Map within a map in terraform variables

Would this syntax be possible? ${var.var[firstchoice[firstAchoice]]}

With Terraform 0.12+ nested blocks are supported seamlessly. Extending @Avichal Badaya's answer to explain it using an example:

# Nested Variable

variable "test" {
  default = {
    firstchoice = {
      firstAChoice = "foo"
      firstBChoice = "bar"
    }

    secondchoice = {
      secondAChoice = "foobar"
      secondBChoice = "barfoo"
    }

    thirdchoice = {
      thirdAChoice = {
          thirdBChoice = {
              thirdKey = "thirdValue"
        }
      }
    }
  }
}


# Outputs

output "firstchoice" {
  value = var.test["firstchoice"]
}

output "FirstAChoice" {
  value = var.test["firstchoice"]["firstAChoice"]
}

output "thirdKey" {
  value = var.test["thirdchoice"]["thirdAChoice"]["thirdBChoice"]["thirdKey"]
}

Applying the above, you can verify that Terraform map nesting is now quite powerful and this makes a lot of things easier.

# Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

# Outputs:

firstchoice = {
  "firstAChoice" = "foo"
  "firstBChoice" = "bar"
}

thirdKey = thirdValue 

For more complex structures and Rich Value Types, see HashiCorp Terraform 0.12 Preview: Rich Value Types

Yes, it's possible to have map variable as value of map variable key. Your variable just needed right indentation. Also I am putting ways to access that variable.

variable "var" {
  default = {
    firstchoice = {
      firstAChoice = "foo"
      firstBChoice = "bar"
    }

    secondchoice = {
      secondAChoice = "foobar"
      secondBChoice = "barfoo"
    }
  }
}

To access entire map value of a map key firstchoice, you can try following

value = "${var.var["firstchoice"]}"

output:
{
  firstAChoice = foo
  firstBChoice = bar
}

To access specific key of that map key (example firstAChoice), you can try

value = "${lookup(var.var["firstchoice"],"firstAChoice")}"

output: foo