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One generic function serves every type its bound allows, and the bound is checked

top_by finds the item with the largest key: the priciest order, the latest date, the longest name. It is written once, for any item type T and any key type K, and the closure you pass picks the key. The bound K < Ord says what top_by needs from a key: that two keys can be compared. The compiler checks the bound at each call, so a key that can't be ordered is rejected where you pass it, not deep inside the function.

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fn top_by[T, K < Ord](items: List[T], key: fn(T) -> K) -> T:    let best = items[0]    for item in items:        if key(item) > key(best):  # ← needs K < Ord            best = item    bestpub fn main() -> void $ Console:    orders := [Order { id: "A-1", cents: 1250 }, Order { id: "A-2", cents: 4999 }]    println(top_by(orders, fn(order): order.cents).id)  # ← K is i32    println(top_by(["ada", "grace"], fn(name): name.len()))# Change the first key to `fn(order): order` and Run. An# order has no ordering, and the call is where it fails:#     unsatisfied-trait-bound: Order does not implement Ord# ── plumbing ──struct Order:    id: string    cents: i32

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