Effects and handlers

What

Effects are built into the language as part of normal flow control: you define an effect, perform it, and handle it at a boundary.

Why

This model separates “what happened” from “how to handle it,” which keeps business logic clearer as projects grow.

let Console := effect {
  let readln () : String;
};

How

Define one effect family, perform operations, and add handlers for policy (logging, fallback, default values, reporting).

perform Console.readln();

When

Use effects for cross-cutting concerns: resource usage, command routing, telemetry, and deferred behavior.

handle perform Console.readln() with Console of (
| value => value
| readln(k) => resume "ok"
);

Analogy

Comparable to middleware stacks in web frameworks, but in expression-level form.

Try it

Read all three snippets end-to-end, then continue to Types and generics.

Continue to Types and generics.