This further alters the object-construction mode so that rather than
producing a flat object it instead produces an object of tuples. All
items that produce the same key are grouped together under the same
key in the result, allowing projections that flip the orientation of
a key/value sequence where the new keys are not necessarily unique.
Attribute-only splat expressions cannot have other splats nested inside,
since we're only interested in supporting how these behaved for HIL when
running inside Hashicorp Terraform. More complex cases should be dealt
with using either full splats (bracketed *) or "for" expressions.
Attribute-only splat expressions, indicated by using the star symbol as
if it were an attribute, are inherited from HIL and are a limited sort of
splat that only works for walking through attributes in each item of its
source.
Later zcl will also get full splat expressions, whose syntax is using
the star symbol as if it were an _index_, which will generalize to
supporting _all_ postfix traversal operations, including indexing and
nested splats.
If we can't find a function in the given EvalContext, we must traverse
the context chain until either we run out of contexts or we find a
matching function.
At present there is no reason for a non-root context to have any
functions, so this will always traverse to the root. This may change in
future if we introduce constructs that define local functions.
This syntax func(arg...) allows the final argument to be a sequence-typed
value that then expands to be one argument for each element of the
value.
This allows applications to define variadic functions where that's
user-friendly while still allowing users to pass tuples to those functions
in situations where the args are chosen dynamically.
Previously operations were an enum, but we've ended up needing to store
a collection of values against each, so an operation being a pointer to
a struct feels more natural.
This in turn allows us to more easily fix the return types of the
operations, so that we don't need to do any unusual work to understand
that (for example) arithmetic always returns a number.
This applies both two the whole of bare expressions and to any nested
expressions within parentheses. The latter means that an attribute value
can span over multiple lines if it's wrapped in parens:
foo = (
1 + 2 + 3 + 4 + 5
)
This is the first non-trivial expression Value implementation. Lots of
code here, so hopefully while implementing other expressions some
opportunities emerge to factor out some of these details.
