bounds

# Bounds: a generic that asks its type parameter for a trait.

trait Priced
  abstract def price : Int32
end

struct Book
  def initialize(@price : Int32)
  end
end

struct Coffee
  def initialize(@price : Int32)
  end
end

impl Priced for Book
  def price : Int32
    @price
  end
end

impl Priced for Coffee
  def price : Int32
    @price
  end
end

# `T : Priced` says: any T, as long as it implements Priced. The bound is
# checked where the call is made, so `total([1, 2])` would not compile.
def total(items : Array(T)) : Int32 forall T : Priced
  items.sum { |item| item.price }
end

puts total([Book.new(120), Book.new(80)])
puts total([Coffee.new(45)])

def cheapest(items : Array(T)) : T forall T : Priced
  items.min_by { |item| item.price }
end

puts cheapest([Coffee.new(45), Coffee.new(30)]).price