effect-io-ai

Package: effect
Module: Types

Types.NarrowReason

Narrows a specific reason variant by its _tag from an error’s reason field.

When to use

Use to preserve the original error shape while narrowing its nested reason field to the matching variant.

Details

Returns never if E has no matching reason variant.

Example (Narrowing a reason variant)

import type { Types } from "effect"

type RateLimitError = { readonly _tag: "RateLimitError"; readonly retryAfter: number }
type QuotaError = { readonly _tag: "QuotaError"; readonly limit: number }
type ApiError = { readonly _tag: "ApiError"; readonly reason: RateLimitError | QuotaError }

type Result = Types.NarrowReason<ApiError, "RateLimitError">
// ApiError & { readonly reason: { readonly _tag: "RateLimitError"; readonly retryAfter: number } }

See

• ExcludeReason
• ReasonOf
• ReasonTags

Signature

type NarrowReason<E, K> = E extends { readonly reason: infer R }
  ? R extends { readonly _tag: infer T } ? K extends T ? E & { readonly reason: R } : never
  : never
  : never

Source

Since v4.0.0