The templatefile function only has two arguments, so ArgErrorf can be
called with only zero or one as the argument index. If we are out of
bounds then HCL itself will panic trying to build the error message for
this call when called as an HCL function.
Unfortunately there isn't really a great layer in Terraform to test for
this class of bug systematically, because we are currently testing these
functions directly rather than going through HCL to do it. For the moment
we'll just live with that, but if we see this class of error arise again
we might consider either reworking the tests in this package to work with
HCL expression source code instead of direct calls or adding some
additional tests elsewhere that do so.
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.
These implementations are adaptations of the existing implementations in
config/interpolate_funcs.go, updated to work with the cty API.
The set of functions chosen here was motivated mainly by what Terraform's
existing context tests depend on, so we can get the contexts tests back
into good shape before fleshing out the rest of these functions.
Martin Atkins