The optional modifier previously accepted a single argument: the
attribute type. This commit adds an optional second argument, which
specifies a default value for the attribute.
To record the default values for a variable's type, we use a separate
parallel structure of `typeexpr.Defaults`, rather than extending
`cty.Type` to include a `cty.Value` of defaults (which may in turn
include a `cty.Type` with defaults, and so on, and so forth).
The new `typeexpr.TypeConstraintWithDefaults` returns a type constraint
and defaults value. Defaults will be `nil` unless there are default
values specified somewhere in the variable's type.
In type constraints, object attributes may be marked as optional in
order to allow them to be omitted from input values. Doing so results in
filling the attribute value with a typed `null`.
This commit adds a new type `typeexpr.Defaults` which mirrors the
structure of a type constraint, storing default values for optional
attributes. This will allow specification of non-`null` default values
for attributes.
The `Defaults` type is a tree structure, each node containing a sub-tree
type, a map of children, and for object nodes, a map of defaults. The
keys in the children map depend on the type of the node:
- Object nodes have children for each attribute;
- Tuple nodes have children for each index, with indices converted to
string values;
- Collection nodes have a single child at the empty string key.
When traversing this tree we must take this structure into account, with
special cases for map input values which may later be converted to
objects.
The traversal defined in this commit uses a pre-order transformer in
order to pre-populate descendent nodes before their defaults are
applied. This allows type nested type default values to be specified
more compactly.
Using the any keyword with arguments (e.g. any(string, bool)) is
invalid, but any is not technically a "primitive type keyword". This
commit corrects the language in the diagnostic and updates the tests.
This builds on an experimental feature in the underlying cty library which
allows marking specific attribtues of an object type constraint as
optional, which in turn modifies how the cty conversion package handles
missing attributes in a source value: it will silently substitute a null
value of the appropriate type rather than returning an error.
In order to implement the experiment this commit temporarily forks the
HCL typeexpr extension package into a local internal/typeexpr package,
where I've extended the type constraint syntax to allow annotating object
type attributes as being optional using the HCL function call syntax.
If the experiment is successful -- both at the Terraform layer and in
the underlying cty library -- we'll likely send these modifications to
upstream HCL so that other HCL-based languages can potentially benefit
from this new capability.
Because it's experimental, the optional attribute modifier is allowed only
with an explicit opt-in to the module_variable_optional_attrs experiment.
