Terraform: Allow multiple conditions for count in terraform ternary operations

[ghost]

### Terraform Version
Terraform v0.11.7

provider.aws v1.42.0
provider.external v1.2.0

Terraform Configuration Files
My configuration for my resource is as below :

count = “${var.size[terraform.workspace] == "big" ? && local.ips[terraform.workspace] != “<none>“} : 10
elseif
${var.size[terraform.workspace] == "small"  && local.ips[terraform.workspace] != “<none>” } :  20
else 0 

Expected Behaviour
Terraform should allow me to set multiple conditions in ternary operations

Actual Behaviour
Terraform does not support multiple ternary conditions.

[teamterraform]

Hi @vforums! Thanks for sharing this use-case.

It looks like you're writing this as a complex chain of if statements against different maps because of limitations of Terraform 0.11, and sadly the featureset of Terraform 0.11 is now fixed and will not change even if there is a future 0.11.x release for maintenance reasons. For that reason, we'll focus this comment on approaches for Terraform 0.12, which is where any possible language improvements would be happening right now.

A direct way to write what you wanted here would be to use a nested conditional as the "else if" condition:

count = (
  var.size[terraform.workspace] == "big" && local.ips[terraform.workspace] != "<none>" ? 10 : (
    var.size[terraform.workspace] == "small" && local.ips[terraform.workspace] != "<none>" ? 20 : 0
  )
)

Not particularly readable, though. A more "Terraform 0.12"-like way to represent this would be to flip these settings around so that it's a single map with the workspace as the key:

variable "workspace_settings" {
  type = map(object({
    size = string
    ips  = set(string)
  }))
}

Using Terraform 0.12 features, we'd probably prefer to break this down into multiple simpler expressions, giving each a name (as a named local value) that can help the reader understand what it's representing. Since your comment didn't fully describe the use-case we're limited in how meaningful these example names can be, but for example:

locals {
  settings = var.workspace_settings[terraform.workspace]

  # The original example didn't include what this was a count for, so we're just calling
  # it "something" here. Better to replace "something" with whatever object type we're
  # producing a count for here.
  workspace_size_something_counts = {
    small = 10
    big   = 20
  }
  something_count = lookup(workspace_size_something_counts, local.settings.size, 0)
}

resource "something" "example" {
  count = length(local.settings.ips) == 0 ? local.something_count : 0

  # ...
}

Here we use a map to represent the relationship between the "sizes" and the counts, and then encode the special case that there must be no IPs as a simpler conditional directly inside the count expression. That could also be written out as one complex expression by inlining most of the locals like this:

locals {
  settings = var.workspace_settings[terraform.workspace]
}

resource "something" "example" {
  count = length(local.settings.ips) == 0 ? lookup({
    small = 10
    big   = 20
  }, local.settings.size, 0) : 0

  # ...
}

We generally want to encourage writing and combining multiple simple expressions, rather than writing compound expressions with many branches. However, we can see that in your particular example the problem is framed in a way that is difficult to decompose into smaller parts, in spite of the examples above.

We'll use this issue to collect other real-world examples where multiple different independent settings all affect the outcome in arbitrary ways, and thus where a simple map lookup is insufficient. If others share examples here, we'd prefer to see a fuller description of the scenario in question: what are you trying to achieve in the broader sense, rather than what specific Terraform configuration are you trying to write? That will help us see if your situation might be solved in a different way by other potential features or existing features.