generic-types

# Generic types: a struct with a type parameter.

struct Stack(T)
  def initialize
    @items = [] of T
  end

  def push(item : T) : Nil
    @items << item
  end

  def pop : T?
    @items.empty? ? nil : @items.pop
  end

  def size : Int32
    @items.size
  end
end

numbers = Stack(Int32).new
numbers.push(1)
numbers.push(2)
puts numbers.size
puts numbers.pop
puts numbers.pop
puts numbers.pop

words = Stack(String).new
words.push("iyi")
puts words.pop

# A trait can be implemented for the generic type once, for every T.
trait Describe
  abstract def describe : String
end

impl Describe for Stack(T) forall T
  def describe : String
    "a stack of #{size}"
  end
end

numbers.push(5)
puts numbers.describe
puts words.describe