variable "amis" {
    default = {
        us-east-1 = "foo"
        us-west-2 = "foo"
    }
}

variable "test_list" {
    type = list(string)
}

variable "test_map" {
    type = map(string)
}

variable "bar" {
    default = "baz"
}

variable "foo" {}

resource "aws_instance" "foo" {
    num  = "2"
    bar  = var.bar
    list = var.test_list
    map  = var.test_map
}

resource "aws_instance" "bar" {
    foo = var.foo
    bar = var.amis[var.foo]
    baz = var.amis["us-east-1"]
}