Terraform has a _lot_ of functions written against HIL's function API, and
we're not ready to rewrite them all yet, so instead we shim the HIL
function API to conform to the HCL2 (really: cty) function API and thus
allow most of our existing functions to work as expected when called from
HCL2-based config files.
Not all of the functions can be fully shimmed in this way due to depending
on HIL implementation details that we can't mimic through the HCL2 API.
We don't attempt to address that yet, and instead just let them fail when
called. We will eventually address this by using first-class HCL2
functions for these few cases, thus avoiding the HIL API altogether where
we need to. (The methodology for that is already illustrated here in the
provision of jsonencode and jsondecode functions that are HCL2-native.)
This loader uses the HCL2 parser and decoder to process a config file,
and then transforms the result into the same shape as would be produced
by the HCL config loader.
To avoid making changes to the existing config structures (which are
depended on across much of the codebase) we first decode into a set of
HCL2-tailored structs and then process them into the public-facing structs
that a loader is expected to return. This is a compromise to keep the
config package API broadly unchanged for now. Once we're ready to remove
the old HCL loader (which implies that we're ready to support HCL2
natively elsewhere in the codebase) we will be able to simplify this
quite considerably.
Due to some mismatches of abstraction between HCL/HIL and HCL2, some
shimming is required to get the required result.
Martin Atkins