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A list or map is typed by what it holds, and a lookup that may miss says so

[...] builds a List and {key: value} a Map, both from the prelude. Every element has the collection's type, so a Map[string, i32] never hands back a string. Indexing a map with wanted[item] = ... sets an entry, but reading one is get, which gives an i32?, because the key may be absent. You decide what a missing key means, here zero on hand. A comprehension puts its clauses before => and the value after it, and iter() starts a chain of steps such as map and sum.

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fn shortages(stock: Map[string, i32], orders: List[Order]) -> List[string]: let mut wanted: Map[string, i32] = {} for order in orders: wanted[order.item] = on_hand(wanted, order.item) + order.quantity [for (item, count) in wanted if count > on_hand(stock, item) => "$item: short ${count - on_hand(stock, item)}"]fn on_hand(stock: Map[string, i32], item: string) -> i32: stock.get(item).unwrap_or(0) # ← a lookup may miss: `get` gives an i32?pub fn main() -> void $ Console: stock := {"lamp": +4, "bulb": +20} # ← a Map[string, i32] orders := [Order { item: "lamp", quantity: 3 }, Order { item: "bulb", quantity: 6 }, Order { item: "lamp", quantity: 2 }] for line in shortages(stock, orders): println(line) let units: i32 = orders.iter().map(fn(order): order.quantity).sum() # ← an iterator chain println("$units units ordered")# Change the body of `on_hand` to `stock.get(item)` and Run. A# lookup may miss, so its result is not a plain i32:# type-mismatch: in result: expected i32, found i32?# ── plumbing ──struct Order: item: string quantity: i32