Reader
Dependency injection without global state
What is Reader?
Reader provides dependency injection without global state. Pass configuration or dependencies through your entire program without globals. Makes your code more testable and explicit about what it needs.
// Instead of globals:
const config = getGlobalConfig()
function doWork() { use(config) }
// Use Reader:
const doWork: Reader<Config, Result> = R.asks(...)Dependency Injection
Pass config without globals
Testable
Easy to mock dependencies
Composable
Chain operations with shared context
Pure Functions
No hidden dependencies
Reader in Action
❌ Global Config
const globalConfig = { apiUrl: '...' }
function getUser(id: number) {
return fetch(`${globalConfig.apiUrl}/users/${id}`)
}
// Global state, hard to test✅ Reader Approach
const getUser = (id: number): Reader<Config, Promise<User>> =>
R.asks(config =>
fetch(`${config.apiUrl}/users/${id}`)
)
// Dependency injection, testablePractice Exercises
Create Reader
Learn about create reader in fp-ts reader
Map
Learn about map in fp-ts reader
Ask
Learn about ask in fp-ts reader
Chain
Learn about chain in fp-ts reader
Dependency Injection
Learn about dependency injection in fp-ts reader
Local
Learn about local in fp-ts reader
Real World Config
Learn about real world config in fp-ts reader
Reader Pipeline
Learn about reader pipeline in fp-ts reader
Why Learn Reader?
Master Reader
Learn the fundamental concepts and patterns that make Reader powerful
8 Exercises
Practice with hands-on exercises from advanced level
Production Ready
Apply Reader patterns to build robust, type-safe applications