Effect

Effect in DXOS

Answer Effect questions from node_modules/effect rather than from recall. What is on disk is pinned to the version this repo compiles against. What you remember is pinned to nothing, and Effect moves.

The pinned copy

Unlike nearly every other package, effect publishes its TypeScript source and its own agent documentation to npm, matched to the exact version this repo compiles against. Its files field ships src/**/*.ts, AGENTS.md, and ai-docs/**, so this is real source on disk, not dist:

effect.website tracks main; the pinned copy tracks what you compile against.

Research that reaches past the pinned copy, to an upstream issue, a migration note or a blog post, has to match the installed version first. pnpm-workspace.yaml pins the effect catalog entry, currently a 4.x release candidate; material written for 3.x describes a different library.

Tests are the one thing npm leaves out: the package has no test/ or dtslint/.

node_modules/effect is a pnpm symlink into .pnpm/effect@<version>/, and it is gitignored, so nothing will surface it for you. Open it by path.

Lookup order

Before writing Effect, in this order, stopping as soon as the answer is settled:

1. Read a neighbour. Find the nearest DXOS file already doing this and copy its shape. House conventions beat upstream defaults.
2. Read node_modules/effect/AGENTS.md, then follow its link to the ai-docs/src/** example for your topic.
3. Grep node_modules/effect/src/ for the exact export. This settles whether an API exists and what its signature is.
4. Check the divergences below. Three DXOS rules override the pinned guide.

Done when every Effect API in the change either came from a neighbouring DXOS file or was confirmed by grep in node_modules/effect/src/.

Where DXOS diverges from the pinned guide

AGENTS.md applies as written except on three points, where following it literally produces broken or off-convention code here.

Domain errors are BaseError.extend, not Schema.TaggedError. BaseError from @dxos/errors supplies a _tag getter, so Effect.catchTag still discriminates, and it carries context, wrap, and is that the rest of the stack expects. Keep the channel typed either way: bare Error or unknown in Effect<A, E, R> erases the recovery catchTag depends on.

import { BaseError } from '@dxos/errors';

export class HttpError extends BaseError.extend('HttpError', 'HTTP request failed') {
  constructor(context: { readonly status: number }) {
    super({ context });
  }
}

Raise BaseError with Effect.fail. AGENTS.md teaches return yield* new SomeError(), which needs an error implementing Cause.YieldableError. Data.Error, Data.TaggedError and Schema.TaggedError all do; BaseError is a plain Error subclass and does not. DXOS domain errors go through Effect.fail(new HttpError({ status: 404 })).

Effect.fnUntraced is the default wrapper. AGENTS.md prefers Effect.fn('name') for the span it attaches. This repo leans the other way, running fnUntraced roughly two to one and reserving the traced form for boundaries worth a span. Either beats a function that returns Effect.gen.

Layer stacks

Composing a layer stack, and test environments above all: read layer-composition.md. One parameterized factory per environment, a flat Layer.empty.pipe(...), provide for private dependencies against provideMerge for shared ones, and the shared memo map, which hands you the same instance across two provides in one run with no compile error to warn you.

Schema, SchemaAST, and pre-migration code

Touching Schema, annotations, toJsonSchema, or anything under SchemaAST: read v4-schema.md. It carries the rule that effect/SchemaAST is imported only through @dxos/effect, the annotation reads that silently return undefined on refined types, mutableKey placement, lost brands, and the emitted-JSON-Schema wire contract.

It also holds a rename table. Reach for it when reading an older file, an old PR or an upstream issue, and when a name you were about to write turns out to have been replaced.