generic-functions

# Generic functions: `forall T` makes a function work for any type.

def first(items : Array(T)) : T forall T
  items[0]
end

puts first([10, 20, 30])
puts first(["a", "b"])

def swap(pair : Tuple(A, B)) : Tuple(B, A) forall A, B
  {pair[1], pair[0]}
end

puts swap({1, "one"})

# The type is fixed per call: `first` of an Int32 array is an Int32, so
# arithmetic on the result compiles.
puts first([1, 2]) + 100

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 generic-functions.wasm, the module the compiler built from this file, in your browser. How this works

$ iyi run generic-functions.iyi