Writer

Writer[W] lets you accumulate values of type W during a computation. This is useful for collecting events, audit logs, diagnostics, or any output that builds up as your logic runs.

Basic usage

import purelogic.*

def process(using Writer[String]): Int = {
  write("Starting")
  write("Processing")
  42
}

val (logs, result) = Writer {
  process
}
// logs: Vector("Starting", "Processing")
// result: 42

Functions

write

Appends a single value to the log:

write("Order validated")

writeAll

Appends multiple values at once:

writeAll(List("Step 1", "Step 2", "Step 3"))

clear

Clears all accumulated values:

write("this will be gone")
clear
write("fresh start")
// only "fresh start" will be in the log

capture

Runs a block in a nested scope, returning both the captured writes and the result. The captured writes are also forwarded to the outer writer:

def process(using Writer[String]): Unit = {
  write("before")
  val (captured, result) = capture {
    write("inside")
    42
  }
  write("after")
  // captured: Vector("inside")
  // outer log: Vector("before", "inside", "after")
}

Running

Writer(body) returns a tuple of the accumulated values and the result:

val (logs: Vector[W], result: A) = Writer {
  myProgram
}