Updating without mutating

You never change a value in place — you derive a new one with with. Point it at the position to replace; its refers to the value you started from:

type Counter = { label: String, n: Int };
fix c = Counter{ label: "hits", n: 0 };
fix c2 = c with n = its.n + 1;
"${c2.label}: ${c2.n} (orig ${c.n})"

c2 is a fresh counter with n bumped to 1 — and c still reads 0, untouched. That’s the whole point: with hands back a copy.

One update is braceless. Several go in braces:

type Box = { w: Int, h: Int };
fix b = Box{ w: 2, h: 3 };
fix bigger = b with { w = its.w * 10, h = its.h * 10 };
"${bigger.w} x ${bigger.h}"

Two more notes: the update uses = (you’re assigning into a copy), not the : of construction — and a path can reach deeper, like doc with line.to = 9.