How to append to a list in Terraform?

You can use the concat function for this. Expanding upon the example in your question:

variable "foo" {
  type = "list"
  default = [ 1,2,3 ]

# assume a value of 4 of type number is the additional value to be appended
resource "bar_type" "bar" {
  bar_field = "${concat(, [4])}"

which appends to the value assigned to bar_field while ensuring remains unchanged.

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