Merge fails in terraform

Question

Consider I have a variable -

[
  {
    "outer_key_1" = [
      {
        "ip_cidr" = "172.16.6.0/24"
        "range_name" = "range1"
      },
      {
        "ip_cidr" = "172.16.7.0/24"
        "range_name" = "range2"
      },
      {
        "ip_cidr" = "172.17.6.0/24"
        "range_name" = "range3"
      },
      {
        "ip_cidr" = "172.17.7.0/24"
        "range_name" = "range4"
      },
    ]
  },
  {
    "outer_key_2" = [
      {
        "ip_cidr" = "172.16.5.0/24"
        "range_name" = "range5"
      },
      {
        "ip_cidr" = "172.17.5.0/24"
        "range_name" = "range6"
      },
    ]
  },
]

I am able to merge the maps inside the list to get this output -

{
    "outer_key_1" = [
      {
        "ip_cidr" = "172.16.6.0/24"
        "range_name" = "range1"
      },
      {
        "ip_cidr" = "172.16.7.0/24"
        "range_name" = "range2"
      },
      {
        "ip_cidr" = "172.17.6.0/24"
        "range_name" = "range3"
      },
      {
        "ip_cidr" = "172.17.7.0/24"
        "range_name" = "range4"
      },
    ]

    "outer_key_2" = [
      {
        "ip_cidr" = "172.16.5.0/24"
        "range_name" = "range5"
      },
      {
        "ip_cidr" = "172.17.5.0/24"
        "range_name" = "range6"
      },
    ]
}

I have done this using

result = merge(variable[0], variable[1])

But when I try this

result = merge(variable[*])

I get an error saying

Call to function "merge" failed: arguments must be maps or objects, got "tuple".

Why does merge fail when I use the splat operator? Is there a better way to merge maps in list as required above?

Answer 1

The merge function is defined to take one or more separate arguments that are each map or object values. When you call merge(variable) (which is the same as merge(variable[*]) if variable is already a list) you're instead giving it one list argument, leading to this error.

To pass the elements of a list or tuple as multiple individual arguments, you can use argument expansion syntax:

result = merge(variable...)

The ... symbol, when given after the last argument in a function call, tells Terraform to evaluate the expression as a list or tuple before calling the function, and then use the elements in that result a separate argument each.

In other words, merge(variable...) is the same as merge(variable[0], variable[1], variable[2]) etc for each element in the list, without needing to know ahead of time how many elements are in the list.

---

Applying the splat operator [*] to a value that is already a list is redundant; the splat operator is useful in this situation only when it's followed by at least one additional attribute name or index, like example[*].id or example[*][0].

There is one situation where a lonely [*] with no following steps can be useful: if you apply it to anything that isn't a list or tuple then Terraform will either wrap it in a single-element list (if it's non-null) or return an empty list (if it's null), which can be useful in some unusual cases where you're given a single value that might be null and need to adapt it into a zero-or-one length list to use with features like resource for_each and in dynamic blocks.