adding concatlist for concatenating lists

[lamdor]

I found myself needing this instead of trying to hack strings together and apart.

The case is this:

variable "a_count" {}
variable "b_count" {}

resource "aws_instance" "a" {
         count = "${var.a_count}"
}

resource "aws_instance" "b" {
         count = "${var.b_count}"
}

resource "aws_instance" "combining-a-b" {
         count = "${var.a_count + var.b_count}"

         provisioner "remote-exec" {
           command = "echo ${element(concatlist(aws_instance.a.*.private_ip, aws_instance.b.*.private_ip), count.index)}"
         }
}

If I want a resource to be created for multiple, but select an element from the total set, it was really difficult. I can see this concatlist being useful for other things as well.

Thoughts?

[knuckolls]

This is significantly cleaner than alternatives he showed me today. Looking forward to seeing this or something like it merged. It solves a need for us when it comes to keeping our tf files DRY but flexible enough to run on multiple "clouds". I can see this being a useful feature for more than just us.

[radeksimko]

See #1790
Since it's my own PR, I won't close this yet, I will let another maintainer decide here, so I don't look too selfish 😄

[lamdor]

@radeksimko Perfect. I'll close this and switch our config to yours. I had played around with the idea of just making concat work both ways; glad someone thinks likewise.

[ghost]

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