Templatefile() function doesn't work with AWS IAM

I have an IAM terraform module that needs to take a list of ARNs as an input and use templatefile() to modify an IAM template. I’m getting an error because terraform lists add a trailing comma which IAM doesn’t like. Is this a limitation of the templatefile() function? If not, how can I use this function with an IAM template?

The error that I am getting is:

Error: "policy" contains an invalid JSON: invalid character ']' looking for beginning of value

  on main.tf line 84, in resource "aws_iam_role_policy" "iam_dynamoDB_policy_rw_list":
  84: resource "aws_iam_role_policy" "iam_dynamoDB_policy_rw_list" {

Here’s the user input:

arn_list = ["arn:aws:dynamodb:*:1234567890:table/taco","arn:aws:dynamodb:*:1234567890:table/taco1","arn:aws:dynamodb:*:1234567890:table/taco2"]

Here’s the relevant portion of the terraform script:

locals {
  arn_fmt         = formatlist("\"%s\",", var.arn_list)
  arn_map         = { arn_list = local.arn_fmt }

resource "aws_iam_role_policy" "iam_dynamoDB_policy_rw_list" {
  count  = var.accesstype == "rw_list" ? length(local.role_names) : 0
  name   = "${var.accesstype}-dynamo-${local.policy_name}-${var.aws_region}-${local.aws_account_id}"
  policy = templatefile("${path.module}/files/iam_policy/taco.json.tpl", local.arn_map)
  role   = element(local.role_names, count.index)

Here’s the IAM template that I’m trying to modify:

    "Version": "2012-10-17",
    "Statement": [
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
            "Resource": [
                %{ for arn in arn_list ~}
                %{ endfor ~}