パターンマッチング

複数の異なるエラー型を持つ Result 型を扱う際、異なるエラーシナリオを効果的に処理するためにパターンマッチングは不可欠です。このガイドでは、複数のエラー型を適切に処理するためのパターンマッチングの使い方を示します。

なぜユニオンエラーにパターンマッチングが必要なのか？

Result が複数の異なるエラー型で失敗する可能性がある場合、各エラー型を適切に処理する体系的な方法が必要です。パターンマッチングは、すべての可能なエラーケースを処理するためのクリーンで型安全なアプローチを提供します。

Tip

@praha/error-factory の詳細については、Custom Errorページを参照してください。

import { const ErrorFactory: {
    <Name extends string = string, Message extends string = string, Fields extends ErrorFields = ErrorFields>(props: {
        name?: Name;
        message: Message | ((fields: Fields) => Message);
        fields?: Fields;
    }): ErrorConstructor<Name, Message, Fields>;
    fields<Fields extends ErrorFields>(): Fields;
}
A factory function that creates a base class for custom error types.
Extend the returned class to define a custom error with a consistent structure,
reducing boilerplate and ensuring type safety across your application.
@typeParamName  - Inferred as a string literal type from props.name when provided,
or defaults to string when name is omitted.@typeParamMessage  - Inferred as a string literal type from props.message when it is a string,
or defaults to string when message is a function.@typeParamFields  - Inferred from props.fields (via ErrorFactory.fields).
Defaults to the base ErrorFields constraint when fields is omitted.@paramprops  - Configuration for the error class.@paramprops .name - The value set as the name property on both the class and each instance.
When omitted, name is inferred as string and set to new.target.name at construction time,
which resolves to the name of the concrete subclass. Note that omitting name disables
type narrowing via the name property; use name explicitly or instanceof for narrowing.@paramprops .message - The error message. Can be a static string or a function that receives
the custom fields and returns a string, enabling dynamic message generation.@paramprops .fields - A type-level placeholder that declares the additional fields the error
instance will carry. Use ErrorFactory.fields to create this value.
When omitted, no additional fields are added to the instance.@returnsAn  abstract base class typed as ErrorConstructor that should be extended
to produce a concrete custom error class.@exampleBasic  usage
class NotFoundError extends ErrorFactory({
name: 'NotFoundError',
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name);    // "NotFoundError"
console.error(error.message); // "Resource not found"
@exampleOmitting  name
class NotFoundError extends ErrorFactory({
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name); // "NotFoundError" (resolved from new.target.name)
@exampleWith  cause
class DatabaseError extends ErrorFactory({
name: 'DatabaseError',
message: 'A database error occurred',
}) {}

const error = new DatabaseError({ cause: new Error('Connection failed') });
console.error(error.cause); // Error: Connection failed
@exampleWith  additional fields
class QueryError extends ErrorFactory({
name: 'QueryError',
message: 'An error occurred while executing a query',
fields: ErrorFactory.fields<{ query: string }>(),
}) {}

const error = new QueryError({ query: 'SELECT * FROM users' });
console.error(error.query); // "SELECT * FROM users"
@exampleDynamic  message
class ValidationError extends ErrorFactory({
name: 'ValidationError',
message: ({ field }) => `Validation failed for field '${field}'`,
fields: ErrorFactory.fields<{ field: string }>(),
}) {}

const error = new ValidationError({ field: 'email' });
console.error(error.message); // "Validation failed for field 'email'"
ErrorFactory } from '@praha/error-factory';
import { import ResultResult } from '@praha/byethrow';

class class PostNotFoundErrorPostNotFoundError extends ErrorFactory<"PostNotFoundError", "The requested post was not found.", ErrorFields>(props: {
    name?: "PostNotFoundError" | undefined;
    message: "The requested post was not found." | ((fields: ErrorFields) => "The requested post was not found.");
    fields?: ErrorFields | undefined;
}): (new (options?: ErrorOptions) => Error & Readonly<{
    name: "PostNotFoundError";
    message: "The requested post was not found.";
}>) & {
    name: "PostNotFoundError";
}
A factory function that creates a base class for custom error types.
Extend the returned class to define a custom error with a consistent structure,
reducing boilerplate and ensuring type safety across your application.
@typeParamName  - Inferred as a string literal type from props.name when provided,
or defaults to string when name is omitted.@typeParamMessage  - Inferred as a string literal type from props.message when it is a string,
or defaults to string when message is a function.@typeParamFields  - Inferred from props.fields (via ErrorFactory.fields).
Defaults to the base ErrorFields constraint when fields is omitted.@paramprops  - Configuration for the error class.@paramprops .name - The value set as the name property on both the class and each instance.
When omitted, name is inferred as string and set to new.target.name at construction time,
which resolves to the name of the concrete subclass. Note that omitting name disables
type narrowing via the name property; use name explicitly or instanceof for narrowing.@paramprops .message - The error message. Can be a static string or a function that receives
the custom fields and returns a string, enabling dynamic message generation.@paramprops .fields - A type-level placeholder that declares the additional fields the error
instance will carry. Use ErrorFactory.fields to create this value.
When omitted, no additional fields are added to the instance.@returnsAn  abstract base class typed as ErrorConstructor that should be extended
to produce a concrete custom error class.@exampleBasic  usage
class NotFoundError extends ErrorFactory({
name: 'NotFoundError',
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name);    // "NotFoundError"
console.error(error.message); // "Resource not found"
@exampleOmitting  name
class NotFoundError extends ErrorFactory({
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name); // "NotFoundError" (resolved from new.target.name)
@exampleWith  cause
class DatabaseError extends ErrorFactory({
name: 'DatabaseError',
message: 'A database error occurred',
}) {}

const error = new DatabaseError({ cause: new Error('Connection failed') });
console.error(error.cause); // Error: Connection failed
@exampleWith  additional fields
class QueryError extends ErrorFactory({
name: 'QueryError',
message: 'An error occurred while executing a query',
fields: ErrorFactory.fields<{ query: string }>(),
}) {}

const error = new QueryError({ query: 'SELECT * FROM users' });
console.error(error.query); // "SELECT * FROM users"
@exampleDynamic  message
class ValidationError extends ErrorFactory({
name: 'ValidationError',
message: ({ field }) => `Validation failed for field '${field}'`,
fields: ErrorFactory.fields<{ field: string }>(),
}) {}

const error = new ValidationError({ field: 'email' });
console.error(error.message); // "Validation failed for field 'email'"
ErrorFactory({
  name?: "PostNotFoundError" | undefinedname: 'PostNotFoundError',
  message: "The requested post was not found." | ((fields: ErrorFields) => "The requested post was not found.")message: 'The requested post was not found.',
}) {}

class class PostPermissionErrorPostPermissionError extends ErrorFactory<"PostPermissionError", "You do not have permission to perform this action.", ErrorFields>(props: {
    name?: "PostPermissionError" | undefined;
    message: "You do not have permission to perform this action." | ((fields: ErrorFields) => "You do not have permission to perform this action.");
    fields?: ErrorFields | undefined;
}): (new (options?: ErrorOptions) => Error & Readonly<{
    name: "PostPermissionError";
    message: "You do not have permission to perform this action.";
}>) & {
    name: "PostPermissionError";
}
A factory function that creates a base class for custom error types.
Extend the returned class to define a custom error with a consistent structure,
reducing boilerplate and ensuring type safety across your application.
@typeParamName  - Inferred as a string literal type from props.name when provided,
or defaults to string when name is omitted.@typeParamMessage  - Inferred as a string literal type from props.message when it is a string,
or defaults to string when message is a function.@typeParamFields  - Inferred from props.fields (via ErrorFactory.fields).
Defaults to the base ErrorFields constraint when fields is omitted.@paramprops  - Configuration for the error class.@paramprops .name - The value set as the name property on both the class and each instance.
When omitted, name is inferred as string and set to new.target.name at construction time,
which resolves to the name of the concrete subclass. Note that omitting name disables
type narrowing via the name property; use name explicitly or instanceof for narrowing.@paramprops .message - The error message. Can be a static string or a function that receives
the custom fields and returns a string, enabling dynamic message generation.@paramprops .fields - A type-level placeholder that declares the additional fields the error
instance will carry. Use ErrorFactory.fields to create this value.
When omitted, no additional fields are added to the instance.@returnsAn  abstract base class typed as ErrorConstructor that should be extended
to produce a concrete custom error class.@exampleBasic  usage
class NotFoundError extends ErrorFactory({
name: 'NotFoundError',
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name);    // "NotFoundError"
console.error(error.message); // "Resource not found"
@exampleOmitting  name
class NotFoundError extends ErrorFactory({
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name); // "NotFoundError" (resolved from new.target.name)
@exampleWith  cause
class DatabaseError extends ErrorFactory({
name: 'DatabaseError',
message: 'A database error occurred',
}) {}

const error = new DatabaseError({ cause: new Error('Connection failed') });
console.error(error.cause); // Error: Connection failed
@exampleWith  additional fields
class QueryError extends ErrorFactory({
name: 'QueryError',
message: 'An error occurred while executing a query',
fields: ErrorFactory.fields<{ query: string }>(),
}) {}

const error = new QueryError({ query: 'SELECT * FROM users' });
console.error(error.query); // "SELECT * FROM users"
@exampleDynamic  message
class ValidationError extends ErrorFactory({
name: 'ValidationError',
message: ({ field }) => `Validation failed for field '${field}'`,
fields: ErrorFactory.fields<{ field: string }>(),
}) {}

const error = new ValidationError({ field: 'email' });
console.error(error.message); // "Validation failed for field 'email'"
ErrorFactory({
  name?: "PostPermissionError" | undefinedname: 'PostPermissionError',
  message: "You do not have permission to perform this action." | ((fields: ErrorFields) => "You do not have permission to perform this action.")message: 'You do not have permission to perform this action.',
}) {}

class class PostAlreadyDeletedErrorPostAlreadyDeletedError extends ErrorFactory<"PostAlreadyDeletedError", "This post has already been deleted.", ErrorFields>(props: {
    name?: "PostAlreadyDeletedError" | undefined;
    message: "This post has already been deleted." | ((fields: ErrorFields) => "This post has already been deleted.");
    fields?: ErrorFields | undefined;
}): (new (options?: ErrorOptions) => Error & Readonly<{
    name: "PostAlreadyDeletedError";
    message: "This post has already been deleted.";
}>) & {
    name: "PostAlreadyDeletedError";
}
A factory function that creates a base class for custom error types.
Extend the returned class to define a custom error with a consistent structure,
reducing boilerplate and ensuring type safety across your application.
@typeParamName  - Inferred as a string literal type from props.name when provided,
or defaults to string when name is omitted.@typeParamMessage  - Inferred as a string literal type from props.message when it is a string,
or defaults to string when message is a function.@typeParamFields  - Inferred from props.fields (via ErrorFactory.fields).
Defaults to the base ErrorFields constraint when fields is omitted.@paramprops  - Configuration for the error class.@paramprops .name - The value set as the name property on both the class and each instance.
When omitted, name is inferred as string and set to new.target.name at construction time,
which resolves to the name of the concrete subclass. Note that omitting name disables
type narrowing via the name property; use name explicitly or instanceof for narrowing.@paramprops .message - The error message. Can be a static string or a function that receives
the custom fields and returns a string, enabling dynamic message generation.@paramprops .fields - A type-level placeholder that declares the additional fields the error
instance will carry. Use ErrorFactory.fields to create this value.
When omitted, no additional fields are added to the instance.@returnsAn  abstract base class typed as ErrorConstructor that should be extended
to produce a concrete custom error class.@exampleBasic  usage
class NotFoundError extends ErrorFactory({
name: 'NotFoundError',
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name);    // "NotFoundError"
console.error(error.message); // "Resource not found"
@exampleOmitting  name
class NotFoundError extends ErrorFactory({
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name); // "NotFoundError" (resolved from new.target.name)
@exampleWith  cause
class DatabaseError extends ErrorFactory({
name: 'DatabaseError',
message: 'A database error occurred',
}) {}

const error = new DatabaseError({ cause: new Error('Connection failed') });
console.error(error.cause); // Error: Connection failed
@exampleWith  additional fields
class QueryError extends ErrorFactory({
name: 'QueryError',
message: 'An error occurred while executing a query',
fields: ErrorFactory.fields<{ query: string }>(),
}) {}

const error = new QueryError({ query: 'SELECT * FROM users' });
console.error(error.query); // "SELECT * FROM users"
@exampleDynamic  message
class ValidationError extends ErrorFactory({
name: 'ValidationError',
message: ({ field }) => `Validation failed for field '${field}'`,
fields: ErrorFactory.fields<{ field: string }>(),
}) {}

const error = new ValidationError({ field: 'email' });
console.error(error.message); // "Validation failed for field 'email'"
ErrorFactory({
  name?: "PostAlreadyDeletedError" | undefinedname: 'PostAlreadyDeletedError',
  message: "This post has already been deleted." | ((fields: ErrorFields) => "This post has already been deleted.")message: 'This post has already been deleted.',
}) {}

// 投稿削除関数の例
type type PostDeleteError = PostNotFoundError | PostPermissionError | PostAlreadyDeletedErrorPostDeleteError = (
  | class PostNotFoundErrorPostNotFoundError
  | class PostPermissionErrorPostPermissionError
  | class PostAlreadyDeletedErrorPostAlreadyDeletedError
);

// 複数のエラー型を返す可能性がある関数
const const deletePost: (postId: string) => Result.ResultAsync<void, PostDeleteError>deletePost = async (postId: stringpostId: string): import ResultResult.type ResultAsync<T, E> = Promise<Result.Result<T, E>>An asynchronous variant of
Result
, wrapped in a Promise.
@typeParamT  - The type of the Success value.@typeParamE  - The type of the Failure value.@exampleimport { Result } from '@praha/byethrow';

const fetchData = async (): Result.ResultAsync<string, Error> => {
  try {
    const data = await fetch('...');
    return { type: 'Success', value: await data.text() };
  } catch (err) {
    return { type: 'Failure', error: err as Error };
  }
};
@categoryCore  TypesResultAsync<void, type PostDeleteError = PostNotFoundError | PostPermissionError | PostAlreadyDeletedErrorPostDeleteError> => {
  // いずれかのエラー型を返す可能性のある実装
};

// パターンマッチングによる結果のハンドリング
await import ResultResult.const pipe: <Result.ResultAsync<void, PostDeleteError>, Result.ResultAsync<"Post deleted successfully!", PostDeleteError>, Result.ResultAsync<"Post deleted successfully!", PostDeleteError>>(a: Result.ResultAsync<void, PostDeleteError>, ab: (a: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<"Post deleted successfully!", PostDeleteError>, bc: (b: Result.ResultAsync<"Post deleted successfully!", PostDeleteError>) => Result.ResultAsync<...>) => Result.ResultAsync<...> (+25 overloads)pipe(
  const deletePost: (postId: string) => Result.ResultAsync<void, PostDeleteError>deletePost('123'),
  import ResultResult.const map: <Result.ResultAsync<void, PostDeleteError>, "Post deleted successfully!">(fn: (a: void) => "Post deleted successfully!") => (result: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<"Post deleted successfully!", PostDeleteError> (+1 overload)map(() => 'Post deleted successfully!'),
  import ResultResult.const inspectError: <Result.ResultAsync<"Post deleted successfully!", PostDeleteError>, void>(fn: (a: PostDeleteError) => void) => (result: Result.ResultAsync<"Post deleted successfully!", PostDeleteError>) => Result.ResultAsync<"Post deleted successfully!", PostDeleteError> (+1 overload)inspectError((error: PostDeleteErrorerror) => {
    // 異なるエラー型を処理するためのパターンマッチング
  }),
);

推奨：`ts-pattern` を使用する

パターンマッチングにはts-patternの使用を強くお勧めします。優れたTypeScriptサポートを提供し、網羅的なマッチングを保証します。

インストール

npm

yarn

pnpm

bun

deno

npm install ts-pattern

`ts-pattern` によるパターンマッチング

import { import ResultResult } from '@praha/byethrow';
import { function match<const input, output = typeof unset>(value: input): Match<input, output>match creates a pattern matching expression.

Use .with(pattern, handler) to pattern match on the input.
Use .exhaustive() or .otherwise(() => defaultValue) to end the expression and get the result.

Read the documentation for match on GitHub
@exampledeclare let input: "A" | "B";
return match(input)
.with("A", () => "It's an A!")
.with("B", () => "It's a B!")
.exhaustive();match } from 'ts-pattern';

await import ResultResult.const pipe: <Result.ResultAsync<void, PostDeleteError>, Result.ResultAsync<void, PostDeleteError>>(a: Result.ResultAsync<void, PostDeleteError>, ab: (a: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError> (+25 overloads)pipe(
  function deletePost(postId: string): Result.ResultAsync<void, PostDeleteError>deletePost('123'),
  import ResultResult.const inspectError: <Result.ResultAsync<void, PostDeleteError>, void>(fn: (a: PostDeleteError) => void) => (result: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError> (+1 overload)inspectError((error: PostDeleteErrorerror) => {
    match<PostDeleteError, typeof unset>(value: PostDeleteError): Match<PostDeleteError, typeof unset>match creates a pattern matching expression.

Use .with(pattern, handler) to pattern match on the input.
Use .exhaustive() or .otherwise(() => defaultValue) to end the expression and get the result.

Read the documentation for match on GitHub
@exampledeclare let input: "A" | "B";
return match(input)
.with("A", () => "It's an A!")
.with("B", () => "It's a B!")
.exhaustive();match(error: PostDeleteErrorerror)
      .with<{
    readonly name: "PostNotFoundError";
}, void, PostNotFoundError>(pattern: {
    readonly name: "PostNotFoundError";
}, handler: (selections: PostNotFoundError, value: PostNotFoundError) => void): Match<PostPermissionError | PostAlreadyDeletedError, typeof unset, [{
    readonly name: "PostNotFoundError";
}], void> (+3 overloads)
.with(pattern, handler) Registers a pattern and an handler function that
will be called if the pattern matches the input value.
Read the documentation for .with() on GitHubwith({ name: "PostNotFoundError"name: 'PostNotFoundError' }, () => {
        var console: Consoleconsole.Console.error(...data: any[]): voidThe console.error() static method outputs a message to the console at the "error" log level. The message is only displayed to the user if the console is configured to display error output. In most cases, the log level is configured within the console UI. The message may be formatted as an error, with red colors and call stack information.
MDN Referenceerror('リクエストされた投稿が見つかりませんでした。');
      })
      .with<{
    readonly name: "PostPermissionError";
}, void, PostPermissionError>(pattern: {
    readonly name: "PostPermissionError";
}, handler: (selections: PostPermissionError, value: PostPermissionError) => void): Match<PostAlreadyDeletedError, typeof unset, [{
    readonly name: "PostNotFoundError";
}, {
    readonly name: "PostPermissionError";
}], void> (+3 overloads)
.with(pattern, handler) Registers a pattern and an handler function that
will be called if the pattern matches the input value.
Read the documentation for .with() on GitHubwith({ name: "PostPermissionError"name: 'PostPermissionError' }, () => {
        var console: Consoleconsole.Console.error(...data: any[]): voidThe console.error() static method outputs a message to the console at the "error" log level. The message is only displayed to the user if the console is configured to display error output. In most cases, the log level is configured within the console UI. The message may be formatted as an error, with red colors and call stack information.
MDN Referenceerror('このアクションを実行する権限がありません。');
      })
      .with<{
    readonly name: "PostAlreadyDeletedError";
}, void, PostAlreadyDeletedError>(pattern: {
    readonly name: "PostAlreadyDeletedError";
}, handler: (selections: PostAlreadyDeletedError, value: PostAlreadyDeletedError) => void): Match<never, typeof unset, [{
    readonly name: "PostNotFoundError";
}, {
    readonly name: "PostPermissionError";
}, {
    readonly name: "PostAlreadyDeletedError";
}], void> (+3 overloads)
.with(pattern, handler) Registers a pattern and an handler function that
will be called if the pattern matches the input value.
Read the documentation for .with() on GitHubwith({ name: "PostAlreadyDeletedError"name: 'PostAlreadyDeletedError' }, () => {
        var console: Consoleconsole.Console.error(...data: any[]): voidThe console.error() static method outputs a message to the console at the "error" log level. The message is only displayed to the user if the console is configured to display error output. In most cases, the log level is configured within the console UI. The message may be formatted as an error, with red colors and call stack information.
MDN Referenceerror('この投稿は既に削除されています。');
      })
      .exhaustive: () => void (+1 overload).exhaustive() checks that all cases are handled, and returns the result value.
If you get a NonExhaustiveError, it means that you aren't handling
all cases. You should probably add another .with(...) clause
to match the missing case and prevent runtime errors.
Read the documentation for .exhaustive() on GitHubexhaustive(); // すべてのケースが処理されることを保証する
  }),
);

代替案：TypeScriptネイティブなパターンマッチング

外部ライブラリを使用したくない場合は、TypeScriptの組み込み機能を使用してパターンマッチングを実装できます。

`instanceof` を使用する

import { import ResultResult } from '@praha/byethrow';

await import ResultResult.const pipe: <Result.ResultAsync<void, PostDeleteError>, Result.ResultAsync<void, PostDeleteError>>(a: Result.ResultAsync<void, PostDeleteError>, ab: (a: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError> (+25 overloads)pipe(
  function deletePost(postId: string): Result.ResultAsync<void, PostDeleteError>deletePost('123'),
  import ResultResult.const inspectError: <Result.ResultAsync<void, PostDeleteError>, void>(fn: (a: PostDeleteError) => void) => (result: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError> (+1 overload)inspectError((error: PostDeleteErrorerror) => {
    if (error: PostDeleteErrorerror instanceof class PostNotFoundErrorPostNotFoundError) {
      var console: Consoleconsole.Console.error(...data: any[]): voidThe console.error() static method outputs a message to the console at the "error" log level. The message is only displayed to the user if the console is configured to display error output. In most cases, the log level is configured within the console UI. The message may be formatted as an error, with red colors and call stack information.
MDN Referenceerror('リクエストされた投稿が見つかりませんでした。');
    }
    if (error: PostDeleteErrorerror instanceof class PostPermissionErrorPostPermissionError) {
      var console: Consoleconsole.Console.error(...data: any[]): voidThe console.error() static method outputs a message to the console at the "error" log level. The message is only displayed to the user if the console is configured to display error output. In most cases, the log level is configured within the console UI. The message may be formatted as an error, with red colors and call stack information.
MDN Referenceerror('このアクションを実行する権限がありません。');
    }
    if (error: PostDeleteErrorerror instanceof class PostAlreadyDeletedErrorPostAlreadyDeletedError) {
      var console: Consoleconsole.Console.error(...data: any[]): voidThe console.error() static method outputs a message to the console at the "error" log level. The message is only displayed to the user if the console is configured to display error output. In most cases, the log level is configured within the console UI. The message may be formatted as an error, with red colors and call stack information.
MDN Referenceerror('この投稿は既に削除されています。');
    }
  }),
);

判別可能なユニオンを使用する

import { import ResultResult } from '@praha/byethrow';

await import ResultResult.const pipe: <Result.ResultAsync<void, PostDeleteError>, Result.ResultAsync<void, PostDeleteError>>(a: Result.ResultAsync<void, PostDeleteError>, ab: (a: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError> (+25 overloads)pipe(
  function deletePost(postId: string): Result.ResultAsync<void, PostDeleteError>deletePost('123'),
  import ResultResult.const inspectError: <Result.ResultAsync<void, PostDeleteError>, void>(fn: (a: PostDeleteError) => void) => (result: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError> (+1 overload)inspectError((error: PostDeleteErrorerror) => {
    switch (error: PostDeleteErrorerror.Error.name: "PostNotFoundError" | "PostPermissionError" | "PostAlreadyDeletedError"name) {
      case 'PostNotFoundError':
        var console: Consoleconsole.Console.error(...data: any[]): voidThe console.error() static method outputs a message to the console at the "error" log level. The message is only displayed to the user if the console is configured to display error output. In most cases, the log level is configured within the console UI. The message may be formatted as an error, with red colors and call stack information.
MDN Referenceerror('リクエストされた投稿が見つかりませんでした。');
        break;
      case 'PostPermissionError':
        var console: Consoleconsole.Console.error(...data: any[]): voidThe console.error() static method outputs a message to the console at the "error" log level. The message is only displayed to the user if the console is configured to display error output. In most cases, the log level is configured within the console UI. The message may be formatted as an error, with red colors and call stack information.
MDN Referenceerror('このアクションを実行する権限がありません。');
        break;
      case 'PostAlreadyDeletedError':
        var console: Consoleconsole.Console.error(...data: any[]): voidThe console.error() static method outputs a message to the console at the "error" log level. The message is only displayed to the user if the console is configured to display error output. In most cases, the log level is configured within the console UI. The message may be formatted as an error, with red colors and call stack information.
MDN Referenceerror('この投稿は既に削除されています。');
        break;
      default:
        // すべてのエラー型がカバーされていれば、ここには到達しないはず
        const const _exhaustiveCheck: never_exhaustiveCheck: never = error: nevererror;
        throw new var Error: ErrorConstructor
new (message?: string, options?: ErrorOptions) => Error (+1 overload)
Error(`Unhandled error type: ${var JSON: JSONAn intrinsic object that provides functions to convert JavaScript values to and from the JavaScript Object Notation (JSON) format.JSON.JSON.stringify(value: any, replacer?: (this: any, key: string, value: any) => any, space?: string | number): string (+1 overload)Converts a JavaScript value to a JavaScript Object Notation (JSON) string.
@paramvalue  A JavaScript value, usually an object or array, to be converted.@paramreplacer  A function that transforms the results.@paramspace  Adds indentation, white space, and line break characters to the return-value JSON text to make it easier to read.@throws{TypeError} If a circular reference or a BigInt value is found.stringify(const _exhaustiveCheck: never_exhaustiveCheck)}`);
    }
  }),
);

#パターンマッチング

#なぜユニオンエラーにパターンマッチングが必要なのか？

#推奨：ts-pattern を使用する

#インストール

#ts-pattern によるパターンマッチング

#代替案：TypeScriptネイティブなパターンマッチング

#instanceof を使用する

#判別可能なユニオンを使用する