Give every variable and output a type and a description

Untyped variables accept anything, so a typo in a .tfvars file shows up as a confusing error deep in a resource, or not at all. The style guide says every variable needs a type and a description, and every output a description too.

Also from the style guide:

• Give optional variables a sensible default; leave required ones without one.
• Only make something a variable if it really changes between deployments. Too many variables make a module harder to use, not more flexible.
• Use validation blocks for unusually strict requirements, with an error message that says how to fix the value.
• Order arguments as type, description, default, sensitive, validation.

To check, run terraform validate and look at your module's generated docs (for example with terraform-docs). Any input without a description is a question someone will ask you later.

variable "environment" {
  type        = string
  description = "Deployment environment: dev, staging or prod."

  validation {
    condition     = contains(["dev", "staging", "prod"], var.environment)
    error_message = "environment must be dev, staging or prod."
  }
}