Reader

Reader[R] provides read-only access to a value of type R. This is typically used for configuration, environment, or any context that your logic needs to read but never modify.

Basic usage

import purelogic.*

case class Config(maxRetries: Int, timeout: Int)

def process(using Reader[Config]): String = {
  val retries = read(_.maxRetries)
  s"Will retry $retries times"
}

val result = Reader(Config(maxRetries = 3, timeout = 5000)) {
  process
}
// result: "Will retry 3 times"

Functions

read

Returns the current reader value:

val config: Config = read

You can also pass a projection function to extract a field directly:

val retries: Int = read(_.maxRetries)

local

Runs a block with a modified reader value. The original value is restored after the block completes:

def process(using Reader[Config]): String = {
  val outer = read(_.maxRetries) // 3
  val inner = local(_.copy(maxRetries = 10)) {
    read(_.maxRetries) // 10
  }
  val after = read(_.maxRetries) // 3
  s"$outer, $inner, $after"
}

focus

Runs a block with a narrowed reader. This is useful when calling a function that only needs part of your reader:

def retryLogic(using Reader[Int]): Unit = {
  val maxRetries = read
  // ...
}

def process(using Reader[Config]): Unit = {
  focus(_.maxRetries) {
    retryLogic // only sees an Int, not the full Config
  }
}

Running

Reader(value)(body) provides the value and returns the result directly (no wrapping):

val result: A = Reader(myConfig) {
  myProgram
}