concatenate filepath prefix and file name in terraform code - loops

Question

I'm trying to create policies in aws with terraform.
variable "path" {
type = "string"
}
variable "policies" {
type = list(object ({
name = string
plcyfilename = string
asmplcyfilename = string
desc = string
ownner = string}))
default = []
}
resource "aws_iam_policy" "policy" {
count = length(var.policies)
name = lookup(var.policies[count.index], "name")
policy = file(lookup(var.policies[count.index], concat("var.path","plcyfilename")))
description = "Policy for ${lookup(var.policies[count.index], "desc")}"
}
and this is how my tfvars looks like:
path = "./../t2/scripts/"
policies = [
{name = "cwpolicy", plcyfilename = "cw.json" , asmplcyfilename ="csasm.json", desc ="vpcflowlogs", ownner ="vpc"},
]
The error that is thrown while I do this is like this:
Error: Invalid function argument
on main.tf line 13, in resource "aws_iam_policy" "policy":
13: policy = file(lookup(var.policies[count.index], "${concat("${var.path}","plcyfilename")}"))
Invalid value for "seqs" parameter: all arguments must be lists or tuples; got
string.
I'm using terraform 0.12.
It works as expected if I change the variable to have complete file path:plcyfilename=./../t2/scripts/cw.json.
However I want to isolate the file path from the file names.
Can someone point me where I am going wrong.

Answer 1

The concat function is for concatenating lists, not for concatenating strings.
To concatenate strings in Terraform, we use template interpolation syntax:
policy = file("${var.path}/${var.policies[count.index].policy_filename}")
Since your collection of policies is not a sequence where the ordering is significant, I'd recommend also changing this to use resource for_each, which will ensure that Terraform tracks the policies using the policy name strings rather than using the positions in the list:
variable "policies" {
type = map(object({
policy_filename = string
assume_policy_filename = string
description = string
owner = string
}))
default = {}
}
resource "aws_iam_policy" "policy" {
for_each = var.policies
name = each.key
policy = file("${var.path}/${each.value.policy_filename}")
description = "Policy for ${each.value.description}"
}
In this case the policies variable is redefined as being a map, so you'd now present the name of each policy as the key within the map rather than as one of the attributes:
policies = {
cw = {
policy_filename = "cw.json"
assume_policy_filename = "csasm.json"
description = "vpcflowlogs"
owner = "vpc"
}
# ...
}
Because the for_each value is the policies map, each.key inside the resource block is a policy name and each.value is the object representing that policy, making the resulting expressions easier to read and understand.
By using for_each, we will cause Terraform to create resource instance addresses like aws_iam_policy.policy["cw"] rather than like aws_iam_policy.policy[1], and so adding and removing elements from the map will cause Terraform to add and remove corresponding instances from the resource, rather than try to update instances in-place to respect the list ordering as it would've done with your example.