In Effect-TS, various mapping functions can be applied to the values inside an Option to transform, replace, or manipulate the contained values. This article explores different mapping functions provided by Effect-TS with practical examples.

Example 1: Basic Mapping with O.map

Use O.map to apply a transformation function to the value inside an Option. If the Option is Some, the function is applied; otherwise, the result is None.

import { Option as O, pipe } from 'effect';

function mapping_ex01() {
  const some = O.some(1); // Create an Option containing the value 1
  const none = O.none(); // Create an Option representing no value
  const increment = (n: number) => n + 1;

  console.log(pipe(some, O.map(increment))); // Output: Some(2) (since some contains 1 and 1 + 1 = 2)
  console.log(pipe(none, O.map(increment))); // Output: None (since none is None)
}

Example 2: Mapping to a Constant Value with O.as

Use O.as to replace the value inside the Option with a provided constant value.

import { Option as O, pipe } from 'effect';

function mapping_ex02() {
  const some = O.some(1); // Create an Option containing the value 1
  const none = O.none(); // Create an Option representing no value

  console.log(pipe(some, O.as('replaced'))); // Output: Some('replaced') (replaces 1 with 'replaced')
  console.log(pipe(none, O.as('replaced'))); // Output: None (since none is None)
}

Explanation:

1. Creating Options: We create two Options, one containing a value (some with 1) and another representing no value (none).
2. Applying O.as: We use O.as to replace the value inside the Option with the constant value 'replaced'.

The output is Some('replaced') for the some Option and None for the none Option, demonstrating how O.as effectively replaces the original value if it exists.

Example 3: Mapping to void with O.asVoid

Use O.asVoid to replace the value inside the Option with undefined.

import { Option as O, pipe } from 'effect';

function mapping_ex03() {
  const some = O.some(1); // Create an Option containing the value 1
  const none = O.none(); // Create an Option representing no value

  console.log(pipe(some, O.asVoid)); // Output: Some(undefined) (replaces 1 with undefined)
  console.log(pipe(none, O.asVoid)); // Output: None (since none is None)
}

Explanation:

1. Creating Options: We create two Options, one containing a value (some with 1) and another representing no value (none).
2. Applying O.asVoid: We use O.asVoid to replace the value inside the Option with undefined.

The output is Some(undefined) for the some Option and None for the none Option, demonstrating how O.asVoid effectively replaces the original value if it exists.

Example 4: FlatMapping with O.flatMap

Use O.flatMap to apply a transformation function that returns an Option to the value if the Option is Some, and flatten the result.

import { Option as O, pipe } from 'effect';

function mapping_ex04() {
  const some = O.some(1); // Create an Option containing the value 1
  const none = O.none(); // Create an Option representing no value
  const doubleIfPositive = (n: number) => (n > 0 ? O.some(n * 2) : O.none());

  console.log(pipe(some, O.flatMap(doubleIfPositive))); // Output: Some(2) (since some contains 1 and 1 > 0)
  console.log(pipe(none, O.flatMap(doubleIfPositive))); // Output: None (since none is None)
}

Explanation:

1. Creating Options: We create two Options, one containing a value (some with 1) and another representing no value (none).
2. Applying O.flatMap: We use O.flatMap to apply a transformation function (doubleIfPositive) that returns an Option. If the value is positive, it doubles the value and wraps it in Some, otherwise it returns None.

The output is Some(2) for the some Option and None for the none Option, demonstrating how O.flatMap flattens the result of the transformation.

Example 5: FlatMapping Nullable Values with O.flatMapNullable

Use O.flatMapNullable to apply a transformation function that may return a nullable value to the value if the Option is Some, and convert the result to an Option.

import { Option as O, pipe } from 'effect';

function mapping_ex05() {
  const some = O.some({ a: { b: { c: 1 } } }); // Create an Option containing a nested object
  const none = O.none(); // Create an Option representing no value
  const getCValue = (obj: { a?: { b?: { c?: number } } }) => obj.a?.b?.c ?? null;

  console.log(pipe(some, O.flatMapNullable(getCValue))); // Output: Some(1) (extracts the nested value)
  console.log(pipe(none, O.flatMapNullable(getCValue))); // Output: None (since none is None)
}

Explanation:

1. Creating Options: We create two Options, one containing a nested object (some) and another representing no value (none).
2. Applying O.flatMapNullable: We use O.flatMapNullable to apply a transformation function (getCValue) that extracts a nested value and may return null. The function returns Some if a value is found, otherwise it returns None.

The output is Some(1) for the some Option and None for the none Option, demonstrating how O.flatMapNullable converts the result of the transformation to an Option.