The next in line is TF Collection Functions and they are very useful in day to day use for writing TF code.

• concat

concat functions take 2 lists as an input and returns a new list.

concat(aws_vpc_endpoint.sts.*.id, [""])[0]

• contains

contains functions checks for the element in the given list or set and returns a boolean.

contains(["a", "b", "c"], "a")
true

• element

element functions return the item from the list given the index

element(var.azs, count.index)

The above code will return an availability zone at index start with count.index usually start from 0 and so on depending upon the count you set.

• flatten

flatten takes a list and replaces any elements that are lists with a flattened sequence of the list contents.

flatten(module.nginx.vpc_security_group_ids)

With the above code, we can flatten the list of lists.

• map

map takes an even number of arguments and returns a map whose elements are constructed from consecutive pairs of arguments.

map('BusinessUnit','XYZ')

• merge

merge takes an arbitrary number of maps and returns a single map that contains a merged set of elements from all of the maps.

If more than one given map defines the same key then the one that is later in the argument sequence takes precedence.

tags = merge(
    {
      "Name" = format("%s-%03d-%s", var.namespace, count.index, var.environment)
    },
    var.tags,
  )

  volume_tags = merge(
    {
      "Name" = format("%s-%03d-%s", var.namespace, count.index, var.environment)
    },
    var.volume_tags,
  )

In the above code, we are merging all maps and passing them as a single map.

The above listed functions are few of the many Collection Functions Terraform have and are opiniaoted depending on my usage.