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Overview

wrapClient creates a reusable wrapper around any Promise-based client (like API clients, database clients, etc.) that converts operations into Effects with consistent error handling.

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Signature

function wrapClient<CLIENT, ERROR extends Error>(opts: {
  client: CLIENT;
  error: (ctx: { cause: Cause; message?: Message }) => ERROR;
}): <OUTPUT, OVERRIDEN_ERROR>(
  func: (client: CLIENT) => Promise<OUTPUT>,
  overrides?: {
    errorHandler?: (cause: Cause) => OVERRIDEN_ERROR;
    errorMessage?: ((cause: Cause) => Message) | Message;
  }
) => Effect.Effect<OUTPUT, ERROR | OVERRIDEN_ERROR>

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Parameters

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opts.client
CLIENT
required
The client instance to wrap. Can be any object with Promise-returning methods.
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opts.error
(ctx: { cause: unknown; message?: string }) => ERROR
required
Factory function that creates your custom error type from a cause and optional message.

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Returns

Returns a wrapper function that accepts:
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func
(client: CLIENT) => Promise<OUTPUT>
required
A function that uses the client to perform an operation.
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overrides.errorHandler
(cause: unknown) => OVERRIDEN_ERROR
Optional custom error handler for this specific operation. Overrides the default error factory.
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overrides.errorMessage
string | (cause: unknown) => string
Optional error message (static string or function). Passed to the error factory.
The wrapper returns an Effect.Effect<OUTPUT, ERROR | OVERRIDEN_ERROR>.

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Basic Usage

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Simple Client Wrapper

import { wrapClient } from 'ff-effect';
import { Effect } from 'effect';

class ApiError extends Error {
  constructor(
    public cause: unknown,
    message?: string
  ) {
    super(message ?? 'API request failed');
  }
}

const apiClient = {
  fetchUser: (id: number) => fetch(`/api/users/${id}`).then(r => r.json()),
  createUser: (data: UserData) => fetch('/api/users', {
    method: 'POST',
    body: JSON.stringify(data)
  }).then(r => r.json())
};

// Create the wrapper once
const wrap = wrapClient({
  client: apiClient,
  error: ({ cause, message }) => new ApiError(cause, message)
});

// Use it for multiple operations
const program = Effect.gen(function* () {
  // Fetch with custom error message
  const user = yield* wrap(
    (client) => client.fetchUser(1),
    { errorMessage: 'Failed to fetch user' }
  );
  
  // Create without custom message
  const newUser = yield* wrap(
    (client) => client.createUser({ name: 'Alice' })
  );
  
  return { user, newUser };
});

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Error Handling Options

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Default Error

Without overrides, uses the error factory provided to wrapClient: 
const wrap = wrapClient({
  client: myClient,
  error: ({ cause, message }) => new MyError(cause, message)
});

const effect = wrap((client) => client.fetchData());
// On failure: throws MyError with no custom message

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Static Error Message

Provide a static string message: 
const effect = wrap(
  (client) => client.fetchData(),
  { errorMessage: 'Failed to fetch data' }
);
// On failure: throws MyError with message 'Failed to fetch data'

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Dynamic Error Message

Compute the error message from the cause: 
const effect = wrap(
  (client) => client.fetchData(),
  {
    errorMessage: (cause) => {
      if (cause instanceof Error) {
        return `API Error: ${cause.message}`;
      }
      return 'Unknown error occurred';
    }
  }
);

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Custom Error Handler

Completely override the error handling for specific operations: 
import { Data } from 'effect';

class ValidationError extends Data.TaggedError('ValidationError')<{
  cause: unknown;
}> {}

const effect = wrap(
  (client) => client.validateUser(data),
  {
    errorHandler: (cause) => new ValidationError({ cause })
  }
);
// On failure: throws ValidationError instead of the default error type

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Real-World Examples

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HTTP Client Wrapper

import { wrapClient } from 'ff-effect';
import { Data, Effect } from 'effect';

class HttpError extends Data.TaggedError('HttpError')<{
  status?: number;
  cause: unknown;
  message: string;
}> {}

interface HttpClient {
  get: (url: string) => Promise<Response>;
  post: (url: string, body: unknown) => Promise<Response>;
  put: (url: string, body: unknown) => Promise<Response>;
  delete: (url: string) => Promise<Response>;
}

const createHttpWrapper = (client: HttpClient) => {
  return wrapClient({
    client,
    error: ({ cause, message }) => {
      const status = cause instanceof Response ? cause.status : undefined;
      return new HttpError({
        status,
        cause,
        message: message ?? 'HTTP request failed'
      });
    }
  });
};

// Usage
const http = createHttpWrapper(myHttpClient);

const program = Effect.gen(function* () {
  const users = yield* http(
    (client) => client.get('/api/users').then(r => r.json()),
    { errorMessage: 'Failed to fetch users' }
  );
  
  const created = yield* http(
    (client) => client.post('/api/users', { name: 'Alice' }).then(r => r.json()),
    { errorMessage: (cause) => `User creation failed: ${cause}` }
  );
  
  return { users, created };
});

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Database Client Wrapper

import { wrapClient } from 'ff-effect';
import { Data, Effect } from 'effect';

class DbError extends Data.TaggedError('DbError')<{
  query?: string;
  cause: unknown;
  message: string;
}> {}

interface DbClient {
  query: (sql: string, params?: unknown[]) => Promise<unknown[]>;
  execute: (sql: string, params?: unknown[]) => Promise<{ affectedRows: number }>;
}

const createDbWrapper = (client: DbClient) => {
  return wrapClient({
    client,
    error: ({ cause, message }) => new DbError({
      cause,
      message: message ?? 'Database operation failed'
    })
  });
};

const db = createDbWrapper(myDbClient);

const program = Effect.gen(function* () {
  const users = yield* db(
    (client) => client.query('SELECT * FROM users WHERE id = ?', [1]),
    { errorMessage: 'Failed to query users' }
  );
  
  return users;
});

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Third-Party API Client

import { wrapClient } from 'ff-effect';
import { Effect } from 'effect';
import { OpenAI } from 'openai';

class OpenAIError extends Error {
  constructor(
    public cause: unknown,
    message?: string
  ) {
    super(message ?? 'OpenAI API error');
  }
}

const createOpenAIWrapper = (client: OpenAI) => {
  return wrapClient({
    client,
    error: ({ cause, message }) => new OpenAIError(cause, message)
  });
};

const openai = createOpenAIWrapper(new OpenAI({ apiKey: process.env.OPENAI_API_KEY }));

const program = Effect.gen(function* () {
  const completion = yield* openai(
    (client) => client.chat.completions.create({
      model: 'gpt-4',
      messages: [{ role: 'user', content: 'Hello!' }]
    }),
    { errorMessage: 'Failed to generate completion' }
  );
  
  return completion.choices[0].message.content;
});

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Type Safety

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Generic Client Types

The wrapper preserves full type information: 
interface MyClient {
  getString: () => Promise<string>;
  getNumber: () => Promise<number>;
  getUser: () => Promise<{ id: number; name: string }>;
}

const wrap = wrapClient<MyClient, MyError>({
  client: myClient,
  error: ({ cause, message }) => new MyError(cause, message)
});

// Return types are fully inferred
const stringEffect: Effect.Effect<string, MyError> = wrap(
  (client) => client.getString()
);

const userEffect: Effect.Effect<{ id: number; name: string }, MyError> = wrap(
  (client) => client.getUser()
);

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Error Type Overrides

When using errorHandler, the error type is updated: 
class CustomError extends Error {}

const wrap = wrapClient<Client, DefaultError>({
  client,
  error: ({ cause }) => new DefaultError(cause)
});

// Type: Effect.Effect<string, CustomError>
const effect = wrap(
  (client) => client.getData(),
  { errorHandler: (cause) => new CustomError('custom') }
);

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See Also