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      • Pattern Matching

      When working with Result types that have union error types, pattern matching becomes essential for handling different error scenarios effectively. This guide demonstrates how to use pattern matching to handle multiple error types gracefully.

      Why Pattern Matching for Union Errors?

      When your Result can fail with multiple different error types, you need a systematic way to handle each error type appropriately. Pattern matching provides a clean, type-safe approach to handle all possible error cases.

      Tip

      For more details about @praha/error-factory, see the Custom Error page.

      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.@example
      Basic  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"
      @example
      Omitting  name
      

      class NotFoundError extends ErrorFactory({
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name); // "NotFoundError" (resolved from new.target.name)
      @example
      With  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
      @example
      With  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"
      @example
      Dynamic  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 Result
      Result       } from '@praha/byethrow';      

class 
      class PostNotFoundError
      PostNotFoundError       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.@example
      Basic  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"
      @example
      Omitting  name
      

      class NotFoundError extends ErrorFactory({
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name); // "NotFoundError" (resolved from new.target.name)
      @example
      With  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
      @example
      With  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"
      @example
      Dynamic  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" | undefined
      name      : '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 PostPermissionError
      PostPermissionError       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.@example
      Basic  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"
      @example
      Omitting  name
      

      class NotFoundError extends ErrorFactory({
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name); // "NotFoundError" (resolved from new.target.name)
      @example
      With  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
      @example
      With  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"
      @example
      Dynamic  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" | undefined
      name      : '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 PostAlreadyDeletedError
      PostAlreadyDeletedError       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.@example
      Basic  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"
      @example
      Omitting  name
      

      class NotFoundError extends ErrorFactory({
message: 'Resource not found',
}) {}

const error = new NotFoundError();
console.error(error.name); // "NotFoundError" (resolved from new.target.name)
      @example
      With  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
      @example
      With  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"
      @example
      Dynamic  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" | undefined
      name      : 'PostAlreadyDeletedError',      
  
      message: "This post has already been deleted." | ((fields: ErrorFields) => "This post has already been deleted.")
      message      : 'This post has already been deleted.',      
}) {}

// Example of a post deletion function
type 
      type PostDeleteError = PostNotFoundError | PostPermissionError | PostAlreadyDeletedError
      PostDeleteError       = (      
  | 
      class PostNotFoundError
      PostNotFoundError      
  | 
      class PostPermissionError
      PostPermissionError      
  | 
      class PostAlreadyDeletedError
      PostAlreadyDeletedError      
);

// Function that can return multiple error types
const 
      const deletePost: (postId: string) => Result.ResultAsync<void, PostDeleteError>
      deletePost       = async (
      postId: string
      postId      : string): 
      import Result
      Result      .
      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.@example
      import { 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  Types
      ResultAsync      <void, 
      type PostDeleteError = PostNotFoundError | PostPermissionError | PostAlreadyDeletedError
      PostDeleteError      > => {      
  // Implementation that can return any of the error types
};

// Handling the result with pattern matching
await 
      import Result
      Result      .
      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 Result
      Result      .
      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 Result
      Result      .
      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: PostDeleteError
      error      ) => {      
    // Pattern matching to handle different error types
  }),
);

      We strongly recommend using the ts-pattern for pattern matching. It provides excellent TypeScript support and ensures exhaustive matching.

      Installation

      npm
      yarn
      pnpm
      bun
      deno
      npm install ts-pattern

      Pattern Matching with ts-pattern

      import { 
      import Result
      Result       } 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
      @example
      declare 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 Result
      Result      .
      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 Result
      Result      .
      const inspectError: <Result.ResultAsync<void, PostDeleteError>, void>(fn: (a: PostDeleteError) => void) => (result: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError> (+1 overload)
      inspectError      ((
      error: PostDeleteError
      error      ) => {      
    
      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
      @example
      declare let input: "A" | "B";
      

      return match(input)
.with("A", () => "It's an A!")
.with("B", () => "It's a B!")
.exhaustive();
      match      (
      error: PostDeleteError
      error      )      
      .
      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 GitHub
      with      ({ 
      name: "PostNotFoundError"
      name      : 'PostNotFoundError' }, () => {      
        
      var console: Console
      console      .
      Console.error(...data: any[]): void
      The 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 Reference
      error      ('The requested post was not found.');      
      })
      .
      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 GitHub
      with      ({ 
      name: "PostPermissionError"
      name      : 'PostPermissionError' }, () => {      
        
      var console: Console
      console      .
      Console.error(...data: any[]): void
      The 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 Reference
      error      ('You do not have permission to perform this action.');      
      })
      .
      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 GitHub
      with      ({ 
      name: "PostAlreadyDeletedError"
      name      : 'PostAlreadyDeletedError' }, () => {      
        
      var console: Console
      console      .
      Console.error(...data: any[]): void
      The 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 Reference
      error      ('This post has already been deleted.');      
      })
      .
      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 GitHub
      exhaustive      (); // Ensures all cases are handled      
  }),
);

      Alternative: Native TypeScript Pattern Matching

      If you prefer not to use external libraries, you can implement pattern matching using TypeScript's built-in features:

      Using instanceof Checks

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

await 
      import Result
      Result      .
      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 Result
      Result      .
      const inspectError: <Result.ResultAsync<void, PostDeleteError>, void>(fn: (a: PostDeleteError) => void) => (result: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError> (+1 overload)
      inspectError      ((
      error: PostDeleteError
      error      ) => {      
    if (
      error: PostDeleteError
      error       instanceof 
      class PostNotFoundError
      PostNotFoundError      ) {      
      
      var console: Console
      console      .
      Console.error(...data: any[]): void
      The 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 Reference
      error      ('The requested post was not found.');      
    }
    if (
      error: PostDeleteError
      error       instanceof 
      class PostPermissionError
      PostPermissionError      ) {      
      
      var console: Console
      console      .
      Console.error(...data: any[]): void
      The 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 Reference
      error      ('You do not have permission to perform this action.');      
    }
    if (
      error: PostDeleteError
      error       instanceof 
      class PostAlreadyDeletedError
      PostAlreadyDeletedError      ) {      
      
      var console: Console
      console      .
      Console.error(...data: any[]): void
      The 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 Reference
      error      ('This post has already been deleted.');      
    }
  }),
);

      Using Discriminated Unions

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

await 
      import Result
      Result      .
      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 Result
      Result      .
      const inspectError: <Result.ResultAsync<void, PostDeleteError>, void>(fn: (a: PostDeleteError) => void) => (result: Result.ResultAsync<void, PostDeleteError>) => Result.ResultAsync<void, PostDeleteError> (+1 overload)
      inspectError      ((
      error: PostDeleteError
      error      ) => {      
    switch (
      error: PostDeleteError
      error      .
      Error.name: "PostNotFoundError" | "PostPermissionError" | "PostAlreadyDeletedError"
      name      ) {      
      case 'PostNotFoundError':
        
      var console: Console
      console      .
      Console.error(...data: any[]): void
      The 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 Reference
      error      ('The requested post was not found.');      
        break;
      case 'PostPermissionError':
        
      var console: Console
      console      .
      Console.error(...data: any[]): void
      The 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 Reference
      error      ('You do not have permission to perform this action.');      
        break;
      case 'PostAlreadyDeletedError':
        
      var console: Console
      console      .
      Console.error(...data: any[]): void
      The 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 Reference
      error      ('This post has already been deleted.');      
        break;
      default:
        // This should never happen if all error types are covered
        const 
      const _exhaustiveCheck: never
      _exhaustiveCheck      : never = 
      error: never
      error      ;      
        throw new 
      var Error: ErrorConstructor
new (message?: string, options?: ErrorOptions) => Error (+1 overload)
      Error      (`Unhandled error type: ${
      var JSON: JSON
      An 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      )}`);      
    }
  }),
);