Terraform set a variable with conditional another variable present - amazon-ec2

Question

I would like to set a variable only if a value is present.
My variables are:
variable "http_tcp_listeners" {
description = "aws_lb_listener"
type = map(any)
default = {
http = {
# load_balancer_arn = aws_lb.nlb_test.arn
port = "80"
protocol = "TCP"
action_type = "forward"
certificate_arn = ""
alpn_policy = ""
},
https = {
# load_balancer_arn = aws_lb.nlb_test.arn
port = "443"
protocol = "TLS"
action_type = "forward"
certificate_arn = "arn:aws:acm:us-east-1:b447fa7953be"
alpn_policy = "HTTP2Preferred"
}
}
}
In case http listener the string alpn_policy = each.value.alpn_policy should be absent. If the string is just empty alpn_policy = "" we will have got error Error: expected alpn_policy to be one of [HTTP1Only HTTP2Only HTTP2Optional HTTP2Preferred None], got
If we set any value we will have the error message that ALPN policy cannot be set for non secure listeners
I would like something like this. Pseudocode.
...
If val.alpn_policy != empty then
certificate_arn = try(each.value.certificate_arn, false)
alpn_policy = each.value.alpn_policy
default_action {
else
certificate_arn = try(each.value.certificate_arn, false)
default_action {
...
resource "aws_lb_listener" "frontend_http_tcp" {
for_each = var.http_tcp_listeners
load_balancer_arn = aws_lb.main.arn
port = each.value.port
protocol = each.value.protocol
certificate_arn = try(each.value.certificate_arn, false)
alpn_policy = each.value.alpn_policy
default_action {
type = each.value.action_type
target_group_arn = aws_lb_target_group.main[each.key].arn
}
depends_on = [
aws_lb.main,
aws_lb_target_group.main,
]
}

Answer 1

Thank #RafaP and #Marcin for the ideas. Finally, the code looks like this.
I have deleted unwanted variables in VAR and uses try Function alpn_policy= try(each.value.alpn_policy, null)
instead of
alpn_policy = lookup(var.https_listeners[count.index], "alpn_policy", null)
variable "http_tcp_listeners" {
description = "aws_lb_listener"
type = map(any)
default = {
http = {
port = "80"
protocol = "TCP"
action_type = "forward"
},
https = {
port = "443"
protocol = "TLS"
action_type = "forward"
certificate_arn = "arn:aws:acm:us-east-1:714154805721:certificate/c3be"
alpn_policy = "HTTP2Preferred"
}
}
}
resource "aws_lb_listener" "frontend_http_tcp" {
for_each = var.http_tcp_listeners
load_balancer_arn = aws_lb.main.arn
port = each.value.port
protocol = each.value.protocol
certificate_arn = try(each.value.certificate_arn, null)
alpn_policy = try(each.value.alpn_policy, null)
default_action {
type = each.value.action_type
target_group_arn = aws_lb_target_group.main[each.key].arn
}
depends_on = [
aws_lb.main,
aws_lb_target_group.main,
]
}