Memoize in Crystal

memoize.cr

macro memoize(type_decl, &block)
  @{{type_decl.var}} : {{ type_decl.type }} | UninitializedMemo = UninitializedMemo::INSTANCE
  
  def {{type_decl.var}}
    if (value = @{{type_decl.var}}).is_a?(UninitializedMemo)
      @{{type_decl.var}} = begin
        {{block.body}}
      end
    else
      value
    end
  end
end

class UninitializedMemo
  INSTANCE = new
end

class ClassName
  memoize expensive_query : Int32 do
    puts "hello"
    "hello".size
  end
end

puts ClassName.new.expensive_query