defer

# `defer`: name the cleanup where you acquire, run it when the scope ends.

def work : Int32
  puts "open"
  defer puts "close"

  puts "working"
  42
end

puts work

# Deferred lines run last-in, first-out.
def nested : Nil
  defer puts "third"
  defer puts "second"
  puts "first"
end

nested

# The scope is the block, so in a loop the cleanup runs every turn.
2.times do |i|
  defer puts "done with #{i}"
  puts "turn #{i}"
end

Press Run.

recorded with iyi 0.18.0 (built on Crystal 1.22.0-dev [2a9653052] (2026-10-18)) · Linux 7.0.0-38-generic x86_64, QEMU Virtual CPU version 2.5+ · 2026-10-19

You can type in the pane: nothing here compiles, so what you type cannot run, and the page says which bytes it is running the moment the text stops being the recorded file. There is no standard input, no filesystem and no threads on this target, and one sample fails on purpose for exactly that reason.

Run executes defer.wasm, the module the compiler built from this file, in your browser. How this works

$ iyi run defer.iyi