Basic demonstration of what can be done with Crystal Generic classes

crystal_generics.cr

class Gen(T)
  getter value : T

  def initialize(@value : T)
  end

  def self.build(value : K) : Gen(K) forall K
      Gen(K).new(value)
  end

  def type : Class
    T
  end

  def test : T
    value
  end
end

gen1 = Gen.build("Foo")
gen2 = Gen.build(1)
gen1.test.class
gen2.test.class

class SpecString < Gen(String)
end

class SpecInt < Gen(Int32)
end

class Container(T)
  def initialize(@gen : Gen(T))
  end

  def type : Class
    T
  end
end

con1 = Container.new(SpecInt.new(1))
con2 = Container.new(SpecString.new("String"))

con1.type
con2.type