types: Consider Method for Checking Whether Collection Values are Fully Known #597

Module version

v1.0.1

Use-cases

Configuration validation logic generally needs to skip over any potentially unknown values. The IsUnknown() bool method checks whether the value itself is unknown, but in the case of collection types such as List, Map, Object, and Set there may be underlying attributes/elements which are unknown. For validators such as the terraform-plugin-framework-validators listvalidator.UniqueValues(), it tries to detect duplicate values in the list based on value type equality. If there are unknown values from differing sources, it may unexpectedly trip up the equality checking since there is no differentiation in the type system between unknown values. For example:

resource "examplecloud_thing" "example" {
  count = 2
}

resource "examplecloud_widget" "example" {
  thing_ids = [for id in examplecloud_thing[*].id: id] # differing unknown strings
}

Proposal

Upstream in terraform-plugin-go, the tftypes.Value.IsFullyKnown() bool method provides additional recursive checks about whether the value contains unknown values. This module could provide something similar for all the framework types or at least the collection types. The implementation may need to lean on using the *Valuable interfaces to handle custom types.