If you like to learn by doing, this tutorial will walk through how to build, compose, and run components through a calculator example. Calculators can conduct many operations: add, subtract, multiply, and so on. In this example, each operation will be a component, that will be composed with an eval-expression component that will evaluate the expression using the expected operator. With one operation per component, this calculator is exaggeratedly granular to show how independent logic of an application can be contained in a component. In production, components will likely have a larger scope than a simple mathematical operation.

Our eventual solution will involve three components: one for the calculator engine, one for the addition operation, and one for the command-line interface. Once we have built these as separate Wasm components, we will compose them into a single runnable component, and test it using the wasmtime CLI.

The calculator interface

For tutorial purposes, we are going to define all our interfaces in one WIT package (in fact, one .wit file). This file defines:

  • An interface for the calculator itself. We'll use this later to carry out calculations. It contains an evaluate function, and an enum that delineates the operations that can be involved in a calculation. In this tutorial, the only operation is add.
  • Interfaces for the various operations the calculator might need to carry out as part of a calculation. For the tutorial, again, the only interface we define is for the "add" operation.
  • A world describing the calculator component. This world exports the calculator interface, meaning that other components can call it to perform calculations. It imports the operation interfaces (such as "add"), meaning it relies on other components to perform those operations.
  • A world describing each operator component. Again, there's just the "adder" world right now, and this exports the "add" interface, meaning that components such as the calculator can call it when they need to add numbers.
  • A world describing the "primary" app component, which imports the "calculate" interface. This is the component will take in command line arguments and pass them to the "eval-expression" function of the calculator component.
// calculator.wit
package docs:calculator@0.1.0;

interface calculate {
    enum op {
    eval-expression: func(op: op, x: u32, y: u32) -> u32;

interface add {
    add: func(a: u32, b: u32) -> u32;

world adder {
    export add;

world calculator {
    export calculate;
    import add;

world app {
    import calculate;

Create an add component

Reference the language guide and authoring components documentation to create a component that implements the adder world of calculator.wit. For reference, see the completed example.

Create a calculator component

Reference the language guide and authoring components documentation to create a component that implements the calculator world of calculator.wit. For reference, see the completed example. The component should import the add function from the adder world and call it if the op enum matches add.

Create a command component

A command is a component with a specific export that allows it to be executed directly by wasmtime (or other wasm:cli hosts). The host expects it to export the wasi:cli/run interface, which is the equivalent of the main function to WASI. cargo-component will automatically resolve a Rust bin package with a main function to a component with wasi:cli/run exported. Scaffold a new Wasm application with a command component:

cargo component new command --command

This component will implement the app world, which imports the calculate interface. In Cargo.toml, point cargo-component to the WIT file and specify that it should pull in bindings for the app world:

path = "../path/to/calculator.wit"
world = "app"

Now, implement a command line application that:

  1. takes in three arguments: two operands and the name of an operator ("1 2 add")
  2. parses the operator name and ensures it is supported in the op enum
  3. calls the calculate interface's eval_expression, passing in the arguments.

For reference, see a completed example.

Composing the calculator

Now, we are ready to bring our components together into one runnable calculator component, using wasm-tools. We will first compose the calculator component with the add component to satisfy it's imports. We then compose that resolved calculator component with the command component to satisfy its calculate imports. The result is a command component that has all its imports satisfied and exports the wasi:cli/run function, which can be executed by wasmtime.

wasm-tools compose calculator.wasm -d adder.wasm -o composed.wasm
wasm-tools compose command.wasm -d composed.wasm -o final.wasm

If you'd prefer to take a more visual approach to composing components, see the documentation on composing components with wasmbuilder.app.

Running the calculator

Now it all adds up! Run the final component with the wasmtime CLI, ensuring you are using a [v14.0.0 or greater release](https://github.com/bytecodealliance/wasmtime/releases), as earlier releases of the wasmtime` command line do not include component model support.

wasmtime run final.wasm 1 2 add
1 + 2 = 3

To infinity and beyond!

To expand the exercise to add more components, modify calculator.wit to add another operator world and expand the op enum. Then, modify the command and calculator components to support the expanded enum.

Another extension of this tutorial could be to remove the op enum and instead modify eval-expression to take in a string that can then be parsed to determine which operator component to call. Maybe this parser is a component of its own?!