apparentlymart
commented
2 years ago Thanks for reporting this, @johakoch.
Indeed, the documentation here is incorrect. I think what happened here is that this function got here by someone at HashiCorp proposing to upstream Terraform's lookup function, which does indeed have an optional final argument for backward-compatibility with how older versions of Terraform (v0.6 and earlier, I think) dealt with map indexing exclusively using functions, whereas modern Terraform has an explicit indexing operator.
As part of reviewing that proposal I requested various changes to the functions to eliminate Terraform-specific quirks given that cty itself has no requirement to be backward-compatible with old versions of Terraform that predate it. I expect I called for lookup to have a mandatory third argument as part of that feedback, because the no-default case is already represented by the function index.
I think the best fix here is to update the documentation to accurately describe the behavior, and perhaps to suggest using index for situations where a default value isn't needed.
If you want the behavior of Terraform's lookup function for your application then you'll need to take the implementation from Terraform itself. I don't think that implementation is available as a library anywhere, but you could copy it directly from the Terraform codebase as long as your project has a compatible open source license.
(Though since your project seems to use HCL, you probably don't actually need the Terraform backward-compatibility behavior: HCL's {a = 1, b = 2}["a"] means the same thing that lookup({a = 1, b = 2}, "a") would achieve in Terraform, and is also the same as cty's index function which HCL's indexing operator is implemented in terms of.)
johakoch
The documentation for the
lookup()stdlib function states:However, if I call
lookup({a = 1, b = 2}, "a")without a third argument, I get the errorThe function specification lists exactly three
Params and noVarParam.