Functype User Guide

Overview

Transform TypeScript code to use functype - a Scala-inspired functional programming library providing type-safe alternatives to null checks, exceptions, and imperative patterns. This skill helps integrate Option, Either, Try, List, IO, Task, and other functional types into projects.

When to Use This Skill

Trigger this skill when users:

•Convert imperative code to functional patterns
•Look up functype APIs or methods
•Handle nullable values or optional chaining
•Replace try-catch with functional error handling
•Work with immutable collections
•Compose side effects, async operations, or dependency injection with IO
•Run async operations with cancellation or progress tracking (Task)
•Debug functype code or understand error messages

Quick Start

Installation

bash

npm install functype
# or
pnpm add functype

Core Imports

typescript

// Import from main bundle
import { Option, Either, Left, Right, Try, List, IO, Tag, Task, Layer } from "functype"

Constructor vs Companion Methods

Functype collections provide multiple ways to create instances:

Method	Use When	Example
`List([...])`	Creating from existing array	`List(existingArray)`
`List.of(...)`	Inline literal values	`List.of(1, 2, 3)`
`List.empty()`	Empty collections (typed)	`List.empty<number>()`

Decision Guide

Use Constructor List([...]) when:

•Converting existing arrays: List(data.items)
•Spreading iterables: List([...set])
•Variables holding arrays: List(myArray)

Use List.of(...) when:

•Inline literal values: List.of(1, 2, 3)
•Cleaner for small fixed lists: List.of("a", "b", "c")
•No need to wrap in array brackets

Use List.empty() when:

•Starting with empty collection: List.empty<User>()
•Type parameter needed but no initial values
•Returns singleton (efficient for repeated calls)

Examples

typescript

// Constructor - wrapping existing data
const users = List(fetchedUsers)
const items = List([...existingSet])

// .of() - inline literals
const colors = List.of("red", "green", "blue")
const primes = Set.of(2, 3, 5, 7, 11)

// .empty() - typed empty collections
const errors = List.empty<string>()
const cache = Map.empty<string, User>()

Pattern Conversion Guide

Null/Undefined Checks → Option

Before (Imperative):

typescript

if (value !== null && value !== undefined) {
  return value.toUpperCase()
}
return ""

After (Functype):

typescript

Option(value)
  .map((v) => v.toUpperCase())
  .orElse("")

Optional Chaining → Option Chain

Before:

typescript

const url = user?.profile?.avatar?.url

After:

typescript

const url = Option(user)
  .flatMap((u) => Option(u.profile))
  .flatMap((p) => Option(p.avatar))
  .map((a) => a.url)
  .orElse("/default-avatar.png")

Try-Catch → Try or Either

Before:

typescript

try {
  return JSON.parse(str)
} catch (e) {
  return null
}

After (with Try):

typescript

Try(() => JSON.parse(str))
  .toOption()
  .orElse(null)

After (with Either):

typescript

Try(() => JSON.parse(str))
  .toEither()
  .fold(
    (error) => `Parse failed: ${error.message}`,
    (data) => data,
  )

Array Operations → List

Before:

typescript

array.filter((x) => x > 0).map((x) => x * 2)

After:

typescript

List(array)
  .filter((x) => x > 0)
  .map((x) => x * 2)
  .toArray()

If-Else Chains → Cond

Before:

typescript

if (x > 10) {
  return "big"
} else if (x > 5) {
  return "medium"
} else {
  return "small"
}

After:

typescript

import { Cond } from "functype"

Cond.start<string>()
  .case(x > 10, "big")
  .case(x > 5, "medium")
  .otherwise("small")

Switch Statements → Match

Before:

typescript

switch (status) {
  case "success":
    return data
  case "error":
    return null
  default:
    return undefined
}

After:

typescript

import { Match } from "functype"

Match(status)
  .case("success", () => data)
  .case("error", () => null)
  .exhaustive()

Common Use Cases

Validation with Either

typescript

import { Either, Left, Right } from "functype"

function validateEmail(email: string): Either<string, string> {
  return email.includes("@") ? Right(email) : Left("Invalid email format")
}

function validateUser(user: any): Either<string, User> {
  return validateEmail(user.email)
    .map((email) => ({ ...user, email }))
    .flatMap((u) => (u.age >= 18 ? Right(u) : Left("Must be 18 or older")))
}

const result = validateUser({ email: "test@example.com", age: 20 }).fold(
  (error) => console.error(error),
  (user) => console.log("Valid user:", user),
)

Safe API Calls with Option

typescript

import { Option } from "functype"

interface User {
  id: string
  name: string
  email?: string
}

function getUserEmail(userId: string): Option<string> {
  return Option(fetchUser(userId))
    .flatMap((user) => Option(user.email))
    .filter((email) => email.includes("@"))
}

const email = getUserEmail("123").orElse("no-reply@example.com")

Error Recovery with Try

typescript

import { Try } from "functype"

const parseConfig = Try(() => JSON.parse(configStr))
  .recover((error) => {
    console.warn("Using default config:", error)
    return defaultConfig
  })
  .map((config) => validateConfig(config))

Collection Pipeline with List

typescript

import { List } from "functype"

const users = List([
  { name: "Alice", hobbies: ["reading", "coding"] },
  { name: "Bob", hobbies: ["gaming", "music"] },
])

const allHobbies = users
  .flatMap((user) => List(user.hobbies))
  .toSet() // Remove duplicates
  .toArray()

Additional Data Structures

IO<R,E,A> - Effect Type

IO is a lazy, composable effect type with typed errors and dependency injection:

•R = Requirements (environment/dependencies needed)
•E = Error type (typed failures)
•A = Success type (value produced on success)

typescript

import { IO, Tag, Layer } from "functype"

// Creation
IO.sync(() => computation()) // Sync operation
IO.succeed(value) // Pure success
IO.fail(error) // Pure failure
IO.async(() => promise) // Async operation
IO.tryPromise({
  // Promise with error mapping
  try: () => fetch(url),
  catch: (e) => new NetworkError(e),
})

// Dependency injection
const Database = Tag<DatabaseService>("Database")
const dbEffect = IO.service(Database) // Access a service
const program = dbEffect.flatMap((db) => IO.sync(() => db.query()))
program.provide(Layer.fromValue(Database, myDb)) // Provide deps

// Generator do-notation (cleaner syntax)
const program = IO.gen(function* () {
  const db = yield* IO.service(Database)
  const user = yield* IO.tryPromise(() => db.findUser(id))
  return user
})

// Execution
await effect.run() // Returns Promise<A>
effect.runSync() // Returns A (throws if async)
await effect.runEither() // Returns Promise<Either<E,A>>
await effect.runExit() // Returns Promise<Exit<E,A>>

Tuple - Type-safe Fixed-length Array

typescript

import { Tuple } from "functype"

const pair = Tuple(42, "hello")
pair.first() // 42
pair.second() // "hello"
pair.mapFirst((x) => x * 2) // Tuple(84, "hello")
pair.swap() // Tuple("hello", 42)
pair.apply((a, b) => a + b.length) // 47
pair.concat(Tuple(true)) // Tuple(42, "hello", true)

Stack - Last-In-First-Out Collection

typescript

import { Stack } from "functype"

Stack.empty<number>()
const stack = Stack.of(1, 2, 3)
stack.push(value) // Returns new Stack
stack.pop() // Returns [Option<T>, Stack<T>]
stack.peek() // Returns Option<T>
stack.match({
  Empty: () => "empty stack",
  NonEmpty: (top, rest) => `top: ${top}`,
})

LazyList - Lazy-evaluated List

typescript

import { LazyList } from "functype"

// Creation
LazyList([1, 2, 3])
LazyList.of(1, 2, 3)
LazyList.empty<number>()

// Infinite sequences
const naturals = LazyList.from(0, (n) => n + 1)
const evens = naturals.filter((n) => n % 2 === 0).take(10)

// Operations are deferred until needed
const result = LazyList(hugeArray)
  .filter((x) => x > 0)
  .map((x) => x * 2)
  .take(5)
  .toArray() // Only processes first 5 matching elements

Do-Notation (Generator Comprehensions)

Scala-like for-comprehensions using JavaScript generators:

typescript

import { Do, DoAsync, $ } from "functype"

// Synchronous comprehensions
const result = Do(function* () {
  const x = yield* $(Option(5))
  const y = yield* $(Option(10))
  return x + y
}) // Option(15)

// Async comprehensions
const asyncResult = await DoAsync(async function* () {
  const user = yield* $(await fetchUserAsync(userId))
  const profile = yield* $(await fetchProfileAsync(user.id))
  return { user, profile }
})

// Cartesian products with List (2.5x-12x faster than nested flatMap)
const pairs = Do(function* () {
  const x = yield* $(List([1, 2, 3]))
  const y = yield* $(List([10, 20]))
  return { x, y }
}) // List([{x:1,y:10}, {x:1,y:20}, {x:2,y:10}, ...])

// Mixed monad types with automatic conversion
const mixed = Do(function* () {
  const a = yield* $(Option(5))
  const b = yield* $(Right<string, number>(10))
  return a + b
})

Note: First monad determines return type. Uses Reshapeable for automatic type conversion.

Companion Pattern & Type Guards

All types provide static type guards for narrowing:

typescript

import { Option, Either, Try } from "functype"

// Option type guards
if (Option.isSome(option)) {
  option.value // TypeScript knows value exists
}
if (Option.isNone(option)) {
  // Handle empty case
}

// Either type guards
if (Either.isRight(either)) {
  either.value // TypeScript knows it's Right
}
if (Either.isLeft(either)) {
  either.value // TypeScript knows it's error value
}

// Try type guards
if (Try.isSuccess(tryVal)) {
  tryVal.value // TypeScript knows it succeeded
}
if (Try.isFailure(tryVal)) {
  tryVal.error // TypeScript knows it failed
}

Serialization

All Serializable types provide JSON, YAML, and binary serialization:

typescript

// Serialization methods
option.serialize().toJSON()
option.serialize().toYAML()
option.serialize().toBinary() // Uint8Array

// Deserialization
Option.fromJSON<string>(jsonString)
Option.fromYAML<string>(yamlString)
Option.fromBinary<string>(binaryData)

Looking Up Functype APIs

Feature Matrix Reference

For a complete overview of which methods are available on each data structure, consult the Feature Matrix at:

•references/feature-matrix.md (included with this skill)
•Or in the functype repo: docs/FUNCTYPE_FEATURE_MATRIX.md

The matrix shows which interfaces (Functor, Monad, Foldable, etc.) each type implements and what methods are available.

Common Methods by Type

Option<T>

•map, flatMap, filter, fold
•orElse, or, orNull, orUndefined, orThrow
•isSome, isNone, contains

Either<L, R>

•map, flatMap, fold
•orElse, or, swap
•isLeft, isRight

Try<T>

•map, flatMap, fold
•recover, recoverWith
•toOption, toEither
•isSuccess, isFailure

List<A>

•map, flatMap, filter, reduce
•foldLeft, foldRight
•append, prepend, concat
•head, tail, isEmpty
•toArray, toSet

Additional Resources

For pattern conversion help, examples, and API reference:

•Feature Matrix: See references/feature-matrix.md for complete interface/method reference
•API Documentation: https://jordanburke.github.io/functype/
•GitHub Repository: https://github.com/jordanburke/functype
•CLI Documentation: Run npx functype for LLM-optimized API reference

Debugging Tips

Understanding Error Messages

"Type 'X' is not assignable to type 'Y'"

•Ensure proper type parameters: Option<string> not Option<any>
•Check that chains maintain type consistency

"Cannot read property 'map' of undefined"

•Remember to construct the type: Option(value) not just value
•Some types require explicit constructors: List([...]), Right(value)

Common Pitfalls

•

Forgetting to extract values

typescript

// Wrong - returns Option<string>
const name = Option(user).map((u) => u.name)

// Correct - returns string
const name = Option(user)
  .map((u) => u.name)
  .orElse("Unknown")

•

Using map instead of flatMap

typescript

// Wrong - returns Option<Option<string>>
Option(user).map((u) => Option(u.email))

// Correct - returns Option<string>
Option(user).flatMap((u) => Option(u.email))

•

Mutating instead of transforming

typescript

// Wrong - mutates original array
const list = List([1, 2, 3])
list.toArray().push(4)

// Correct - creates new List
const newList = list.append(4)

Resources

references/

•feature-matrix.md - Complete interface and method reference
•common-patterns.md - Additional pattern examples and recipes
•quick-reference.md - Cheat sheet for functype APIs

For more examples and detailed documentation, visit:

•GitHub: https://github.com/jordanburke/functype
•Docs: https://jordanburke.github.io/functype/