The Effect<A, E, R> type

Effect v3.12.0

If you can read three type parameters, you can read Effect, and that is most of the battle. Start where it feels familiar, with a plain function whose only output is the value it hands back on success. That success value, here just a number, is the A in Effect of A, E, R.

Wrap the same function in an Effect and the whole shape appears. A is the success and E is every way it can fail. Here that is a number or a DivByZero, so a failure is just a value you return, spelled out in the type instead of buried in a stack trace.

The third slot, R, is the one that took me longest to trust. Give an effect a dependency and R stops being never: the effect now refuses to run until you hand it that context. That single rule is the seed of a whole later chapter, so for now just remember the shape.

import { Data, Effect } from "effect";

class DivByZero extends Data.TaggedError("DivByZero")<{}> {}

// Start with a plain function. Its result is the A — the success value.
const divide = (a: number, b: number) => a / b;

const result = divide(10, 2);
//    ^? number

❚❚ beat