test.cr

class  Any
  alias Type = Array(Int32) | String | Nil
  getter raw : Type

  def initialize(raw)
    @raw = raw.as Type
  end

  def foo
    if @raw.responds_to?(:each)
      @raw.each do |e|
        yield e
      end
    end
  end
end

Any.new("a").foo  {|x| puts x}
Any.new(nil).foo  {|x| puts x}
Any.new([1,2,3]).foo {|x| puts x}

$ crystal test.cr 
Error in ./test.cr:18: instantiating 'Any#foo()'

Any.new("a").foo  {|x| puts x}
             ^~~

in ./test.cr:11: undefined method 'each' for String (compile-time type is (Array(Int32) | String | Nil))

      @raw.each do |e|
           ^~~~

Rerun with --error-trace to show a complete error trace.

test2.cr

alias Type = Array(Int32) | String | Nil

def foo(r)
  raw = r.as Type
  if raw.responds_to?(:each)
    raw.each do |e|
      yield e
    end
  end
end

foo("a")  {|x| puts x}
foo(nil)  {|x| puts x}
foo([1,2,3]) {|x| puts x}


$ crystal test2.cr 
1
2
3

https://play.crystal-lang.org/#/r/1bs6

class  Any(T)
  getter raw : T

  def initialize(@raw : T)
  end

  def foo
    if (raw = @raw).responds_to?(:each)
      raw.each do |e|
        yield e
      end
    end
  end
end

Any.new([1,2,3]).foo {|x| puts x}
Any.new("str").foo {|x| puts x}