Terraform example for process a list of names, adding a domain and join all in a single string.

main.tf

# File: main.tf
locals {
  # Static variables
  domain = "domain.com"
  name_list = [
	"name1",
	"name2",
	"name3"
  ]

  # Formatted variables
  list_format = formatlist("%s.${local.domain}", local.name_list)

  string_variable = join(",", formatlist("%s.${local.domain}", local.name_list))
}


$ terraform console
> local.domain
"domain.com"
> local.name_list
[
  "name1",
  "name2",
  "name3",
]
> local.list_format
tolist([
  "name1.domain.com",
  "name2.domain.com",
  "name3.domain.com",
])
> local.string_variable
"name1.domain.com,name2.domain.com,name3.domain.com"