Type Boundary Patterns

The Problem

Vendors like BetterAuth provide $Infer types:

type Session = typeof auth.$Infer.Session;

This gives compile-time shape. Runtime data is still UNTRUSTED until parsed.

The Lie

// DANGEROUS
const session = await auth.api.getSession({ headers });
return session as AuthSession; // LIES TO COMPILER

The Solution

import { Schema, Effect } from "effect";

// 1. Define Schema matching expected shape
// CRITICAL: Use DateFromSelf for vendor Date objects, Date for ISO strings
const AuthSessionSchema = Schema.Struct({
  session: Schema.Struct({
    id: Schema.String,
    userId: Schema.String,
    token: Schema.String,
    expiresAt: Schema.DateFromSelf, // Vendor returns Date, not string
  }),
  user: Schema.Struct({
    id: Schema.String,
    email: Schema.String,
    phoneNumber: Schema.optional(Schema.String),
  }),
});

// 2. Derive type FROM schema
type AuthSession = typeof AuthSessionSchema.Type;

// 3. Parse at boundary
const getSession = (header: string | undefined) =>
  Effect.gen(function* () {
    const raw = yield* betterAuthGetSession(header);
    if (!raw) return null;

    return yield* Schema.decodeUnknown(AuthSessionSchema)(raw).pipe(
      Effect.mapError((e) => new AuthError({ code: 'INTERNAL', message: String(e) }))
    );
  });

Schema.Date vs Schema.DateFromSelf

BetterAuth, Drizzle ORM, and most Node libraries return actual Date objects. Use Schema.DateFromSelf for these.

Key Principle