Should concat() support lists of complex types?

[phinze]

Over in #7145 I've submitted a fix for the reported panic in #7030. This fix is the stop gap measure of properly reporting the lack of support in concat() for anything but lists of strings.

We should decide what we want the semantics of concat() to be for the following:

• List of maps
• List of lists
• Arbitrarily nested versions of the above

It would also help us decide how to prioritize making this functionality work if we could gather together a few use cases for the various scenarios.

[phinze]

@oillio did you have a use case in mind when you ran into the panic in #7030?

[oillio]

I don't have a use case that actually works :)

I ran into the crash when I was trying to find a way to use multiple sources of values to configure an aws_elastic_beanstalk_environment.setting field. I couldn't find a way to use variable data in the setting, so I don't currently have a use for concat on complex variable types.
I will want to use concat like this if a feature like #7034 is ever implemented.

[jen20]

This is fairly incompatible with the type checker in HIL, since concat has to explicitly declare the return type (hence the backward compatibility note of no longer being able to use concat() with strings). We should probably consider whether to make the type checker smarter or whether to add differently named mechanisms for concatenating other data types.

[phinze]

I think the return type for concat() will always be a TypeList here - it's just the type of the contents of said list that change.

A shot at target semantics:

• concat() - combine lists, e.g. concat(["foo", "bar"], ["baz"]) => ["foo, "bar", "baz"]
• merge() - combine maps e.g. merge({foo = 1}, {bar = 2}) => {foo = 1, bar = 2}

[tecnobrat]

Looks like this was fixed by the mentioned pr?

[phinze]

Indeed, this was taken care of in #7528 by @jbardin

[ju2wheels]

@phinze Im not following what the overall decision here was. Is concat supposed to work on lists of maps or only lists of strings?

Using 0.9.6, Im trying to do the following:

resource "aws_rds_cluster_parameter_group" "my_rcpg" {
  description = "My Cluster - ${var.my_module_instance_name}"
  family      = "${var.my_rds_cluster_parameter_group_family}"
  name        = "mymodule-${var.my_module_instance_name}"
  parameter   = "${concat(list(map("apply_method", "immediate", "name", "aws_default_s3_role", "value", aws_iam_role.my_role.arn)), var.my_rds_cluster_parameter_group_parameters)}"
  tags        = "${var.my_rds_cluster_parameter_group_tags}"
}

# tfvars
my_rds_cluster_parameter_group_parameters = []

Although im getting the following error:

* aws_rds_cluster_parameter_group.my_rcpg: parameter: should be a list

Im pretty sure this will work if I just pass it a var of list of maps instead of trying to construct a default list with a map object and appending any additional maps to it. Is this a separate bug or expected concat behavior?

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.