[FEATURE PROPOSAL] add an alert when key argument in lookup() call contains dot (.)

[landure]

Introduction

lookup() look for a key in a map. Terraform beginners might be tempted to use it like:

locals {
   my_map = { parent-key = { child-key = "value" } }
   lookup_result = lookup(local.my_map, "parent-key.child-key", "default")
}

output "lookup-returns-default" {
  value = local.lookup_result
}

while this doesn't trigger an error, it always return 'default', instead of local.my_map['parent-key']['child-key'].

Using try() gives the expected result:

locals {
    try_result = try(local.my_map.parent-key.child-key, "default")
}

output "try-returns-value" {
  value = local.try_result
}

Proposal

Add an alert when a lookup() call key contains a dot, that encourage to use try() instead.

References

• lookup Function @ OpenTofu documentation.

[bendrucker]

Keys with literal dots in them are perfectly valid. Maybe this is a common pitfall you want to enforce in you org but it doesn't meet the criteria for this ruleset.

[landure]

Hi,

It just a pitfall I encountered with AI generated code, with data loaded from a yaml file.
The code was valid, but lookup always returned default.

While key with dots in them are valid syntax,
terraform beginners coming from other languages can intuitively think that lookup will know to split the key on the dot.
This is why an alert for such issues would be nice, particularly for hard coded key lookups.

[bendrucker]

This is why an alert for such issues would be nice

Except when you have filenames as keys in your maps like index.html and then the alert is noise you have to suppress or even worse your coding agent dutifully follows the recommendation to use try and breaks your code.

This is why this ruleset focuses on enforcing documented recommendations and correctness issues.