Terraform make variable nullable based on other variable value

I'm working with Okta provider and I want to implement app_oauth. There is an attribute like redirect_uris (documentation) and this attrobute is required for all application types except service. I was trying to solve this like that in my vars.tf:

variable "type" {
  description = "Application Type"
  type        = string
  nullable    = false
}

variable "redirect_uris" {
  description = "Application Grant Types"
  type        = list(string)
  nullable    = var.type == "service" ? true : false
}

Unfortunately while running terragrunt plan I get:

 Error: Variables not allowed
│ 
│   on vars.tf line 23, in variable "redirect_uris":
│   23:   nullable    = var.type == "service" ? true : false
│ 
│ Variables may not be used here.
╵

╷
│ Error: Unsuitable value type
│ 
│   on vars.tf line 23, in variable "redirect_uris":
│   23:   nullable    = var.type == "service" ? true : false
│ 
│ Unsuitable value: value must be known

Can somebody give me any tips how to deal with that?

EDIT: working solution

resource "okta_app_oauth" "app" {
  label          = var.label
  type           = var.type
  grant_types    = var.grant_types
  redirect_uris  = var.type != "service" ? var.redirect_uris : null
  response_types = var.response_types
}

Solution

Since you have not posted a reproducible code block, I think you could achieve what you want by doing the following:

redirect_uris = var.type != "service" ? var.redirect_uris : null