etl4s

Side Effects

Observing with .tap()

Peek at values mid-pipeline without modifying them:

import etl4s._

val greeting = Node("hello world")
val split    = Node[String, Array[String]](_.split(" "))

val pipeline = greeting
  .tap(x => println(s"Got: $x"))
  ~> split

pipeline.unsafeRun(())
Prints: 
Got: hello world
and returns Array("hello", "world").

Chain taps at different stages:

val pipeline = fetchData
  .tap(files => println(s"Fetched ${files.size} files"))
  ~> processFiles
  .tap(count => println(s"Processed $count files"))

Sequential Effects with >>

Run multiple nodes in order, same input to each. Only the last result is kept:

val logStart = Node[String, Unit](s => println(s"Start: $s"))
val logEnd   = Node[String, Unit](s => println(s"End: $s"))
val process  = Node[String, Int](_.length)

val pipeline = logStart >> logEnd >> process

pipeline.unsafeRun("hello")
Prints: 
Start: hello
End: hello
and returns 5.

Common for setup/teardown:

val clearCache = Node { println("Clearing cache...") }
val warmCache  = Node { println("Warming cache...") }

val pipeline = clearCache >> warmCache >> mainPipeline

Or audit logging:

val auditStart = Node[Request, Unit](r => log(s"Started ${r.id}"))
val auditEnd   = Node[Request, Unit](r => log(s"Finished ${r.id}"))

val pipeline = auditStart >> processRequest >> auditEnd

Laziness

Side effects are lazy - nothing executes until .unsafeRun(). Build and compose freely without triggering effects.