@systemfsoftware/effect-gherkin-spec

Write Gherkin-style behaviour tests with Effect.

makeFeature({ it }) gives you a Feature builder for organising integration tests as Given/When/Then step pipelines. A Then body is (state, expect) => one check: it asserts the state the last Given or When produced, exactly once — several facts about that state become one check over a record. Every scenario runs under the simulation kernel by default (the zero-preemption schedule, then the profile's seeded schedules); a scenario that must stay on the live clock declares a required reason, which the run report lists.

import { Gherkin, Given, it, makeFeature, Then, When } from '@systemfsoftware/effect-gherkin-spec'
import { Effect } from 'effect'

const Feature = makeFeature({ it })

Feature('Borrowing a library book')
  .withScenarioLayer(BookCheckoutLive)
  .body(({ scenario }) => {
    scenario(
      'A member borrows an available book',
      Gherkin.Do.pipe(
        Given('an available book on the shelf')('book', () => BookCheckoutLive.current()),
        When('the member borrows the book')(() => BookCheckoutLive.borrow('member-42')),
        Then('the book is checked out to the member')((_scope, expect) =>
          Effect.map(BookCheckoutLive.holder(), (holder) => expect(holder).toBe('member-42'))
        ),
      ),
    )

    scenario(
      'A notification that leaves the building uses the live clock',
      { live: 'the notification leaves the process for a real socket' },
      Gherkin.Do.pipe(
        Given('a notification on its way out')('notice', () => Effect.succeed('due-reminder')),
        Then('the socket receives the notice')((scope, expect) => expect(scope.notice).toBe('due-reminder')),
      ),
    )
  })

A whole feature stays on the live clock with Feature('Name').live('the reason it waits on real I/O'), and a scenario outline takes the same reason through opts.live. A live declaration without a reason fails type-checking; there is no boolean form.

Scenario outlines parameterise the same behaviour across example rows, each row running through the same kernel exploration:

Feature('Calculating late fees')
  .withScenarioLayer(LoanLedgerLive)
  .body(({ scenarioOutline }) => {
    scenarioOutline(
      'A late book accrues its daily fee',
      [{ daysLate: 1, fee: 25 }, { daysLate: 3, fee: 75 }],
      ({ daysLate, fee }) =>
        Gherkin.Do.pipe(
          Given('a book returned late')(() => LoanLedgerLive.recordReturn({ daysLate })),
          Then('the ledger shows the accrued fee')((_scope, expect) =>
            Effect.map(LoanLedgerLive.fee(), (charged) => expect(charged).toBe(fee))
          ),
        ),
    )
  })

Install

pnpm add -D @systemfsoftware/effect-gherkin-spec@workspace:^

[!NOTE] This package is private: true — it exists to exercise the effect v4 RC inside the workspace and is never published.

[!NOTE] effect, @systemfsoftware/vitest, and vitest are peer dependencies — you bring your own.