How to iterative over public/private subnets in the Terraform VPC module?

I'm trying to define public and private subnets as input variables to the Terraform vpc module. How can I reference my private/public subnet variables in an iterative statement rather than hard-coding in the "list[x]" elements?

I have the vpc module (only two AZs at the moment) defined in main.tf.

module "vpc" {
  source          = "terraform-aws-modules/vpc/aws"
  name            = "my-vpc"
  cidr            = var.my_cidr

  azs             = ["eu-west-1a", "eu-west-1b"]
  private_subnets = [var.my_private_subnets[0], var.my_private_subnets[1]]
  public_subnets  = [var.my_public_subnets[0], var.my_public_subnets[1]]
  #                      ^
  #                      +--- can these subnets be iterated over in a for_each here?
  ...
}

My subnet input variables are defined like this:

variable "my_cidr" {
  description = "The IPv4 CIDR block for the VPC"
  type        = string
  default     = "10.10.0.0/16"
}

variable "my_private_subnets" {
  type = list
  description = "private subnet within vpc cidr block"
  default = ["10.10.20.0/24", "10.10.30.0/24"]
}

variable "my_public_subnets" {
  type = list
  description = "public subnet within vpc cidr block"
  default = ["10.10.100.0/24", "10.10.200.0/24"]
}

First update the variables type to list(string)

    variable "my_private_subnets" {
    type = list(string)
    description = "private subnets within vpc cidr block"
    default = ["10.10.20.0/24", "10.10.30.0/24"]
    }

    variable "my_public_subnets" {
    type = list(string)
    description = "public subnets within vpc cidr block"
    default = ["10.10.20.0/24", "10.10.30.0/24"]
    }

Then pass these as variables to module as following:

    module "vpc" {
    source = "terraform-aws-modules/vpc/aws"
    name   = "my-vpc"
    cidr   = "10.6.0.0/16"

    azs = ["eu-west-1a", "eu-west-1b"]
    private_subnets = var.my_private_subnets
    public_subnets  = var.my_public_subnets
    
    }