Validation condition of a variable based on another variable

[sankara911]

Terraform Version

terraform version
Terraform v1.3.2
on darwin_arm64

Use Cases

Validation of a variable based on another variable.
Currently, while declaring a variable there is a

variable "somevar"{
type=string
default=""

validation {
condition = var.somevar == "" || var.somevar == "hello"
error = "variable should be either null or hello"
}

But, if I use that validation to check another variable as below,

variable "somevar"{
type=string
default=""

validation {
condition = var.somevar == "" || var.someothervar == "hello"
error = "variable should be either null or hello"
}

I get the error -

The condition for variable "somevar" can only refer to the variable
│ itself, using var.somevar.

---

Would be a good addition to enable such a feature.

Attempted Solutions

But, if I use that validation to check another variable as below,

variable "somevar"{
type=string
default=""

validation {
condition = var.somevar == "" || var.someothervar == "hello"
error = "variable should be either null or hello"
}

I get the error -

The condition for variable "somevar" can only refer to the variable
│ itself, using var.somevar

Proposal

Proposing that a condition in a validation block be allowed to check the validation for another variable and set this one accordingly.

References

No response

[jbardin]

Hi @sankara911,

While this is a duplicate of #25609, the overall functionality of validating multiple variables in a single expression can be done with check blocks.

Thanks!

[briceburg]

@jbardin AFAIK the terraform 1.5 check blocks are essentially post-apply/postcondition validation, whereas input validation is typically a pre-apply/pre-condition step. is there a way to have check blocks fire before a resource attempts application?

[jbardin]

@briceburg, check blocks will be evaluated during plan, but yes, they will not block application of that plan. If you need to halt execution as early as possible, you can create a precondition at the point where the variables are used, or insert an empty resource like terraform_data to give you a location for a precondition where both variables are in scope.

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