Implementing the config loader for Terraform led to the addition of some
special static analysis operations for expressions, separate from the
usual action of evaluating an expression to produce a value.
These operations are useful for building application-specific language
constructs within HCL syntax, and so they are now included as part of the
specification in order to help developers of other applications understand
their behaviors and the implications of using them.
Previously we allowed arrays only at the "leaf" of a set of objects
describing a block and its labels. This is not sufficient because it is
therefore impossible to preserve the relative ordering of a sequence
of blocks that have different block types or labels.
The spec now allows arrays of objects to be used in place of single
objects when that value is representing either an HCL body or a set of
labels on a nested block. This relaxing does not apply to JSON objects
interpreted as expressions or bodies interpreted in dynamic attributes
mode, since there is no requirement to preserve attribute ordering or
support duplicate property names in those scenarios.
This new model imposes additional constraints on the underlying JSON
parser used to interpret JSON HCL: it must now be able to retain the
relative ordering of object keys and accept multiple definitions of the
same key. This requirement is not imposed on _producers_, which are free
to use the allowance for arrays of objects to force ordering and duplicate
keys with JSON-producing libraries that are unable to make these
distinctions.
Since we are now requiring a specialized parser anyway, we also require
that it be able to represent numbers at full precision, whereas before
we made some allowances for implementations to not support this.
This is a super-invasive update since the "zcl" package in particular
is referenced all over.
There are probably still a few zcl references hanging around in comments,
etc but this takes care of most of it.
