Functype Library Developer

Overview

Guide for contributing to the functype TypeScript library. This skill provides architectural patterns, development workflows, and tooling for implementing new functional data structures following functype's Scala-inspired design.

When to Use This Skill

Trigger this skill when:

• •Creating new data structures or types
• •Adding methods to existing data structures
• •Implementing functional interfaces (Functor, Monad, Foldable, etc.)
• •Adding or fixing tests
• •Understanding library architecture
• •Debugging functional type implementations
• •Following the Base pattern and type class system
• •Working with the Feature Matrix

Development Workflow

Prerequisites

• •Node.js: ≥ 18.0.0
• •pnpm: 10.12.1
• •TypeScript: Strict mode enabled

Essential Commands

# Install dependencies
pnpm install

# Development (build with watch)
pnpm dev

# Before committing (MUST PASS)
pnpm validate

# Run tests
pnpm test

# Run specific test file
pnpm vitest run test/specific.spec.ts

# Check types without building
pnpm compile

Pre-Commit Checklist

Always run before committing:

pnpm validate

This runs:

1. •Format: Prettier formatting
2. •Lint: ESLint checks
3. •Test: All Vitest tests
4. •Build: Production build

Core Architecture

Scala-Inspired Constructor Pattern

All types follow this pattern:

// Constructor function returns object with methods
const option = Option(value) // Constructor
option.map((x) => x + 1) // Instance methods
Option.none() // Companion methods

NOT class-based:

// ❌ Don't do this
class Option<T> {
  constructor(value: T) { ... }
}

// ✅ Do this
export function Option<T>(value: T | null | undefined): OptionType<T> {
  return value == null ? none() : some(value)
}

Base Pattern

Use the Base function to add common functionality to all types:

import { Base } from "@/core/base"

export function Option<T>(value: T | null | undefined): OptionType<T> {
  if (value == null) {
    return Base("None", {
      // methods here
      map: <B>(f: (val: T) => B) => Option.none<B>(),
      // ...
    })
  }

  return Base("Some", {
    // methods here
    map: <B>(f: (val: T) => B) => Option(f(value)),
    // ...
  })
}

Base provides:

• •Typeable interface (type metadata)
• •Standard toString() method
• •Consistent object structure

Type System Foundation

Base constraint:

import type { Type } from "@/functor"

// Use Type instead of any
function process<T extends Type>(value: T): void {
  // ...
}

Never use any:

• •Use unknown for uncertain types
• •Use Type for generic constraints
• •Use proper type definitions

Functional Interfaces

Every container type should implement these when applicable:

Core interfaces (see references/architecture.md for details):

• •Functor - map over values
• •Applicative - apply wrapped functions
• •Monad - flatMap for sequencing
• •Foldable - extract via pattern matching
• •Traversable - collection operations
• •Serializable - JSON/YAML/binary serialization

Reference the Feature Matrix: Check docs/FUNCTYPE_FEATURE_MATRIX.md to see which interfaces each type implements and what methods are required.

Creating a New Data Structure

Step-by-Step Guide

1. Research existing patterns

# Use the Explore agent to understand the codebase
# Look at Option, Either, or Try as reference implementations

2. Create module structure

mkdir -p src/mynewtype
touch src/mynewtype/index.ts
touch test/mynewtype.spec.ts

3. Use the template script

# Run the new-type-template script
./claude/skills/functype-developer/scripts/new-type-template.sh MyNewType

This generates:

• •Basic type structure
• •Required interface implementations
• •Test file boilerplate

4. Implement the type

Follow the constructor pattern:

// src/mynewtype/index.ts
import { Base } from "@/core/base"
import type { Functor } from "@/functor"

export type MyNewType<T> = Functor<T> & {
  // Your methods here
  getValue(): T
}

export function MyNew<T>(value: T): MyNewType<T> {
  return Base("MyNewType", {
    // Functor
    map: <B>(f: (val: T) => B): MyNewType<B> => {
      return MyNew(f(value))
    },

    // Your methods
    getValue: () => value,

    // Pipe for composition
    pipe: () => ({
      map: (f: (val: T) => any) => MyNew(value).map(f),
    }),
  })
}

// Companion methods
MyNew.empty = <T>() => MyNew<T>(null as any)

5. Add exports

Update src/index.ts:

export { MyNew } from "./mynewtype"
export type { MyNewType } from "./mynewtype"

Update package.json exports:

{
  "exports": {
    "./mynewtype": {
      "import": "./dist/esm/mynewtype/index.js",
      "require": "./dist/cjs/mynewtype/index.js",
      "types": "./dist/types/mynewtype/index.d.ts"
    }
  }
}

6. Write comprehensive tests

// test/mynewtype.spec.ts
import { describe, expect, it } from "vitest"
import { MyNew } from "@/mynewtype"

describe("MyNewType", () => {
  describe("Construction", () => {
    it("should create from value", () => {
      const value = MyNew(5)
      expect(value.getValue()).toBe(5)
    })
  })

  describe("Functor", () => {
    it("should map over values", () => {
      const result = MyNew(5).map((x) => x * 2)
      expect(result.getValue()).toBe(10)
    })
  })

  // More tests...
})

7. Update Feature Matrix

Add your type to docs/FUNCTYPE_FEATURE_MATRIX.md showing which interfaces it implements.

8. Validate

pnpm validate

Interface Implementation Checklist

When implementing interfaces, refer to this checklist:

Functor:

• • map<B>(f: (value: A) => B): Functor<B>

Applicative (extends Functor):

• • ap<B>(ff: Applicative<(value: A) => B>): Applicative<B>

Monad (extends Applicative):

• • flatMap<B>(f: (value: A) => Monad<B>): Monad<B>

Foldable:

• • fold<B>(onEmpty: () => B, onValue: (value: A) => B): B
• • foldLeft<B>(z: B): (op: (b: B, a: A) => B) => B
• • foldRight<B>(z: B): (op: (a: A, b: B) => B) => B

See references/architecture.md for complete interface definitions.

Testing Patterns

Test Structure

Use Vitest with describe/it pattern:

describe("TypeName", () => {
  describe("Feature Group", () => {
    it("should do specific thing", () => {
      // Arrange
      const input = createInput()

      // Act
      const result = performOperation(input)

      // Assert
      expect(result).toBe(expected)
    })
  })
})

Property-Based Testing

Use fast-check for property tests:

import { fc, test } from "@fast-check/vitest"

test.prop([fc.integer()])("should always return positive", (n) => {
  const result = Math.abs(n)
  expect(result).toBeGreaterThanOrEqual(0)
})

Edge Cases to Test

Always test:

• •Empty/null/undefined inputs
• •Type inference correctness
• •Method chaining
• •Error cases
• •Immutability (original unchanged)

Code Style Guidelines

Imports

// Type-only imports when possible
import type { Type } from "@/functor"
import { Option } from "@/option"

// Organized with simple-import-sort (automatic)

Types

// ✅ Use Type for constraints
function process<T extends Type>(value: T): void

// ✅ Prefer types over interfaces
export type MyType<T> = {
  value: T
}

// ✅ Explicit type annotations
function transform<T>(input: T): MyType<T> {
  return { value: input }
}

Naming

// PascalCase for types
type OptionType<T> = { ... }

// camelCase for functions/variables
const someValue = Option(5)
function mapOption<T>(opt: OptionType<T>) { ... }

// Constructor functions are PascalCase
Option(value)
Either(value)

Functional Style

// ✅ Immutability
const newList = list.append(item)

// ❌ Mutation
list.push(item)

// ✅ Pure functions
function double(x: number): number {
  return x * 2
}

// ❌ Side effects
function double(x: number): number {
  console.log(x) // side effect
  return x * 2
}

Pattern Matching

// ✅ Use Cond for conditionals
import { Cond } from "@/cond"

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

// ✅ Use Match for switches
import { Match } from "@/match"

Match(status)
  .case("success", () => handleSuccess())
  .case("error", () => handleError())
  .done()

// ❌ Avoid early returns
// Use Cond or Option instead

Common Development Tasks

Adding a Helper Method

// Add to type definition
export type MyType<T> = {
  // existing methods...

  // New helper
  isEmpty(): boolean
}

// Implement in constructor
export function MyType<T>(value: T): MyType<T> {
  return Base("MyType", {
    // existing methods...

    isEmpty: () => value == null,
  })
}

Implementing Serialization

import { createSerializable } from "@/core/serializable"

export function MyType<T>(value: T): MyType<T> {
  return Base("MyType", {
    // other methods...

    serialize: () =>
      createSerializable({
        type: "MyType",
        value: value,
      }),
  })
}

Adding Do-Notation Support

export const MyTypeCompanion = {
  Do: <T>(gen: () => Generator<MyType<any>, T, any>): MyType<T> => {
    // Implementation here
    // See Option or Either for reference
  },
}

export const MyType = Object.assign(MyTypeConstructor, MyTypeCompanion)

Debugging Tips

TypeScript Errors

"Type instantiation is excessively deep"

• •Check for circular type references
• •Simplify nested generic types
• •Use type aliases to break up complex types

"Property 'X' does not exist on type 'Y'"

• •Ensure all interfaces are properly implemented
• •Check that types are correctly exported
• •Verify Base pattern includes all required methods

Test Failures

# Run specific test with verbose output
pnpm vitest run test/mytype.spec.ts --reporter=verbose

# Run tests in watch mode
pnpm test:watch

# Check test coverage
pnpm test:coverage

Build Issues

# Clean and rebuild
pnpm clean && pnpm build

# Check TypeScript compilation only
pnpm compile

# Analyze bundle size
pnpm analyze:size

Variance (covariance — <out T>)

Most functype containers are declared covariant in their type parameter(s) — List<out A>, Either<out L, out R>, etc. When adding a new container or a new method, follow these four rules and the variance declaration will hold:

1. •Element-query / removal (contains, indexOf, remove, has) → accept unknown. Equality with an unrelated value is a safe false at runtime.
2. •Additive (add, prepend, concat, push, set) → widen via <B>(item: B): C<A | B>. The return union accurately reflects runtime.
3. •Aggregation (reduce, reduceRight) → guard with Widen<A, B>:
   ts

   import { type Widen, reduceWiden } from "@/typeclass"
   reduce: <B = A>(op: (b: Widen<A, B>, a: Widen<A, B>) => Widen<A, B>): Widen<A, B> =>
     reduceWiden<A, Widen<A, B>>(array, op)
   

   Widen<A, B> = A extends B ? B : never. Blocks list.reduce<UnrelatedType>(...) at compile time.
4. •Recovery / fallback (or, orElse, recover) → widen via <T2>(...): T | T2 or C<T | T2>.

When to stay invariant: mutable cells (Ref<A>), record types with keyof dependence (Obj<T>), equality-sensitive keys (Map<K, _>). Document the choice in JSDoc with a brief "why".

When TS rejects <out>: the compiler names the offending method. Fix it using the rules above and re-annotate.

Regression tests: add test/<path>/<type>-variance.spec.ts patterned after test/either/either-variance.spec.ts (comprehensive) or test/list/list-variance.spec.ts (minimal). Type-only assertions run in milliseconds and catch silent regressions.

Full recipe: docs/variance-guide.md.

Resources

scripts/

• •new-type-template.sh - Generate boilerplate for new types
• •validate.sh - Run the full validation workflow

references/

• •architecture.md - Detailed architecture and patterns
• •adding-types.md - Complete guide for adding new data structures
• •adding-methods.md - Checklist for adding methods to existing types
• •testing-patterns.md - Testing strategies and examples

External Links

• •GitHub: https://github.com/jordanburke/functype
• •Docs: https://jordanburke.github.io/functype/
• •Feature Matrix: docs/FUNCTYPE_FEATURE_MATRIX.md
• •Variance Guide: docs/variance-guide.md