Previously the templatefile function would permit any arbitrary string as
a variable name, but due to the HCL template syntax it would be impossible
to refer to one that isn't a valid HCL identifier without causing an
HCL syntax error.
The HCL syntax errors are correct, but don't really point to the root
cause of the problem. Instead, we'll pre-verify that the variable names
are valid before we even try to render the template, and given a
specialized error message that refers to the vars argument expression as
the problematic part, which will hopefully make the resolution path
clearer for a user encountering this situation.
The syntax error still remains for situations where all of the variable
names are correct but e.g. the user made a typo referring to one, which
makes sense because in that case the problem _is_ inside the template.
It's a common source of errors to try to produce JSON or YAML syntax
using string concatenation via our template language but to miss some
details like correct string escaping, quoting, required commas, etc.
The jsonencode and yamlencode functions are a better way to generate JSON
and YAML, but it's not immediately obvious that both of these functions
are available for use in external templates (via templatefile) too.
Given that questions related to this come up a lot in our community forum
and elsewhere, it seems worth having a documentation section to show the
pattern of having a template that consists only of a single function call.
This function is similar to the template_file data source offered by the
template provider, but having it built in to the language makes it more
convenient to use, allowing templates to be rendered from files anywhere
an inline template would normally be allowed:
user_data = templatefile("${path.module}/userdata.tmpl", {
hostname = format("petserver%02d", count.index)
})
Unlike the template_file data source, this function allows values of any
type in its variables map, passing them through verbatim to the template.
Its tighter integration with Terraform also allows it to return better
error messages with source location information from the template itself.
The template_file data source was originally created to work around the
fact that HIL didn't have any support for map values at the time, and
even once map support was added it wasn't very usable. With HCL2
expressions, there's little reason left to use a data source to render
a template; the only remaining reason left to use template_file is to
render a template that is constructed dynamically during the Terraform
run, which is a very rare need.
Martin Atkins