flatten terraform list of list using for loop without using flatten function

Question

I know my expected result can be made using flatten built in function( which I have given below). But I want to know is there any way to create them only using nested for loops.

  output "languages" {
  value = flatten([ 
    for user in var.users: [
      for language in user.languages: language
    ]
  ])
}

This is my variable

variable "users" {

  default = {
   
    "sam" = {
      "name" = "sam"
      "age"  = 22
      "languages" = ["python","nodejs","perl","ruby"]
      
    },
    "john" = {
      "name" = "john"
      "age"  = 36
      "languages" = ["python","html","cpp","golang","perl","ruby"] 
    },
    "kevin" = {
      "name" = "kevin"
      "age"  = 30
      "languages" = ["python","nodejs","bash","cpp","ruby"] 
    },
    "devid" = {
      "name" = "devid"
      "age"  = 40
      "languages" = ["python","bash","perl","ruby"] 
    },
    "devon" = {
      "name" = "devon"
      "age"  = 25
      "languages" = ["python"] 
    },
    "jain" = {
      "name" = "jain"
      "age"  = 42
      "languages" = ["python","bash","perl","ruby"] 
    }

  } }

This is mycode

output "languages" {
  value = [ 
    for user in var.users: [
      for language in user.languages: language
    ]
  ]
}

Current Result

languages = [
  [
    "python",
    "bash",
    "perl",
    "ruby",
  ],
  [
    "python",
  ],
  [
    "python",
    "bash",
    "perl",
    "ruby",
  ],
  [
    "python",
    "html",
    "cpp",
    "golang",
    "perl",
    "ruby",
  ],
  [
    "python",
    "nodejs",
    "bash",
    "cpp",
    "ruby",
  ],
  [
    "python",
    "nodejs",
    "perl",
    "ruby",
  ],
]

Excepted Result

languages = [
    "python",
    "bash",
    "perl",
    "ruby",
    "python",
    "python",
    "bash",
    "perl",
    "ruby",
    "python",
    "html",
    "cpp",
    "golang",
    "perl",
    "ruby",
    "python",
    "nodejs",
    "bash",
    "cpp",
    "ruby",
    "python",
    "nodejs",
    "perl",
    "ruby",

]

Hi, I just want to know whether it is possible or not , academic interest only. — Fuji Komalan, Jan 01 '23 at 13:45

score 1 · Answer 1 · answered Jan 01 '23 at 14:58

I'm not entirely sure if this is what you want, but without using flatten function you can accomplish something similar with a for loop, concat and with the ellipsis expression:

output "languages" {
  value = concat([for item in var.users : item.languages]...)
}

The output result will be the following:

languages = [
  "python",
  "bash",
  "perl",
  "ruby",
  "python",
  "python",
  "bash",
  "perl",
  "ruby",
  "python",
  "html",
  "cpp",
  "golang",
  "perl",
  "ruby",
  "python",
  "nodejs",
  "bash",
  "cpp",
  "ruby",
  "python",
  "nodejs",
  "perl",
  "ruby",
]

Keep in mind, Terraform mainly relies on using pure functions. Moreover, while you can have local variables, they cannot act as intermediary in case of loops and expressions.

Thanks Brother. So its not possible to create the intended result only using two for loops, I mean without any built in function. — Fuji Komalan, Jan 02 '23 at 11:39

score 0 · Answer 2 · answered Jan 04 '23 at 02:57

Each for expression always constructs either a new tuple or a new object. There is no way to use two for expressions together to construct a single tuple or object.

The flatten function is the way to achieve this in the Terraform language. Although there are other functions that can potentially achieve a similar result, the flatten function is the one designed for this purpose and the one idiomatic to use, so is the one that other readers of your configuration will probably find most intuitive if they are familiar with Terraform from elsewhere.

flatten terraform list of list using for loop without using flatten function

2 Answers2