effect-io-ai

Package: effect
Module: Match

Match.when

Defines a condition for matching values.

When to use

Use to add one positive pattern case to a Match.type or Match.value pipeline when a direct value, predicate, or structured object pattern should run a handler for matching input.

Details

Supports both direct value comparisons and predicate functions. If the pattern matches, the associated function is executed and the matched input is removed from the remaining cases tracked by the matcher.

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: number) => age > 18 },
    (user: { age: number }) => `Age: ${user.age}`
  ),
  // Match when age is exactly 18
  Match.when({ age: 18 }, () => "You can vote"),
  // Fallback case for all other ages
  Match.orElse((user: { age: number }) => `${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 for handling any one of several patterns with the same handler
• whenAnd for requiring all provided patterns to match before running a handler
• not for handling inputs that do not match a pattern
• orElse for providing a fallback when no pattern case matches

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 v4.0.0