Structure instances are mutable by default. Golo generates a factory function with the Immutable prefix to directly build immutable instances:

module test

struct Point = { x, y }

function main = |args| {

  let p = ImmutablePoint(1, 2)
  println(p)

  try {
    # Fails! (p is immutable)
    p: x(100)
  } catch (expected) {
    println(expected: getMessage())
  }
}