Purify Mastery

Purify is a practical functional library for TypeScript focusing on ergonomics and real-world usability, offering algebraic data types and tools for composable error handling and data transformations.

Installation and Setup

npm install purify-ts

import { Maybe, Either, EitherAsync, MaybeAsync } from 'purify-ts';
import { Codec, string, number } from 'purify-ts/Codec';
import { List } from 'purify-ts/List';
import { Tuple } from 'purify-ts/Tuple';

Philosophy

Purify focuses on:

• Practicality: Easier learning curve than fp-ts
• Ergonomics: Cleaner syntax, method chaining
• Real-world use cases: Built for production TypeScript
• Type safety: Leverages TypeScript's type system
• Async-first: First-class async support with EitherAsync/MaybeAsync

Maybe Type

Represents optional values safely.

Construction

import { Maybe, Just, Nothing } from 'purify-ts';

// Create Maybe values
const just = Just(42);              // Just(42)
const nothing = Nothing;             // Nothing
const fromNullable = Maybe.fromNullable(maybeValue);
const fromPredicate = Maybe.fromPredicate(
  (n: number) => n > 0,
  5
); // Just(5)

// fromFalsy - treats falsy values as Nothing
Maybe.fromFalsy(0);        // Nothing
Maybe.fromFalsy('hello');  // Just('hello')
Maybe.fromFalsy('');       // Nothing

Core Operations

// map - transform the value
Just(5)
  .map(n => n * 2);  // Just(10)

Nothing
  .map(n => n * 2);  // Nothing

// chain (flatMap) - sequence operations
Just(5)
  .chain(n => n > 0 ? Just(n * 2) : Nothing);  // Just(10)

// Method chaining for readability
Just(5)
  .map(n => n * 2)
  .chain(n => Just(n + 1))
  .map(n => n.toString());  // Just("11")

// orDefault - extract with default
Just(5).orDefault(0);      // 5
Nothing.orDefault(0);       // 0

// caseOf - pattern matching
Just(5).caseOf({
  Just: n => `Value: ${n}`,
  Nothing: () => 'No value'
}); // "Value: 5"

// filter
Just(5)
  .filter(n => n > 3);  // Just(5)

Just(2)
  .filter(n => n > 3);  // Nothing

// alt - provide alternative Maybe
Nothing
  .alt(Just(42));  // Just(42)

// ap - apply function in Maybe to value in Maybe
Just((n: number) => n * 2)
  .ap(Just(5));  // Just(10)

Utility Methods

// isJust and isNothing
const maybe = Just(5);
if (maybe.isJust()) {
  // TypeScript knows this is Just
  const value = maybe.extract();  // Safe to extract
}

// extract - unsafe! Throws if Nothing
Just(5).extract();  // 5
// Nothing.extract();  // Throws error!

// toEither - convert to Either
Just(5).toEither('error');  // Right(5)
Nothing.toEither('error');   // Left('error')

// ifJust and ifNothing - side effects
Just(5).ifJust(n => console.log(n));  // Logs 5
Nothing.ifNothing(() => console.log('Empty'));

// sequence - convert array of Maybe to Maybe of array
Maybe.sequence([Just(1), Just(2), Just(3)]);  // Just([1, 2, 3])
Maybe.sequence([Just(1), Nothing, Just(3)]);  // Nothing

// catMaybes - filter out Nothing values
Maybe.catMaybes([Just(1), Nothing, Just(3)]);  // [1, 3]

// mapMaybe - map and filter in one pass
Maybe.mapMaybe(
  (n: number) => n > 0 ? Just(n * 2) : Nothing,
  [1, -1, 2, -2, 3]
);  // [2, 4, 6]

// encase - wrap throwing function
Maybe.encase(() => JSON.parse(jsonString));

Practical Examples

// Safe array access
const head = <A>(arr: A[]): Maybe<A> =>
  Maybe.fromNullable(arr[0]);

// Safe property access
const getProp = <T, K extends keyof T>(obj: T, key: K): Maybe<T[K]> =>
  Maybe.fromNullable(obj[key]);

// Chaining optional operations
type User = { name: string; address?: { city?: string } };

const getCity = (user: User): Maybe<string> =>
  Maybe.fromNullable(user.address)
    .chain(addr => Maybe.fromNullable(addr.city));

// Safe parsing
const parseNumber = (s: string): Maybe<number> =>
  Maybe.encase(() => {
    const n = parseFloat(s);
    if (isNaN(n)) throw new Error('Not a number');
    return n;
  });

// Complex data extraction
const extractUserEmail = (data: unknown): Maybe<string> =>
  Maybe.fromNullable(data)
    .chain(d => typeof d === 'object' ? Just(d as any) : Nothing)
    .chain(obj => Maybe.fromNullable(obj.user))
    .chain(user => Maybe.fromNullable(user.email))
    .filter(email => typeof email === 'string' && email.includes('@'));

Either Type

Represents computations that can fail.

Construction

import { Either, Left, Right } from 'purify-ts';

// Create Either values
const right = Right(42);              // Right(42)
const left = Left('error');           // Left('error')
const fromPredicate = Either.fromPredicate(
  (n: number) => n > 0,
  n => `${n} is not positive`,
  5
); // Right(5)

// encase - wrap throwing function
Either.encase(() => JSON.parse(jsonString));  // Either<Error, unknown>

Core Operations

// map - transform right value
Right(5)
  .map(n => n * 2);  // Right(10)

Left('error')
  .map(n => n * 2);  // Left('error')

// mapLeft - transform left value
Left('error')
  .mapLeft(e => e.toUpperCase());  // Left('ERROR')

// chain (flatMap) - sequence Either operations
Right(5)
  .chain(n => n > 0 ? Right(n * 2) : Left('negative'));  // Right(10)

// Method chaining
Right(5)
  .map(n => n * 2)
  .chain(n => Right(n + 1))
  .map(n => n.toString());  // Right("11")

// caseOf - pattern matching
Right(5).caseOf({
  Left: error => `Error: ${error}`,
  Right: value => `Success: ${value}`
}); // "Success: 5"

// orDefault - extract with default
Right(5).orDefault(0);     // 5
Left('error').orDefault(0); // 0

// orDefaultLazy - lazy default
Left('error').orDefaultLazy(() => expensiveComputation());

// swap - exchange left and right
Right(5).swap();  // Left(5)
Left('error').swap();  // Right('error')

// bimap - map both sides
Right<string, number>(5).bimap(
  e => e.toUpperCase(),
  n => n * 2
);  // Right(10)

// alt - provide alternative Either
Left('error')
  .alt(Right(42));  // Right(42)

Error Handling Patterns

// try-catch wrapping
const safeParse = (json: string): Either<Error, unknown> =>
  Either.encase(() => JSON.parse(json));

// Validation
const validateEmail = (email: string): Either<string, string> =>
  email.includes('@')
    ? Right(email)
    : Left('Invalid email');

const validateAge = (age: number): Either<string, number> =>
  age >= 18
    ? Right(age)
    : Left('Must be 18 or older');

// Chain validations
const validateUser = (email: string, age: number): Either<string, User> =>
  validateEmail(email)
    .chain(validEmail =>
      validateAge(age).map(validAge => ({
        email: validEmail,
        age: validAge
      }))
    );

// Multiple validations with sequence
Either.sequence([
  validateEmail('test@example.com'),
  validateAge(25).mapLeft(() => 'Age validation failed')
]);  // Right(['test@example.com', 25])

// lefts and rights - extract from array
const eithers = [Right(1), Left('error'), Right(2)];
Either.lefts(eithers);   // ['error']
Either.rights(eithers);  // [1, 2]

// isLeft and isRight - type guards
const either = Right(5);
if (either.isRight()) {
  const value = either.extract();  // Safe to extract
}

// extract - unsafe! Throws if Left
Right(5).extract();  // 5
// Left('error').extract();  // Throws error!

// extractLeft - unsafe! Throws if Right
Left('error').extractLeft();  // 'error'
// Right(5).extractLeft();  // Throws error!

// toMaybe - convert to Maybe
Right(5).toMaybe();  // Just(5)
Left('error').toMaybe();  // Nothing

// ifLeft and ifRight - side effects
Right(5).ifRight(n => console.log(n));
Left('error').ifLeft(e => console.error(e));

Practical Examples

// API response handling
type ApiError = { message: string; code: number };

const parseApiResponse = <T>(
  response: Response
): Either<ApiError, T> =>
  response.ok
    ? Either.encase(() => response.json())
        .mapLeft(e => ({ 
          message: 'Parse error', 
          code: 500 
        }))
    : Left({ message: 'Request failed', code: response.status });

// Form validation with error accumulation
type ValidationError = { field: string; message: string };

const validateForm = (
  email: string,
  age: number,
  name: string
): Either<ValidationError[], User> => {
  const errors: ValidationError[] = [];
  
  if (!email.includes('@')) {
    errors.push({ field: 'email', message: 'Invalid email' });
  }
  if (age < 18) {
    errors.push({ field: 'age', message: 'Must be 18+' });
  }
  if (name.length === 0) {
    errors.push({ field: 'name', message: 'Required' });
  }
  
  return errors.length > 0
    ? Left(errors)
    : Right({ email, age, name });
};

// Railway-oriented programming
const processUser = (data: unknown): Either<string, string> =>
  parseUserData(data)
    .chain(validateUser)
    .map(normalizeUser)
    .chain(saveUser)
    .map(formatResponse);

EitherAsync Type

Async operations with error handling.

Construction

import { EitherAsync } from 'purify-ts';

// Create EitherAsync
const fetchUser = (id: number): EitherAsync<Error, User> =>
  EitherAsync(async ({ liftEither, throwE }) => {
    try {
      const response = await fetch(`/api/users/${id}`);
      if (!response.ok) {
        return throwE(new Error('Not found'));
      }
      return response.json();
    } catch (error) {
      return throwE(error as Error);
    }
  });

// From Promise
const fromPromise = EitherAsync.fromPromise(
  async () => fetch('/api/data').then(r => r.json())
);

// liftEither - convert Either to EitherAsync
const eitherAsync = EitherAsync.liftEither(
  validateData(data)
);

Core Operations

// map - transform success value
fetchUser(1)
  .map(user => user.name);  // EitherAsync<Error, string>

// mapLeft - transform error
fetchUser(1)
  .mapLeft(error => ({ 
    message: error.message, 
    code: 500 
  }));

// chain (flatMap) - sequence async operations
fetchUser(1)
  .chain(user => fetchPosts(user.id));  // EitherAsync<Error, Post[]>

// Method chaining
fetchUser(1)
  .map(user => user.id)
  .chain(fetchPosts)
  .map(posts => posts.length);  // EitherAsync<Error, number>

// run - execute and get Promise<Either>
await fetchUser(1).run();  // Promise<Either<Error, User>>

// caseOf - pattern matching (async)
await fetchUser(1)
  .run()
  .then(either => either.caseOf({
    Left: error => console.error(error),
    Right: user => console.log(user)
  }));

// Helpers in async callback
EitherAsync<Error, Result>(async ({ liftEither, fromPromise, throwE }) => {
  // liftEither - convert Either to value
  const validated = await liftEither(validateData(data));
  
  // fromPromise - wrap promise
  const result = await fromPromise(fetch('/api/data'));
  
  // throwE - short-circuit with Left
  if (condition) {
    return throwE(new Error('Failed'));
  }
  
  return result;
});

Practical Examples

// Fetch with validation
const fetchValidatedUser = (id: number): EitherAsync<Error, User> =>
  EitherAsync(async ({ liftEither, throwE }) => {
    const response = await fetch(`/api/users/${id}`);
    if (!response.ok) {
      return throwE(new Error(`HTTP ${response.status}`));
    }
    
    const data = await response.json();
    const validated = await liftEither(validateUser(data));
    return validated;
  });

// Parallel requests
const fetchUserData = (id: number): EitherAsync<Error, UserData> =>
  EitherAsync.liftEither(
    Either.sequence([
      Right(fetchUser(id)),
      Right(fetchPosts(id)),
      Right(fetchComments(id))
    ])
  ).chain(async ([userAsync, postsAsync, commentsAsync]) => {
    const [user, posts, comments] = await Promise.all([
      userAsync.run(),
      postsAsync.run(),
      commentsAsync.run()
    ]);
    
    return Either.sequence([user, posts, comments])
      .map(([u, p, c]) => ({ user: u, posts: p, comments: c }));
  });

// Retry logic
const fetchWithRetry = <T>(
  fetch: EitherAsync<Error, T>,
  retries: number = 3
): EitherAsync<Error, T> =>
  EitherAsync(async ({ throwE }) => {
    let lastError: Error | null = null;
    
    for (let i = 0; i < retries; i++) {
      const result = await fetch.run();
      if (result.isRight()) {
        return result.extract();
      }
      lastError = result.extractLeft();
      await new Promise(resolve => setTimeout(resolve, 1000 * (i + 1)));
    }
    
    return throwE(lastError!);
  });

// Transaction pattern
const createUserTransaction = (
  userData: UserData
): EitherAsync<Error, User> =>
  EitherAsync(async ({ liftEither, throwE }) => {
    // Validate
    const validated = await liftEither(validateUserData(userData));
    
    // Create user
    const user = await liftEither(
      await createUser(validated).run()
    );
    
    // Send welcome email
    const emailResult = await sendWelcomeEmail(user.email).run();
    if (emailResult.isLeft()) {
      // Rollback user creation
      await deleteUser(user.id).run();
      return throwE(new Error('Failed to send email'));
    }
    
    return user;
  });

MaybeAsync Type

Async operations that may not return a value.

Construction and Usage

import { MaybeAsync } from 'purify-ts';

// Create MaybeAsync
const findUser = (id: number): MaybeAsync<User> =>
  MaybeAsync(async ({ liftMaybe }) => {
    const response = await fetch(`/api/users/${id}`);
    if (!response.ok) return liftMaybe(Nothing);
    
    const data = await response.json();
    return liftMaybe(Maybe.fromNullable(data));
  });

// map and chain
findUser(1)
  .map(user => user.name)
  .chain(name => findUserByName(name));

// run - execute and get Promise<Maybe>
await findUser(1).run();  // Promise<Maybe<User>>

// toEitherAsync - convert to EitherAsync
findUser(1)
  .toEitherAsync(new Error('User not found'));  // EitherAsync<Error, User>

// Example: cache lookup
const getCachedValue = <T>(key: string): MaybeAsync<T> =>
  MaybeAsync(async ({ liftMaybe }) => {
    const value = await cache.get(key);
    return liftMaybe(Maybe.fromNullable(value));
  });

const getOrFetch = <T>(
  key: string,
  fetch: () => Promise<T>
): MaybeAsync<T> =>
  getCachedValue<T>(key)
    .alt(MaybeAsync(async () => {
      const value = await fetch();
      await cache.set(key, value);
      return value;
    }));

Codec - Runtime Type Validation

Type-safe encoding/decoding with runtime validation.

Basic Codecs

import { Codec, string, number, boolean, nullType } from 'purify-ts/Codec';
import { Left, Right } from 'purify-ts';

// Primitive codecs
string.decode('hello');     // Right('hello')
string.decode(123);         // Left('Expected a string')

number.decode(42);          // Right(42)
boolean.decode(true);       // Right(true)
nullType.decode(null);      // Right(null)

// Custom primitive codec
const positiveNumber = Codec.custom<number>({
  decode: (input) =>
    typeof input === 'number' && input > 0
      ? Right(input)
      : Left('Expected positive number'),
  encode: (input) => input
});

Object Codecs

import { GetType } from 'purify-ts/Codec';

// Define codec
const UserCodec = Codec.interface({
  id: number,
  name: string,
  email: string,
  age: number
});

// Infer TypeScript type from codec
type User = GetType<typeof UserCodec>;
// type User = { id: number; name: string; email: string; age: number }

// Decode
const result = UserCodec.decode({
  id: 1,
  name: 'John',
  email: 'john@example.com',
  age: 30
});  // Right({ id: 1, name: 'John', email: 'john@example.com', age: 30 })

const invalid = UserCodec.decode({
  id: 1,
  name: 'John'
  // Missing required fields
});  // Left('Problem at key "email": missing key')

// Encode (typically identity for simple codecs)
UserCodec.encode(user);  // user as-is

Advanced Codecs

import { array, optional, oneOf, exactly, lazy } from 'purify-ts/Codec';

// Array codec
const NumberArrayCodec = array(number);
NumberArrayCodec.decode([1, 2, 3]);  // Right([1, 2, 3])

// Optional fields
const UserWithOptionalEmailCodec = Codec.interface({
  id: number,
  name: string,
  email: optional(string)
});

// Union types with oneOf
const StatusCodec = oneOf([
  exactly('pending'),
  exactly('active'),
  exactly('completed')
]);

type Status = GetType<typeof StatusCodec>;
// type Status = 'pending' | 'active' | 'completed'

// Discriminated unions
const ShapeCodec = oneOf([
  Codec.interface({
    type: exactly('circle'),
    radius: number
  }),
  Codec.interface({
    type: exactly('rectangle'),
    width: number,
    height: number
  })
]);

// Recursive types with lazy
interface TreeNode {
  value: number;
  children: TreeNode[];
}

const TreeNodeCodec: Codec<TreeNode> = Codec.interface({
  value: number,
  children: lazy(() => array(TreeNodeCodec))
});

// Record codec
import { record } from 'purify-ts/Codec';

const StringToNumberCodec = record(number);
StringToNumberCodec.decode({ a: 1, b: 2 });  // Right({ a: 1, b: 2 })

// Nested objects
const AddressCodec = Codec.interface({
  street: string,
  city: string,
  zipCode: string
});

const PersonCodec = Codec.interface({
  name: string,
  age: number,
  address: AddressCodec
});

Custom Validation

// Add validation to existing codec
const emailCodec = string.compose(
  Codec.custom<string>({
    decode: (email) =>
      email.includes('@')
        ? Right(email)
        : Left('Invalid email format'),
    encode: (email) => email
  })
);

// Complex validation
const PasswordCodec = string.compose(
  Codec.custom<string>({
    decode: (password) => {
      if (password.length < 8) {
        return Left('Password must be at least 8 characters');
      }
      if (!/[A-Z]/.test(password)) {
        return Left('Password must contain uppercase letter');
      }
      if (!/[0-9]/.test(password)) {
        return Left('Password must contain number');
      }
      return Right(password);
    },
    encode: (password) => password
  })
);

// Type refinement
const NonEmptyStringCodec = string.compose(
  Codec.custom<string>({
    decode: (s) =>
      s.length > 0 ? Right(s) : Left('String cannot be empty'),
    encode: (s) => s
  })
);

Practical Codec Patterns

// API response validation
const ApiResponseCodec = Codec.interface({
  data: UserCodec,
  meta: Codec.interface({
    page: number,
    total: number
  })
});

const fetchUser = async (id: number): Promise<Either<string, User>> => {
  const response = await fetch(`/api/users/${id}`);
  const data = await response.json();
  
  return ApiResponseCodec.decode(data)
    .mapLeft(error => `Validation failed: ${error}`)
    .map(result => result.data);
};

// Form data validation
const SignupFormCodec = Codec.interface({
  email: emailCodec,
  password: PasswordCodec,
  age: positiveNumber,
  terms: exactly(true)
});

const validateSignupForm = (
  data: unknown
): Either<string, SignupForm> =>
  SignupFormCodec.decode(data);

// Environment variables
const EnvCodec = Codec.interface({
  NODE_ENV: oneOf([
    exactly('development'),
    exactly('production'),
    exactly('test')
  ]),
  PORT: number,
  DATABASE_URL: string,
  API_KEY: NonEmptyStringCodec
});

const env = EnvCodec.decode(process.env).unsafeCoerce();
// Throws at startup if env vars invalid

// Transform data while decoding
const DateCodec = Codec.custom<Date>({
  decode: (input) => {
    if (typeof input !== 'string') {
      return Left('Expected ISO date string');
    }
    const date = new Date(input);
    return isNaN(date.getTime())
      ? Left('Invalid date')
      : Right(date);
  },
  encode: (date) => date.toISOString()
});

const EventCodec = Codec.interface({
  name: string,
  date: DateCodec,
  attendees: array(UserCodec)
});

List Type

Functional list operations.

import { List } from 'purify-ts/List';

// Create lists
const list = List.of(1, 2, 3, 4, 5);
const fromArray = List([1, 2, 3]);

// head and tail
list.head();  // Just(1)
list.tail();  // Just(List(2, 3, 4, 5))

// map and filter
list
  .map(n => n * 2)
  .filter(n => n > 5);  // List(6, 8, 10)

// reduce
list.reduce((acc, n) => acc + n, 0);  // 15

// find
list.find(n => n > 3);  // Just(4)

// toArray
list.toArray();  // [1, 2, 3, 4, 5]

// cons (prepend)
list.cons(0);  // List(0, 1, 2, 3, 4, 5)

// at - safe index access
list.at(2);  // Just(3)
list.at(10);  // Nothing

// Useful operations
list.length();  // 5
list.isEmpty();  // false
list.reverse();  // List(5, 4, 3, 2, 1)
list.take(3);  // List(1, 2, 3)
list.drop(2);  // List(3, 4, 5)
list.zip(List.of('a', 'b', 'c'));  // List([1,'a'], [2,'b'], [3,'c'])

Tuple Type

Fixed-size heterogeneous collections.

import { Tuple } from 'purify-ts/Tuple';

// Create tuples
const tuple = Tuple(1, 'hello', true);

// Access elements (type-safe!)
tuple.fst();  // 1
tuple.snd();  // 'hello'

// map (applies to last element)
Tuple(1, 'hello').map(s => s.toUpperCase());  // Tuple(1, 'HELLO')

// mapFirst
Tuple(1, 'hello').mapFirst(n => n * 2);  // Tuple(2, 'hello')

// bimap (map both elements)
Tuple(1, 'hello').bimap(
  n => n * 2,
  s => s.toUpperCase()
);  // Tuple(2, 'HELLO')

// swap
Tuple(1, 'hello').swap();  // Tuple('hello', 1)

// toArray
Tuple(1, 'hello', true).toArray();  // [1, 'hello', true]

// fanout - apply two functions to same input
Tuple.fanout(
  (n: number) => n * 2,
  (n: number) => n + 1
)(5);  // Tuple(10, 6)

Function Utilities

import { identity, constant, compose } from 'purify-ts/Function';

// identity
identity(5);  // 5

// constant - always returns same value
const alwaysFive = constant(5);
alwaysFive();  // 5
alwaysFive(10);  // 5

// compose - right-to-left composition
const double = (n: number) => n * 2;
const increment = (n: number) => n + 1;
const toString = (n: number) => n.toString();

const process = compose(toString, increment, double);
process(5);  // "11"

Non-Empty Lists and Arrays

Type-safe non-empty collections.

import { NonEmptyList } from 'purify-ts/NonEmptyList';

// Must have at least one element
const nel = NonEmptyList.fromArray([1, 2, 3]);  // Just(NonEmptyList)
const empty = NonEmptyList.fromArray([]);  // Nothing

// Always safe operations (no Maybe needed)
nel.map(list => list.head());  // Just(1) - no Maybe!
nel.map(list => list.last());  // Just(3)

// Type guarantees non-emptiness
const getFirst = (nel: NonEmptyList<number>): number =>
  nel.head();  // Always safe!

Practical Patterns

API Client with Error Handling

type ApiError = 
  | { type: 'network'; message: string }
  | { type: 'validation'; errors: string[] }
  | { type: 'server'; status: number; message: string };

class ApiClient {
  constructor(private baseUrl: string) {}

  request<T>(
    path: string,
    options?: RequestInit
  ): EitherAsync<ApiError, T> {
    return EitherAsync(async ({ throwE }) => {
      try {
        const response = await fetch(`${this.baseUrl}${path}`, options);
        
        if (!response.ok) {
          return throwE({
            type: 'server',
            status: response.status,
            message: await response.text()
          });
        }
        
        return await response.json();
      } catch (error) {
        return throwE({
          type: 'network',
          message: error instanceof Error ? error.message : 'Unknown error'
        });
      }
    });
  }

  get<T>(path: string): EitherAsync<ApiError, T> {
    return this.request<T>(path);
  }

  post<T>(path: string, body: unknown): EitherAsync<ApiError, T> {
    return this.request<T>(path, {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify(body)
    });
  }
}

// Usage
const api = new ApiClient('https://api.example.com');

const createUser = (userData: UserData): EitherAsync<ApiError, User> =>
  api.post<unknown>('/users', userData)
    .chain(data =>
      EitherAsync.liftEither(
        UserCodec.decode(data).mapLeft(error => ({
          type: 'validation',
          errors: [error]
        }))
      )
    );

Form Validation with Codecs

const LoginFormCodec = Codec.interface({
  email: string.compose(
    Codec.custom<string>({
      decode: (email) =>
        email.includes('@')
          ? Right(email)
          : Left('Invalid email'),
      encode: identity
    })
  ),
  password: string.compose(
    Codec.custom<string>({
      decode: (password) =>
        password.length >= 8
          ? Right(password)
          : Left('Password must be at least 8 characters'),
      encode: identity
    })
  )
});

type LoginForm = GetType<typeof LoginFormCodec>;

const validateLoginForm = (data: unknown): Either<string, LoginForm> =>
  LoginFormCodec.decode(data);

// In React component
const handleSubmit = (data: unknown) => {
  validateLoginForm(data).caseOf({
    Left: error => setError(error),
    Right: form => submitLogin(form)
  });
};

Async Pipeline with EitherAsync

const processUserSignup = (
  signupData: unknown
): EitherAsync<string, User> =>
  EitherAsync.liftEither(
    SignupFormCodec.decode(signupData)
      .mapLeft(error => `Validation error: ${error}`)
  )
    .chain(form =>
      EitherAsync(async ({ liftEither, throwE }) => {
        // Check if email exists
        const exists = await checkEmailExists(form.email).run();
        if (exists.extract()) {
          return throwE('Email already registered');
        }

        // Hash password
        const hashedPassword = await hashPassword(form.password);

        // Create user
        const user = await liftEither(
          await createUser({
            ...form,
            password: hashedPassword
          }).run()
        );

        // Send welcome email
        await sendWelcomeEmail(user.email).run();

        return user;
      })
    );

// Usage
await processUserSignup(formData)
  .run()
  .then(result => result.caseOf({
    Left: error => console.error(error),
    Right: user => console.log('Created:', user)
  }));

Caching with MaybeAsync

class Cache<T> {
  private store = new Map<string, T>();

  get(key: string): MaybeAsync<T> {
    return MaybeAsync(async ({ liftMaybe }) => {
      const value = this.store.get(key);
      return liftMaybe(Maybe.fromNullable(value));
    });
  }

  set(key: string, value: T): Promise<void> {
    this.store.set(key, value);
    return Promise.resolve();
  }

  getOrFetch<E>(
    key: string,
    fetch: () => EitherAsync<E, T>
  ): EitherAsync<E, T> {
    return this.get(key)
      .toEitherAsync(null as any)
      .alt(
        fetch().chainFirst(value =>
          EitherAsync.fromPromise(() => this.set(key, value))
        )
      );
  }
}

// Usage
const cache = new Cache<User>();

const getUser = (id: number): EitherAsync<Error, User> =>
  cache.getOrFetch(
    `user:${id}`,
    () => fetchUser(id)
  );

Best Practices

1. Use method chaining: Purify's API is designed for readability through chaining
2. Leverage Codecs: Use Codecs for all external data validation
3. Prefer EitherAsync over Promise: Explicit error handling prevents surprises
4. Use caseOf for exhaustive handling: Pattern matching ensures all cases covered
5. Type-safe with GetType: Derive TypeScript types from Codecs
6. Lazy defaults with orDefaultLazy: Avoid expensive computations in happy path
7. Chain with flatMap: Avoid nested Maybe/Either with chain
8. Use NonEmptyList: When guaranteed non-empty, use type system to enforce
9. Sequence for all-or-nothing: Use Maybe.sequence and Either.sequence
10. Test with property-based testing: Algebraic laws make great properties

Comparison with fp-ts

Purify advantages:

• Cleaner, more ergonomic API
• Method chaining feels more natural
• Built-in Codec system for validation
• Easier learning curve
• Better async support with EitherAsync/MaybeAsync

fp-ts advantages:

• More comprehensive type class system
• Better for advanced FP patterns
• Larger ecosystem
• Integration with Effect-TS
• More generic abstractions

Choose Purify when:

• Team new to functional programming
• Need practical, production-ready code quickly
• Want built-in validation with Codecs
• Prefer method chaining syntax

Choose fp-ts when:

• Need advanced FP abstractions
• Building on Effect-TS ecosystem
• Team comfortable with Haskell/Scala concepts
• Need maximum type safety

Migration Tips

From Promises to EitherAsync

// Before
async function fetchUser(id: number): Promise<User> {
  const response = await fetch(`/api/users/${id}`);
  if (!response.ok) throw new Error('Not found');
  return response.json();
}

// After
function fetchUser(id: number): EitherAsync<Error, User> {
  return EitherAsync(async ({ throwE }) => {
    const response = await fetch(`/api/users/${id}`);
    if (!response.ok) return throwE(new Error('Not found'));
    return response.json();
  });
}

From null/undefined to Maybe

// Before
function getFirstName(user: User | null): string | null {
  return user?.name?.split(' ')[0] ?? null;
}

// After
function getFirstName(user: User | null): Maybe<string> {
  return Maybe.fromNullable(user)
    .chain(u => Maybe.fromNullable(u.name))
    .map(name => name.split(' ')[0]);
}

From try-catch to Either

// Before
function parseJSON<T>(json: string): T {
  try {
    return JSON.parse(json);
  } catch (error) {
    throw new Error(`Parse failed: ${error}`);
  }
}

// After
function parseJSON<T>(json: string): Either<Error, T> {
  return Either.encase(() => JSON.parse(json))
    .mapLeft(error => new Error(`Parse failed: ${error}`));
}