An educational pure functional programming library in TypeScript
Replace type casting with type guards and helper functions
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+2
docs/guides/concepts/branded-types.md
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docs/guides/concepts/branded-types.md
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The compiler catches the bug before the code runs.
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> **Note on casting:** The examples above use `as` for clarity, but in application code, prefer the `brand()` helper function which encapsulates the cast. See the [Smart Constructors](#smart-constructors) section below.
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### How It Works
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The `Brand<B>` type adds a phantom property that only exists at the type level:
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-3
docs/guides/tutorial/07-the-effect-system.md
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-3
docs/guides/tutorial/07-the-effect-system.md
···
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| { _tag: "NotFound"; url: string }
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| { _tag: "ServerError"; status: number }
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// Type guard for ApiError
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const isApiError = (e: unknown): e is ApiError =>
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e !== null &&
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typeof e === "object" &&
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"_tag" in e &&
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(e._tag === "NetworkError" || e._tag === "NotFound" || e._tag === "ServerError")
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const fetchJson = <T>(url: string): Eff<T, ApiError, unknown> =>
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pipe(
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fromPromise(() => fetch(url)),
···
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: fromPromise(() => response.json())
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),
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catchAll(e =>
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e && typeof e === "object" && "_tag" in e
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? fail(e as ApiError)
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isApiError(e)
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? fail(e)
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: fail({ _tag: "NetworkError", message: String(e) })
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)
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) as Eff<T, ApiError, unknown>
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)
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// Usage
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type User = { id: number; name: string }
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-1
docs/guides/tutorial/10-building-a-complete-app.md
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-1
docs/guides/tutorial/10-building-a-complete-app.md
···
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)
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// Mark task as done
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// Helper to create Done status (avoids type assertion)
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const doneStatus = (completedAt: Date): TaskStatus => ({ _tag: "Done", completedAt })
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const completeTask = (id: TaskId): Eff<Task, AppError, AppEnv> =>
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pipe(
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loadTasks(),
···
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(task: Task) => {
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const updated = updateTask(tasks, id, t => ({
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...t,
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status: { _tag: "Done", completedAt: new Date() } as TaskStatus,
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status: doneStatus(new Date()),
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}))
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return pipe(
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saveTasks(updated),
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-8
examples/http-client/with-purus.ts
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examples/http-client/with-purus.ts
···
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pipe,
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match,
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runPromise,
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runPromiseExit,
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} from "../../src/index"
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// =============================================================================
···
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email: string
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}
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// Type guard - validates the shape at runtime without casting
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const isUser = (data: unknown): data is User =>
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typeof data === "object" &&
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data !== null &&
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"id" in data &&
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"name" in data &&
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"email" in data
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// =============================================================================
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// SECTION 2: Fetch as an Effect
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// =============================================================================
···
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fetch(url, { signal: controller.signal })
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.then(async (response) => {
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if (response.ok) {
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const data = await response.json()
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resume(Exit.succeed(data as User))
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const data: unknown = await response.json()
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// Use type guard instead of casting - validates at runtime
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isUser(data)
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? resume(Exit.succeed(data))
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: resume(Exit.fail(HttpError.serverError(500)))
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} else if (response.status === 404) {
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resume(Exit.fail(HttpError.notFound(url)))
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} else {
···
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)
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)
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try {
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const user = await runPromise(request1)
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console.log(`Got user: ${user.name} (${user.email})\n`)
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} catch (e) {
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console.log(`Error: ${handleError(e as HttpError)}\n`)
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}
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// Use runPromiseExit instead of try/catch - no type casting needed
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const exit1 = await runPromiseExit(request1)
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exit1._tag === "Success"
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? console.log(`Got user: ${exit1.value.name} (${exit1.value.email})\n`)
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: exit1._tag === "Failure"
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? console.log(`Error: ${handleError(exit1.error)}\n`)
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: console.log("Interrupted\n")
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// ---------------------------------------------------------------------------
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// Test 2: Short timeout - will fail, but we recover