The existing "type" argument allows specifying a type constraint that
allows for some basic validation, but often there are more constraints on
a variable value than just its type.
This new feature (requiring an experiment opt-in for now, while we refine
it) allows specifying arbitrary validation rules for any variable which
can then cause custom error messages to be returned when a caller provides
an inappropriate value.
variable "example" {
validation {
condition = var.example != "nope"
error_message = "Example value must not be \"nope\"."
}
}
The core parts of this are designed to do as little new work as possible
when no validations are specified, and thus the main new checking codepath
here can therefore only run when the experiment is enabled in order to
permit having validations.
Traditionally we've preferred to release new language features in major
releases only, because we can then use the beta cycle to gather feedback
on the feature and learn about any usability challenges or other
situations we didn't consider during our design in time to make those
changes before inclusion in a stable release.
This "experiments" feature is intended to decouple the feedback cycle for
new features from the major release rhythm, and thus allow us to release
new features in minor releases by first releasing them as experimental for
a minor release or two, adjust for any feedback gathered during that
period, and then finally remove the experiment gate and enable the feature
for everyone.
The intended model here is that anything behind an experiment gate is
subject to breaking changes even in patch releases, and so any module
using these experimental features will be broken by a future Terraform
upgrade.
The behavior implemented here is:
- Recognize a new "experiments" setting in the "terraform" block which
allows module authors to explicitly opt in to experimental features.
terraform {
experiments = [resource_for_each]
}
- Generate a warning whenever loading a module that has experiments
enabled, to avoid accidentally depending on experimental features and
thus risking unexpected breakage on next Terraform upgrade.
- We check the enabled experiments against the configuration at module
load time, which means that experiments are scoped to a particular
module. Enabling an experiment in one module does not automatically
enable it in any other module.
This experiments mechanism is itself an experiment, and so I'd like to
use the resource for_each feature to trial it. Because any configuration
using experiments is subject to breaking changes, we are free to adjust
this experiments feature in future releases as we see fit, but once
for_each is shipped without an experiment gate we'll be blocked from
making significant changes to it until the next major release at least.
