effect-io-ai

Package: effect
Module: Match

Match.when

Defines a condition for matching values.

Details

This function enables pattern matching by checking whether a given value satisfies a condition. It supports both direct value comparisons and predicate functions. If the condition is met, the associated function is executed.

This function is useful when defining matchers that need to check for specific values or apply logical conditions to determine a match. It works well with structured objects and primitive types.

Example (Matching with Values and Predicates)

import { Match } from "effect"

// Create a matcher for objects with an "age" property
const match = Match.type<{ age: number }>().pipe(
  // Match when age is greater than 18
  Match.when({ age: (age) => age > 18 }, (user) => `Age: ${user.age}`),
  // Match when age is exactly 18
  Match.when({ age: 18 }, () => "You can vote"),
  // Fallback case for all other ages
  Match.orElse((user) => `${user.age} is too young`)
)

console.log(match({ age: 20 }))
// Output: "Age: 20"

console.log(match({ age: 18 }))
// Output: "You can vote"

console.log(match({ age: 4 }))
// Output: "4 is too young"

See

• whenOr Use this when multiple patterns should match in a single condition.
• whenAnd Use this when a value must match all provided patterns.
• orElse Provides a fallback when no patterns match.

Signature

declare const when: <R, const P extends Types.PatternPrimitive<R> | Types.PatternBase<R>, Ret, Fn extends (_: Types.WhenMatch<R, P>) => Ret>(pattern: P, f: Fn) => <I, F, A, Pr>(self: Matcher<I, F, R, A, Pr, Ret>) => Matcher<I, Types.AddWithout<F, Types.PForExclude<P>>, Types.ApplyFilters<I, Types.AddWithout<F, Types.PForExclude<P>>>, A | ReturnType<Fn>, Pr, Ret>

Source

Since v1.0.0