In order to be able to predict a result type even if arguments are not yet
known, coalesce requires all of its arguments to be of the same type. Our
usual automatic conversion rules mean that in some cases the result is
a silent type conversion rather than an explicit error, so we'll at least
document that so that folks who encounter it can understand what is
causing the likely-surprising behavior.
If we were building this function over again today I expect we'd make it
always return an error under type mismatch, but to do so now would be a
breaking change and the potential cost of that seems too high for
something that doesn't seem to arise incredibly often in practice.
As of this commit, that layout doesn't exist yet, but I'm isolating the one-line
changes to their own commit to try and keep your eyes from glazing over.
* funcs/coalesce: return the first non-null, non-empty element from a
sequence.
The go-cty coalesce function, which was originally used here, returns the
first non-null element from a sequence. Terraform 0.11's coalesce,
however, returns the first non-empty string from a list of strings.
This new coalesce function aims to preserve terraform's documented
functionality while adding support for additional argument types. The
tests include those in go-cty and adapted tests from the 0.11 version of
coalesce.
* website/docs: update coalesce function document
Martin Atkins