functype

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, 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
• Debug functype code or understand error messages

Quick Start

Installation

npm install functype
# or
pnpm add functype

Core Imports

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

Constructor vs Companion Methods

Functype collections provide multiple ways to create instances:

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

// 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):

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

After (Functype):

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

Optional Chaining → Option Chain

Before:

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

After:

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:

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

After (with Try):

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

After (with Either):

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

Array Operations → List

Before:

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

After:

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

If-Else Chains → Cond

Before:

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

After:

import { Cond } from "functype"

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

Switch Statements → Match

Before:

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

After:

import { Match } from "functype"

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

Common Use Cases

Validation with Either

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

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

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

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()

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

1. Forgetting to extract values

   // 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")
   

2. Using map instead of flatMap

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

3. Mutating instead of transforming

   // 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/