wit for name.wasm is:
```wit
package example:name;
world name {
/// Exporting a 'name' function that returns a name to greet.
export name: func() -> string;
}
```
And the wit for greeter.wasm is:
```wit
package example:greeter;
world greeter {
/// Importing a 'name' function that returns the name to greet.
import name: func() -> string;
/// Exporting a 'greet' function that returns a greeting using the name.
export greet: func() -> string;
}
```
The following is an example of a wac file that composes these two components together
by plugging name.wasm's "name" export into greeter.wasm's "name" import.
```wac
package example:composition;
// Instantiate the `name` component
let n = new example:name {};
// Instantiate the `greeter` component by plugging its `name`
// import with the `name` export of the `name` component.
let greeter = new example:greeter {
name: n.name,
};
// Export the greet function from the greeter component
export greeter.greet;
```
The result of encoding this composition is a single component that
does not import anything and only exports the "greet" function.
For a full description of the `wac` language see [the language guide](LANGUAGE.md).
## Installation
To install the `wac` CLI from source, run the following command:
```
cargo install wac-cli
```
If you have the [cargo-binstall](https://github.com/cargo-bins/cargo-binstall)
utility installed, `wac` CLI can also be installed via a prebuilt
release artifact, saving time on the installation:
```
cargo binstall wac-cli
```
## Usage
The `wac` CLI tool has the following commands:
* `wac plug` - Plugs the imports of a component with one or more other components.
* `wac compose` - Compose WebAssembly components using the provided WAC source file.
* `wac targets` - Determines whether a given component conforms to the supplied wit world.
* `wac parse` - Parses a composition into a JSON representation of the AST.
* `wac resolve` - Resolves a composition into a JSON representation.
### Quick & Easy Compositions
To do simple compositions, use the `wac plug` command:
```
wac plug my-socket.wasm --plug my-plug.wasm -o plugged.wasm
```
Or mixing in packages published to a Warg registry:
```
wac plug my-namespace:package-name --plug some-namespace:other-package-name -o plugged.wasm
```
### Checking Whether a Component Implements a World
To see whether a given component implements a given world, use the `wac targets` command:
```
wac targets my-component.wasm my-wit.wit
```
If `my-component.wasm` implements the world defined in `my-wit.wit` then the command will succeed. Otherwise, an error will be returned.
If `my-wit.wit` has multiple world definitions, you can disambiguate using the `--world` flag.
### Encoding Compositions
To perform a composition, use the `wac compose` command:
```
wac compose -t input.wac
```
This will use `input.wac` to perform the composition and write the text
representation of the component to stdout.
```
wac compose -o output.wasm input.wac
```
This will perform the composition specified in `input.wac` and output a WebAssembly component named `output.wasm`.
#### Dependencies
By default, `wac` will create a component that embeds its dependencies (i.e. packages
referenced in a WAC source file) inside of itself rather than importing those dependencies;
to cause dependencies to be imported in the output component, use the
`--import-dependencies` flag:
```
wac compose --import-dependencies -o output.wasm input.wac
```
Dependencies may be located within a `deps` subdirectory, with an expected structure of:
```
deps/
├─ /
│ ├─ .wasm
``````
The dependency may be also be a WIT file or a directory containing a WIT package:
```
deps/
├─ /
│ ├─ /
│ │ ├─ a.wit
│ │ ├─ ...
```
The `--deps-dir` CLI option may be used to specify a different directory to
search for dependencies.
The location of specific dependencies may also be specified with the `--dep` CLI option:
```
wac compose --dep foo:bar=./baz.wasm -o output.wasm input.wac
```
By default, dependencies must be binary-encoded WebAssembly components; to
enable support for WAT files, use the `wat` build-time feature.
If built with default features, then dependencies may be
automatically resolved from a Warg registry and do not need to exist in the
`deps` subdirectory or specified via the `--dep` CLI option.
