Declare static type for variables.

[msrn]

Currently most of the input variables are declared as implicit variables.

While type constraints are optional, we recommend specifying them; they can serve as helpful reminders for users of the module, and they allow Terraform to return a helpful error message if the wrong type is used.

Example:

variable "name" {
  type = string
  default =""
}

Type Constraints @ Terraform Docs