Terraform: count == true - terraform

Question

I have used this previously in my TF code:
count = "${var.whatever == "true" ? 1 : 0}"
Which works great for what I wanted to use. However, I'm thinking of how best to use something similar to say, if var.whatever is equal to true, then count is equal to length(var.whatever).
Would this work?
count = "${var.whatever == "true" ? ${length(var.whatever) : 0}"
It's just that I was always under the impression with TF that you can't nest interpolations.
Also, this kind of leads me to another interesting question.. what do you guys use to test syntax? I don't mean to lint the TF Code, I mean something to run the code against to test things like this? I don't want to just deploy to dev, just to test the output of my TF code. I was wondering if there was something, shell like, which I could literally test this stuff as is it were bash or something. Any thoughts?

Answer 1

Your original idea won't work because if var.whatever is a list, then it can't also be a string (i.e. = "true").
However, the good news is that interpolations can be used inside the general ternary operator.
So you can have something like:
count = "${var.bool == "true" ? length(var.whatever) : 0}"
A good way to test out interpolations before dropping them into your final code is by using Terraform's console feature (i.e. terraform console).
Set up your vars in a file, say console.tf in a directory without any other TF code.
variable "whatever" {
type = "list"
default = ["1", "2", "foo", "bar" ]
}
variable "bool" {
default = "true"
}
Now from the command line, run terraform console.
$ terraform console
> var.whatever
[
"1",
"2",
"foo",
"bar",
]
> length(var.whatever)
4
> "${var.bool == "true" ? length(var.whatever) : 0}"
4
> exit
$
You can see that Terraform performs the interpolations, then runs the ternary operator on those evaluated values.