Composing effects — pipe, map, andThen

Effect v3.12.0

A real program is many effects wired together, and the wire is pipe. It threads a value left to right, and Effect.map transforms the success channel with a plain function while leaving any failure untouched.

Here is the mistake that cost me an embarrassing week. When the function you hand back returns its own effect, map does not flatten it, so you end up with an effect of an effect, a value you cannot use. Reach for flatMap and it flattens that right back into a single, usable effect.

Most days you will simply sequence steps, each one returning an effect, and that is exactly what andThen is for: parse the string, then add one. And the quiet payoff of this whole chapter is that a failure in any step rides along in E for free, still visible, never thrown.

import { Effect } from "effect";

// pipe threads a value left→right; map transforms the success channel.
const program = Effect.succeed(2).pipe(
  Effect.map((n) => n + 1),
  Effect.map((n) => n * 10),
);
//    ^? Effect<number, never, never>

console.log(Effect.runSync(program)); // 30

❚❚ beat