gistfile1.txt

module "test" {
  source "./mymodule"
  # This seems to work
  security_groups_as_a_list = ["12341234"]
  
  # But this produces an error: aws_instance.vm: security_groups: should be a list
  security_groups_as_a_list = ["${aws_security_group.terminal.id}"]
  
  # This would also work, but it's ugly
  a_single_security_group = "${aws_security_group.terminal.id}"
}

mymodule main.tf

variable "security_groups_as_a_list" {
	type = "list"
}

variable "a_single_security_group" {}

resource "aws_instance" "vm" {
    security_groups = "${var.security_groups_as_a_list}"
    
    # This would also work
    security_groups = ["${var.a_single_security_group}"]
}