Effect-TS Testing Skill

Use this skill when writing or reviewing tests for Effect-based code.

Effect testing rules (for *.spec.* files)

When testing a method that returns an Effect type, use the @effect/vitest package.

• Use import { it } from '@effect/vitest' as opposed to importing it from regular vitest.
• Usage:
  typescriptCopy

  it.effect('test name', () => {
     return Effect.gen(...)
  })
  

• Use it.layer() to provide test dependencies when your Effect requires specific services:
  typescriptCopy

  it.layer(Layer.fresh(IncrementalIdGenerator))((it) => {
    it.effect('test name', () => {
      return Effect.gen(...)
    })
  })
  

Testing for type-safety

Use tsafe package for compile-time type assertions to validate type safety in tests.

• Import: import { assert } from 'tsafe' and import type { Equals } from 'tsafe'
• Usage: assert<Equals<ActualType, ExpectedType>>() - validates exact type equality at compile time

Test time helpers

Never use new Date() in tests. Always use yield* DateTime.now and convert to Date if needed.

import { DateTime, Effect } from 'effect';

const nowDateTime = yield* DateTime.now;
const now = DateTime.toDateUtc(nowDateTime);

Schema decoding with decodeUnknown

When decoding unknown values using Effect schemas, always use the decodeUnknown function exported from zerospin instead of Schema.decodeUnknown directly.

• Import: import { decodeUnknown } from 'zerospin'
• Always specify onExcessProperty: 'error', 'ignore', or 'preserve' based on your needs
• Map errors to ZerospinError with code 'failed-to-decode-unknown'

In Effect.gen contexts:

import { decodeUnknown } from 'zerospin';
import { ZerospinError } from '@zerospin/error';
import { Effect } from 'effect';

const decoded = yield* decodeUnknown({
  onExcessProperty: 'error', // or 'ignore' or 'preserve'
  schema: MySchema,
  value: unknownValue,
}).pipe(
  Effect.mapError((error) => {
    return new ZerospinError({
      code: 'failed-to-decode-unknown',
      message: error.message,
      cause: error,
    });
  })
);

In async/await contexts:

import { decodeUnknown } from 'zerospin';
import { ZerospinError } from '@zerospin/error';
import { Effect } from 'effect';

const decoded = await Effect.runPromise(
  decodeUnknown({
    onExcessProperty: 'error',
    schema: MySchema,
    value: unknownValue,
  }).pipe(
    Effect.mapError((error) => {
      return new ZerospinError({
        code: 'failed-to-decode-unknown',
        message: error.message,
        cause: error,
      });
    })
  )
);