Create WebAssembly Command Components

Contributor name: Brian Sletten

Difficulty level: Intermediate

In the “Learn How wasmtime Works with WebAssembly Modules and Components” Shortcut, I showed you some of the mechanics that go into a host application consuming services from WebAssembly components. You saw the host looking specifically for a method called add that took two integers and returned another one. The ability to define behavior and invoke it makes it possible for you to create whatever kind of plug-in interface you might like to build around these composable units. It would get a little old to have to go to all that work for each new interface though.

Remember, in WebAssembly Interface Type (WIT) files, a world is a coordinated set of imports and exports that define a particular shape of a component. Thanks to the standards process, we are starting to see reusable worlds that behave consistently and have common interfaces. WebAssembly command components are one such example. In this Shortcut, I am going to show you how to create them.

A command component adheres to the wasi:cli/command world, which looks like this:

package wasi:cli@0.2.0;
 
world command {
  include imports;
 
  export run;
}

Basically, the command component is provided with common interfaces through the include imports statement above. To avoid redundancy, they are not shown everywhere they are imported, but I will show you momentarily what is included. As per the command world definition, it also exports ...