debug-effect-programs

description: Use Effect.tap and logging to inspect values without changing program flow. globs: "**/*.ts" alwaysApply: true

Debug Effect Programs

Rule: Use Effect.tap and logging to inspect values without changing program flow.

Example

import { Effect, pipe } from "effect"

// ============================================
// 1. Using tap to inspect values
// ============================================

const fetchUser = (id: string) =>
  Effect.succeed({ id, name: "Alice", email: "[email protected]" })

const processUser = (id: string) =>
  fetchUser(id).pipe(
    // tap runs an effect for its side effect, then continues with original value
    Effect.tap((user) => Effect.log(`Fetched user: ${user.name}`)),
    Effect.map((user) => ({ ...user, processed: true })),
    Effect.tap((user) => Effect.log(`Processed: ${JSON.stringify(user)}`))
  )

// ============================================
// 2. Debug a pipeline
// ============================================

const numbers = [1, 2, 3, 4, 5]

const pipeline = Effect.gen(function* () {
  yield* Effect.log("Starting pipeline")

  const step1 = numbers.filter((n) => n % 2 === 0)
  yield* Effect.log(`After filter (even): ${JSON.stringify(step1)}`)

  const step2 = step1.map((n) => n * 10)
  yield* Effect.log(`After map (*10): ${JSON.stringify(step2)}`)

  const step3 = step2.reduce((a, b) => a + b, 0)
  yield* Effect.log(`After reduce (sum): ${step3}`)

  return step3
})

// ============================================
// 3. Debug errors
// ============================================

const riskyOperation = (shouldFail: boolean) =>
  Effect.gen(function* () {
    yield* Effect.log("Starting risky operation")

    if (shouldFail) {
      yield* Effect.log("About to fail...")
      return yield* Effect.fail(new Error("Something went wrong"))
    }

    yield* Effect.log("Success!")
    return "result"
  })

const debugErrors = riskyOperation(true).pipe(
  // Log when operation fails
  Effect.tapError((error) => Effect.log(`Operation failed: ${error.message}`)),

  // Provide a fallback
  Effect.catchAll((error) => {
    return Effect.succeed(`Recovered from: ${error.message}`)
  })
)

// ============================================
// 4. Trace execution flow
// ============================================

const step = (name: string, value: number) =>
  Effect.gen(function* () {
    yield* Effect.log(`[${name}] Input: ${value}`)
    const result = value * 2
    yield* Effect.log(`[${name}] Output: ${result}`)
    return result
  })

const tracedWorkflow = Effect.gen(function* () {
  const a = yield* step("Step 1", 5)
  const b = yield* step("Step 2", a)
  const c = yield* step("Step 3", b)
  yield* Effect.log(`Final result: ${c}`)
  return c
})

// ============================================
// 5. Quick debug with console
// ============================================

// Sometimes you just need console.log
const quickDebug = Effect.gen(function* () {
  const value = yield* Effect.succeed(42)
  
  // Effect.sync wraps side effects
  yield* Effect.sync(() => console.log("Quick debug:", value))
  
  return value
})

// ============================================
// 6. Run examples
// ============================================

const program = Effect.gen(function* () {
  yield* Effect.log("=== Tap Example ===")
  yield* processUser("123")

  yield* Effect.log("\n=== Pipeline Debug ===")
  yield* pipeline

  yield* Effect.log("\n=== Error Debug ===")
  yield* debugErrors

  yield* Effect.log("\n=== Traced Workflow ===")
  yield* tracedWorkflow
})

Effect.runPromise(program)