Native Effect fixtures

This package supports Effect 4: its peer range is effect >=4.0.0 <5, and each release from 4.0.0 through 4.0.1 is tested against the packed adapter. Effect 4 replaced the bundled fast-check with its own effect/Arbitrary engine, which Effect marks as unstable, so a newer Effect 4 release can change generated values. The adapter's metadata names the loaded Effect release, and mimlet doctor reports an untested release as PEER_VERSION_UNTESTED. Newer releases usually work; pin a tested release when you need a tested setup, and report problems in the issue tracker. Toolkit 0.1.0-alpha.3 and earlier target effect@3.22.2; Effect 3 projects should stay on that train.

Pin exact versions when you need to reproduce fixtures; see Getting started.

# Effect 4
npm install --save-dev @mimlet/core @mimlet/effect effect@4.0.1
# Effect 3
npm install --save-dev @mimlet/core@0.1.0-alpha.3 @mimlet/effect@0.1.0-alpha.3 effect@3.22.2

Loaded with Effect 3, this release throws a TypeError that names the required version, and mimlet doctor reports PEER_VERSION_UNSUPPORTED. Effect 4's own type declarations reference DOM types and Disposable. Without skipLibCheck, a project needs the DOM lib plus either the ESNext.Disposable lib or @types/node; either of those alone is not enough in a project without DOM.

import * as S from 'effect/Schema';
import { createSession } from '@mimlet/core';
import { fromEffect } from '@mimlet/effect';
const schema = S.Struct({ age: S.NumberFromString });
const session = createSession({ seed: 123, fingerprint: 'person/v1', provider: 'effect@4.0.0' });
const people = fromEffect(schema);
const input = people.with({ age: '42' }).build(session);
const output = people.with({ age: '42' }).buildValidated(session);

A native arbitrary generates a decoded output and the original encoder turns it into fixture input. This retains native declarations and transformation rules instead of forcing them through JSON. fromEffectAsync uses the native asynchronous encoder; use it for codecs with asynchronous encoding. Synchronous builds over such a codec raise a TypeError that points to fromEffectAsync. fromEffectFactory accepts an explicit input factory for one-way codecs, unsupported arbitrary derivations and application-specific fixture logic.

Generated values come from Effect's arbitrary engine, which favors edge cases: strings such as __proto__, toString, empty strings, lone surrogates and control characters, and dates at the limits of the Date range. For presentable or persisted data, set fields with .with() or use fromEffectFactory. Annotate the factory's session parameter; unannotated, TypeScript infers it as never.

import type { GenerationSession } from '@mimlet/core';
import { fromEffectFactory } from '@mimlet/effect';
const adults = fromEffectFactory(schema, (session: GenerationSession) => ({
  age: String(session.integer(18, 99)),
}));
const adult = adults.buildValidated(session);

A generic helper's return type can stay inferred. When a lint rule such as @typescript-eslint/explicit-function-return-type requires one, name the builder for a Schema.Codec<A, I>: fromEffect(schema) returns EffectBuilder<A, I> and fromEffectFactory(schema, factory) returns EffectFactoryBuilder<A, I, F>, where F is the factory's type, for example (session: GenerationSession) => I. fromEffectAsync(schema) returns AsyncSchemaBuilder<I, A, [session: GenerationSession]> from @mimlet/core. Each type is exactly what its function returns. While I is unresolved, TypeScript cannot tell whether the factory returns a promise, so call .with() and .withFactory() on a factory builder where the helper is called.

effectFields(schema) lists a struct's top-level keys as encoded, which is what builders take: a key renamed with Schema.encodeKeys is listed by its encoded name. Pass it to fluent() from @mimlet/core for a withX() setter per field. It reads the schema's AST only, so it works with every entry point and in a generic helper:

import { fluent } from '@mimlet/core';
import type * as S from 'effect/Schema';
import { effectFields, fromEffect } from '@mimlet/effect';
function rows<A, I extends object>(schema: S.Codec<A, I>) {
  return fluent(fromEffect(schema), effectFields(schema));
}
const person = rows(schema).withAge('42').buildValidated(session);

Non-struct schemas such as unions throw a TypeError; symbol keys get no setter, and structs with index signatures (records, StructWithRest) get no typed setters. See named setters.

effectAdapter exposes the original source, native Standard Schema validation, input/output checks, sync/async encode/decode, and native Effect 4 input/output arbitraries. Input checking checks the encoded shape, not the success of a subsequent decode. Input-arbitrary shrinking re-encodes each native output shrink. Property tests run through Effect's own Arbitrary.checkEffect, which shrinks and returns a replay token; these arbitraries are not fast-check arbitraries and are not passed to @mimlet/fast-check.

import * as A from 'effect/Arbitrary';
import * as E from 'effect/Effect';
import * as S from 'effect/Schema';
import { effectAdapter } from '@mimlet/effect';
const counts = effectAdapter(S.NumberFromString.pipe(S.decodeTo(S.Int)));
const report = E.runSync(
  A.checkEffect(counts.inputArbitrary(), (input) => Number(input) < 5, { seed: 1 })
);
// A failing report carries shrunkInput and a replay token for checkEffect({ replay }).

inputArbitrary() encodes each sample synchronously, so it cannot be used with codecs that encode asynchronously. For those, check outputArbitrary() with an effectful property that calls adapter.encodeAsync, and run it with E.runPromise.

Transformations and arbitrary annotations must remain pure and terminating for property testing. Encoding failures propagate, not trigger hidden retries. Validation invokes the native Standard Schema entry once per validated build, after builder overrides. The original schema owns parsing behavior; excess object properties default to errors and can be configured through parseOptions. Each adapter converts its own Standard Schema wrapper, so two adapters over one schema keep their own parse options and the caller's schema object is not modified.

Schemas requiring Effect services are not accepted by these constructors; provide services in your own factory/validation wrapper. This restriction is type-tested. Async schema execution is supported through async build methods, not detected by inspecting private AST nodes. A schema whose native generation is asynchronous raises a TypeError from synchronous builds that points to fromEffectAsync. Native failures and issues remain available and may contain application values; no reports are logged or transmitted by this package.

Sessions are explicit and caller-versioned: include the schema, annotations, codec behavior and native dependency versions in your replay identity. Unlike the JSON Schema-based adapters, there is no default session. fromEffect and fromEffectAsync require one for every build and list call; omitting it is a type error and, from JavaScript, raises a TypeError before any generation. To opt in, pass your own defaultSession, such as () => createSession({ seed: 1, fingerprint: 'person/v1', provider: 'effect@4.0.1' }); the builders and fromEffectFactory type it, and builds may then omit the session. Every native sampling option (count, size, discards and seed) is passed explicitly, so Arbitrary.configureGlobal cannot change generated fixtures, and the adapter does not modify native configuration. Effect does not promise identical samples across releases; include the Effect version (for example effect@4.0.1, also in effectAdapter(schema).metadata.version) in your session provider. Native schemas/annotations are trusted code; this is not an interruptible worker or a guarantee of arbitrary termination for opaque user filters.

Native peer declarations use Web platform types. Their compatibility consumers include DOM types (and Node types for ArkType); this does not add DOM dependencies to the schema-free core. The pinned Zod Standard Schema implementation probes async refinements synchronously before retrying them asynchronously. The generic standards path calls its validator entry once but does not override that native behavior. Use an explicit safeParseAsync validation wrapper for effectful Zod refinements that must not be probed twice.