Hyperlinkv0.8.0-beta.28

← Match

Match.discriminatorStartsWithconsteffect/Match.ts:757
<D extends string>(field: D): <
  R,
  P extends string,
  Ret,
  Fn extends (_: Extract<R, Record<D, `${P}${string}`>>) => Ret
>(
  pattern: P,
  f: Fn
) => <I, F, A, Pr>(
  self: Matcher<I, F, R, A, Pr, Ret>
) => Matcher<
  I,
  Types.AddWithout<F, Extract<R, Record<D, `${P}${string}`>>>,
  Types.ApplyFilters<
    I,
    Types.AddWithout<F, Extract<R, Record<D, `${P}${string}`>>>
  >,
  A | ReturnType<Fn>,
  Pr,
  Ret
>

Matches values where a specified field starts with a given prefix.

When to use

Use to match string discriminator values by prefix instead of exact value.

Details

Instead of checking for exact matches, this helper matches values that share a common prefix. For example, if the discriminant field contains hierarchical names like "A", "A.A", and "B", a single "A" rule can match both "A" and "A.A".

Example (Matching discriminator prefixes)

import { Match, pipe } from "effect"

const match = pipe(
  Match.type<{ type: "A" } | { type: "B" } | { type: "A.A" } | {}>(),
  Match.discriminatorStartsWith("type")("A", (_) => 1 as const),
  Match.discriminatorStartsWith("type")("B", (_) => 2 as const),
  Match.orElse((_) => 3 as const)
)

console.log(match({ type: "A" })) // 1
console.log(match({ type: "B" })) // 2
console.log(match({ type: "A.A" })) // 1
Defining patternsdiscriminator
Source effect/Match.ts:75715 lines
export const discriminatorStartsWith: <D extends string>(
  field: D
) => <R, P extends string, Ret, Fn extends (_: Extract<R, Record<D, `${P}${string}`>>) => Ret>(
  pattern: P,
  f: Fn
) => <I, F, A, Pr>(
  self: Matcher<I, F, R, A, Pr, Ret>
) => Matcher<
  I,
  Types.AddWithout<F, Extract<R, Record<D, `${P}${string}`>>>,
  Types.ApplyFilters<I, Types.AddWithout<F, Extract<R, Record<D, `${P}${string}`>>>>,
  A | ReturnType<Fn>,
  Pr,
  Ret
> = internal.discriminatorStartsWith