flowchart TD
  Iso --> Lens
  Iso --> Prism
  Lens --> ReversedPrism
  ReversedPrism --> Optional
  Prism --> Optional
  Optional --> Getter
  Optional --> Setter
  Getter --> Optic
  Setter --> Optic

Features

• Unified Representation Of Optics. All optics compose the same way because they are all instances of the same data type (Optic)
• Integration. Built-in optics for effect data structures, like Option and Either.

Introduction

@fp-ts/optic is a library that makes it easy to modify parts of larger data structures based on a single representation of an optic as a combination of a getter and setter.

@fp-ts/optic features a unified representation of optics, deep effect integration, helpful error messages.

Credits and sponsorship

This library was inspired by the following projects:

• zio-optics
• monocle-ts

A huge thanks to my sponsors who made the development of @fp-ts/optic possible.

If you also want to become a sponsor to ensure this library continues to improve and receive maintenance, check out my GitHub Sponsors profile

Requirements

• TypeScript 5.4 or newer
• The strict flag enabled in your tsconfig.json file

{
  // ...
  "compilerOptions": {
    // ...
    "strict": true
  }
}

Getting started

To install the alpha version:

npm install @fp-ts/optic

Warning. This package is primarily published to receive early feedback and for contributors, during this development phase we cannot guarantee the stability of the APIs, consider each release to contain breaking changes.

Once you have installed the library, you can import the necessary types and functions from the @fp-ts/optic module.

import * as Optic from "@fp-ts/optic";

Summary

Let's say we have an employee and we need to upper case the first character of his company street name.

import * as Option from "effect/Option";

interface Street {
  readonly num: number;
  readonly name: Option.Option<string>;
}
interface Address {
  readonly city: string;
  readonly street: Street;
}
interface Company {
  readonly name: string;
  readonly address: Address;
}
interface Employee {
  readonly name: string;
  readonly company: Company;
}

const from: Employee = {
  name: "john",
  company: {
    name: "awesome inc",
    address: {
      city: "london",
      street: {
        num: 23,
        name: Option.some("high street"),
      },
    },
  },
};

const to: Employee = {
  name: "john",
  company: {
    name: "awesome inc",
    address: {
      city: "london",
      street: {
        num: 23,
        name: Option.some("High street"),
      },
    },
  },
};

Let's see what could we do with @fp-ts/optic

import * as Optic from "@fp-ts/optic";
import * as StringOptic from "@fp-ts/optic/String";
import * as String from "effect/String";
import * as assert from "node:assert";

const _firstChar: Optic.Optional<Employee, string> = Optic.id<Employee>()
  .at("company")
  .at("address")
  .at("street")
  .at("name")
  .some()
  .compose(StringOptic.index(0));

const capitalizeName = Optic.modify(_firstChar)(String.toUpperCase);

assert.deepStrictEqual(capitalizeName(from), to);

Understanding Optics

@fp-ts/optic is based on a single representation of an optic as a combination of a getter and a setter.

export interface Optic<
  in GetWhole,
  in SetWholeBefore,
  in SetPiece,
  out GetError,
  out SetError,
  out GetPiece,
  out SetWholeAfter,
> {
  readonly getOptic: (
    GetWhole: GetWhole
  ) => Either<readonly [GetError, SetWholeAfter], GetPiece>;
  readonly setOptic: (
    SetPiece: SetPiece
  ) => (
    SetWholeBefore: SetWholeBefore
  ) => Either<readonly [SetError, SetWholeAfter], SetWholeAfter>;
}

The getter can take some larger structure of type GetWhole and get a part of it of type GetPiece. It can potentially fail with an error of type GetError because the part we are trying to get might not exist in the larger structure.

The setter has the ability, given some piece of type SetPiece and an original structure of type SetWholeBefore, to return a new structure of type SetWholeAfter. Setting can fail with an error of type SetError because the piece we are trying to set might not exist in the structure.

Lens

A Lens is an optic that accesses a field of a product type, such as a tuple or a struct.

The GetError type of a Lens is never because we can always get a field of a product type. The SetError type is also never because we can always set the field of a product type to a new value.

In this case the GetWhole, SetWholeBefore, and SetWholeAfter types are the same and represent the product type. The GetPiece and SetPiece types are also the same and represent the field.

Thus, we have:

export interface Lens<in out S, in out A>
  extends Optic<S, S, A, never, never, A, S> {}

The simplified signature is:

export interface Lens<in out S, in out A> {
  readonly getOptic: (s: S) => Either<never, A>;
  readonly setOptic: (a: A) => (s: S) => Either<never, S>;
}

This conforms exactly to our description above. A lens is an optic where we can always get part of the larger structure and given an original structure we can always set a new value in that structure.

Prism

A Prism is an optic that accesses a case of a sum type, such as the Left or Right cases of an Either.

Getting part of a larger data structure with a prism can fail because the case we are trying to access might not exist. For example, we might be trying to access the right side of an Either but the either is actually a Left.

We use the data type Error to model the different ways that getting or setting with an optic can fail. So the GetError type of a prism will be Error.

The SetError type of a prism will be never because given one of the cases of a product type we can always return a new value of the product type since each case of the product type is an instance of the product type.

A prism also differs from a lens in that we do not need any original structure to set. A product type consists of nothing but its cases so if we have a new value of the case we want to set we can just use that value and don't need the original structure.

We represent this by using unknown for the SetWholeBefore type, indicating that we do not need any original structure to set a new value.

Thus, the definition of a prism is:

export interface Prism<in out S, in out A>
  extends Optic<S, unknown, A, Error, never, A, S> {}

And the simplified signature is:

export interface Prism<in out S, in out A> {
  readonly getOptic: (s: S) => Either<Error, A>;
  readonly setOptic: (a: A) => (s: unknown) => Either<never, S>;
}

Again this conforms exactly to our description. A prism is an optic where we might not be able to get a value but can always set a value and in fact do not require any original structure to set.

Other

@fp-ts/optic supports a wide variety of other optics:

• Optional. An Optional is an optic that accesses part of a larger structure where the part being accessed may not exist and the structure contains more than just that part. Both the GetError and SetError types are Error because the part may not exist in the structure and setting does require the original structure since it consists of more than just this one part.
• Iso. An Iso is an optic that accesses a part of a structure where the structure consists of nothing but the part. Both the GetError and SetError types are never and the SetWholeBefore type is unknown.
• Getter. A Getter is an optic that only allows getting a value. The SetWholeBefore and SetPiece types are never because it is impossible to ever set.
• Setter. A Setter is an optic that only allows setting a value. The GetWhole type is never because it is impossible to ever get.

There are also more polymorphic versions of each optic that allow the types of the data structure and part before and after to differ. For example, a PolyPrism could allow us to access the right case of an Either<A, B> and set a C value to return an Either<A, C>.

Cheatsheet

Optic constructors

| Optic | Name | Given | To | | -------- | ------------ | --------------------------------------------------------------- | -------------------------- | | Iso | iso | S => A, A => S | Iso<S, A> | | | id | | Iso<S, S> | | Lens | lens | S => A, A => S => S | Lens<S, A> | | | at | Key | Lens<S, S[Key]> | | | pick | Key | Lens<S, Pick<S, Key>> | | | omit | Key | Lens<S, Omit<S, Key>> | | Prism | prism | S => Either<Error, A>, A => S | Prism<S, A> | | | polyPrism | S => Either<[Error, T], A>, B => T | PolyPrism<S, T, A, B> | | | cons | | Prism<A[], [A, A[]]> | | | nonNullable | | Prism<A, NonNullable<A>> | | | some | | Prism<Option<A>, A> | | | filter | Predicate<S> | Prism<S, S> | | | filter | Refinement<S, A> | Prism<S, A> | | Optional | optional | S => Either<Error, A>, A => S => Either<Error, S> | Optional<S, A> | | | polyOptional | S => Either<[Error, T], A>, B => S => Either<[Error, T], T> | PolyOptional<S, T, A, B> | | | index | number | Optional<A[], A> | | | key | string | Optional<{ []: A }, A> | | | head | | Optional<A[], A> | | | tail | | Optional<A[], A[]> | | | findFirst | Predicate<A> | Optional<A[], A> | | | findFirst | Refinement<A, B> | Optional<A[], B> |

Getter / Setter APIs

| Name | Given | To | | ------------- | ------------------------------- | ------------------ | | get | Lens<S, A>, S | A | | decode | Prism<S, A>, S | Either<Error, A> | | encode | Prism<S, A>, A | S | | getOrModify | PolyOptional<S, T, A, B>, S | Either<T, A> | | modify | Optional<S, A>, A => A | S => S | | replace | Setter<S, A>, A, S | S | | replaceOption | Setter<S, A>, A, S | Option<S> | | getOption | Getter<S, A>, S | Option<A> |

Basic usage

id

The id optic is a special optic that represents the identity function, which simply returns its input unchanged. It can be thought of as a "base case" for optics, from which more complex optics can be built.

The id optic is defined as a singleton type, meaning that there is only one possible value for it. This makes it easy to use as a starting point for building larger optics, as it does not require any arguments or configuration.

import * as Optic from "@fp-ts/optic";

interface Whole {
  readonly a: string;
  readonly b: number;
  readonly c: boolean;
}

// create an iso that focuses on the 'Whole' data structure
const _a: Optic.Iso<Whole, Whole> = Optic.id<Whole>();

compose

The compose method is a utility function that allows you to combine two or more optics into a single optic.

import { pipe } from "effect";
import * as Optic from "@fp-ts/optic";

// This is the type of the data structure that the lens will be operating on.
interface Whole {
  readonly a: string;
  readonly b: number;
  readonly c: boolean;
}

// This creates a lens that focuses on the 'a' field within the 'Whole' object.
const _a: Optic.Lens<Whole, string> =
  // The 'id' function creates an identity lens for the 'Whole' type.
  Optic.id<Whole>()
    // The 'compose' method combines the identity lens with an 'at' lens,
    // which selects the 'a' field within the 'Whole' object.
    .compose(Optic.at("a"));

// Now we can use the '_a' lens to view and modify the 'a' field of a 'Whole' object.

const whole: Whole = {
  a: "foo",
  b: 42,
  c: true,
};

// Use the 'get' function to view the 'a' field of the 'whole' object.
const result: string = pipe(whole, Optic.get(_a)); // returns "foo"

// Use the 'replace' function to update the 'a' field of the 'whole' object.
const updated: Whole = pipe(whole, Optic.replace(_a)("bar")); // returns { a: "bar", b: 42, c: true }

at

The at method is a utility function that creates an optic that focuses on a specific field within a data structure.

import { pipe } from "effect";
import * as Optic from "@fp-ts/optic";

// This is the type of the data structure that the lens will be operating on.
interface Whole {
  readonly a: string;
  readonly b: number;
  readonly c: boolean;
}

// This creates a lens that focuses on the 'a' field within the 'Whole' object.
const _a: Optic.Lens<Whole, string> =
  // The 'id' function creates an identity lens for the 'Whole' type.
  Optic.id<Whole>()
    // The 'at' method selects the 'a' key within the 'Whole' object,
    // resulting in a lens that is focused on that field.
    .at("a");

// Now we can use the '_a' lens to view and modify the 'a' field of a 'Whole' object.

const whole: Whole = {
  a: "foo",
  b: 42,
  c: true,
};

// Use the 'get' function to view the 'a' field of the 'whole' object.
const result: string = pipe(whole, Optic.get(_a)); // returns "foo"

// Use the 'replace' function to update the 'a' field of the 'whole' object.
const updated: Whole = pipe(whole, Optic.replace(_a)("bar")); // returns { a: "bar", b: 42, c: true }

pick

The pick method is a utility function that creates an optic that focuses on a group of keys within a data structure.

import { pipe } from "effect";
import * as Optic from "@fp-ts/optic";

// This is the type of the data structure that the lens will be operating on.
interface Whole {
  readonly a: string;
  readonly b: number;
  readonly c: boolean;
}

// This creates a lens that focuses on the 'a' and 'b' fields within the 'Whole' object.
const _ab: Optic.Lens<Whole, { readonly a: string; readonly b: number }> =
  // The 'id' function creates an identity lens for the 'Whole' type.
  Optic.id<Whole>()
    // The 'pick' method selects the 'a' and 'b' keys within the 'Whole' object,
    // resulting in a lens that is focused on those fields.
    .pick("a", "b");

// Now we can use the '_ab' lens to view and modify the 'a' and 'b' fields of a 'Whole' object.

const whole: Whole = {
  a: "foo",
  b: 42,
  c: true,
};

// Use the 'get' function to view the 'a' and 'b' fields of the 'whole' object.
const result: { readonly a: string; readonly b: number } = pipe(
  whole,
  Optic.get(_ab)
); // returns { a: "foo", b: 42 }

// Use the 'replace' function to update the 'a' and 'b' fields of the 'whole' object.
const updated: Whole = pipe(whole, Optic.replace(_ab)({ a: "bar", b: 23 })); // returns { a: "bar", b: 23, c: true }

omit

The omit method is a utility function that creates a lens that excludes a group of keys from a struct. This can be useful when you want to focus on a subset of a data structure and ignore certain fields.

import { pipe } from "effect";
import * as Optic from "@fp-ts/optic";

interface Whole {
  readonly a: string;
  readonly b: number;
  readonly c: boolean;
}

// This creates a lens that focuses on the 'a' and 'c' fields within the 'Whole' object.
const _ac: Optic.Lens<Whole, { readonly a: string; readonly c: boolean }> =
  // The 'id' function creates an identity lens for the 'Whole' type.
  Optic.id<Whole>()
    // The 'omit' method excludes the 'b' key within the 'Whole' object,
    // resulting in a lens that is focused on the 'a' and 'c' fields.
    .omit("b");

// Now we can use the '_ac' lens to view and modify the 'a' and 'c' fields of a 'Whole' object.

const whole: Whole = {
  a: "foo",
  b: 42,
  c: true,
};

// Use the 'get' function to view the 'a' and 'c' fields of the 'whole' object.
const result: { readonly a: string; readonly c: boolean } = pipe(
  whole,
  Optic.get(_ac)
); // returns { a: "foo", c: true }

// Use the 'replace' function to update the 'a' and 'c' fields of the 'whole' object.
const updated: Whole = pipe(whole, Optic.replace(_ac)({ a: "bar", c: false })); // returns { a: "bar", b: 42, c: false }

filter

The filter method is a utility function that creates an optic that focuses on the elements of a data structure that match a specified predicate.

import { pipe } from "effect";
import * as Optic from "@fp-ts/optic";
import type { Option } from "effect/Option";

// This is the type of the data structure that the prism will be operating on.
interface Whole {
  readonly a: number;
}

// This creates an `Optional` that focuses on the 'a' field within the 'Whole' object,
// and only includes values that are even numbers.
const _evenA: Optic.Optional<Whole, number> =
  // The 'id' function creates an identity prism for the 'Whole' type.
  Optic.id<Whole>()
    // The 'at' method selects the 'a' key within the 'Whole' object,
    // resulting in a `Lens` that is focused on that field.
    .at("a")
    // The 'filter' method only includes values that are even numbers.
    .filter((a) => a % 2 === 0);

// Now we can use the '_evenA' `Optional` to view and modify the 'a' field of a 'Whole' object,
// but only if the value is an even number.

const whole: Whole = {
  a: 2,
};

// Use the 'getOption' function to view the 'a' field of the 'whole' object.
const result: Option<number> = pipe(whole, Optic.getOption(_evenA)); // returns some(2)

// Use the 'replace' function to update the 'a' field of the 'whole' object.
const updated: Whole = pipe(whole, Optic.replace(_evenA)(4)); // returns { a: 4 }

nonNullable

The nonNullable method is a utility function that creates a Prism that focuses on the non-nullable values of a nullable type. This is useful when you want to manipulate or extract the value of a nullable type, but want to ignore the null values.

import { pipe } from "effect";
import * as Optic from "@fp-ts/optic";
import type { Option } from "effect/Option";

const _nonNullString: Optic.Prism<string | null, string> = Optic.id<
  string | null
>().nonNullable();

const result1: Option<string> = pipe("foo", Optic.getOption(_nonNullString)); // returns some("foo")
const result2: Option<string> = pipe(null, Optic.getOption(_nonNullString)); // returns none

some

The some method is a utility function that creates an optic that focuses on the Some case of an Option data type. This optic allows you to view and modify the value contained within the Some case of an Option.

import { pipe } from "effect";
import * as Option from "effect/Option";
import * as Optic from "@fp-ts/optic";

// This creates a prism that focuses on the 'Some' case of the 'Option<number>' object.
const _some: Optic.Prism<Option.Option<number>, number> = Optic.id<
  Option.Option<number>
>().some();

const option: Option.Option<number> = Option.some(42);

const result: Option.Option<number> = pipe(option, Optic.getOption(_some)); // returns some(42)

const updated: Option.Option<number> = pipe(option, Optic.replace(_some)(23)); // returns some(23)

index

The index method creates an Optional optic that focuses on a specific index in a ReadonlyArray. The Optional optic allows you to view the value at the specified index, or None if the index does not exist. You can also use the Optional optic to update the value at the specified index, if it exists.

import { pipe } from "effect";
import * as Optic from "@fp-ts/optic";
import type { Option } from "effect/Option";

const _index2: Optic.Optional<ReadonlyArray<number>, number> = Optic.id<
  ReadonlyArray<number>
>().index(2);

const arr: ReadonlyArray<number> = [1, 2, 3, 4];

const result: Option<number> = pipe(arr, Optic.getOption(_index2)); // some(3)
const updated1: ReadonlyArray<number> = pipe(arr, Optic.replace(_index2)(42)); // [1, 2, 42, 4]
const updated2: ReadonlyArray<number> = pipe(arr, Optic.replace(_index2)(10)); // [1, 2, 10, 4]
const updated3: ReadonlyArray<number> = pipe([], Optic.replace(_index2)(10)); // []

key

The key method is a utility function that allows you to create an Optional optic that focuses on a specific key of an index signature (a type with a string index signature).

import { pipe } from "effect";
import type { Option } from "effect/Option";
import * as Optic from "@fp-ts/optic";

interface Data {
  readonly [key: string]: number;
}

// Create an Optional optic that focuses on the 'foo' key.
const _foo: Optic.Optional<Data, number> = Optic.id<Data>().key("foo");

const data: Data = {
  foo: 1,
  bar: 2,
};

// Use the 'getOption' function to view the value of the 'foo' key.
const fooValue: Option<number> = pipe(data, Optic.getOption(_foo)); // returns some(1)

// Use the 'replace' function to update the value of the 'foo' key.
const updatedData: Data = pipe(data, Optic.replace(_foo)(10)); // returns { foo: 10, bar: 2 }

Documentation

• API Reference

License

The MIT License (MIT)