This allows us to round-trip Body to JSON and back without any loss as
long as the expression source codes are always valid UTF-8, and we require
that during expression parsing anyway so that is a fine restriction.
The JSON encoding is a little noisy to read due to the extra annotations
required to be lossless (including source ranges) but still relatively
compact due to the base64-VLQ encoding of the source location information.
This allows the static analysis functions in the main HCL package to dig
through our wrapper to get the native expression object needed for most
analyses.
For example, this allows an expression with a native expression source
type whose source contains valid tuple constructor syntax to be used with
hcl.ExprList.
hclpack can potentially be used as an intermediary to more easily bring
non-HCL input into the HCL API, and so allowing a literal value in JSON
format as an expression type means that such a transcoder doesn't need to
worry about formatting values it encounters using HCL syntax and can
instead just encode directly to JSON, but at the expense of then not being
able to use the full expression/template syntax.
This is a straightforward way to get a hclpack.Body in the common case
where the input is already native syntax source code. Since the native
syntax is unambiguous about structure, the whole structure can be packed
in a single pass with no further information.
In most applications it's possible to fully evaluate configuration at the
beginning and work only with resolved values after that, but in some
unusual cases it's necessary to split parsing and decoding between two
separate processes connected by a pipe or network connection.
hclpack is intended to provide compact wire formats for sending bodies
over the network such that they can be decoded and evaluated and get the
same results. This is not something that can happen fully automatically
because a hcl.Body is an abstract node rather than a physical construct,
and so access to the original source code is required to construct such
a representation, and to interpret any source ranges that emerged from
the final evaluation.
