tasks

# Tasks: run work at the same time, inside a group that waits for it all.

def slow_square(n : Int32) : Int32
  sleep 10   # milliseconds
  n * n
end

# `group` opens a scope. Every `spawn` inside it is a task, and the
# group's `end` waits for every one of them: no task outlives its group.
results = [] of Int32

group do |g|
  (1..4).each do |n|
    g.spawn do
      results << slow_square(n)
      0
    end
  end
end

puts results.sorted

# `end!` turns the group into a value: the tuple of what its tasks answered.
def both : Tuple(String, String) | Panicked
  group do |g|
    a = g.spawn { "first" }
    b = g.spawn { "second" }
  end!
end

case both
in Tuple(String, String) then puts it
in Panicked              then puts "a task panicked"
end

[1, 4, 9, 16]
{"first", "second"}

exit 0, on Linux 7.0.0-38-generic x86_64, QEMU Virtual CPU version 2.5+

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

This program does not compile for the browser's target, so this page shows what it printed when the compiler ran it natively, on the machine named above. You can still type in the pane, and the page will say when what is in it is no longer the file the recording is about.

Tasks and channels run on Linux, macOS arm64 and Windows x86-64, and not on wasm32 yet: a program that names `group` or `Channel` there does not compile, because src/iyi/concurrency.iyi refuses to ship a spelling without the feature (SPEC.md III.4.8). What you see is what the program printed when the compiler ran it natively. How this works

$ iyi run tasks.iyi