For a while now we've had information equivalent to this in various
internal documents that we've referred to when designing features such as
config-driven refactoring, the "replace" planning option, and so forth.
However, so far we've not put that information in any sort of durable
public place that we can easily find and refer to when having design
discussions on GitHub and similar.
This is therefore an attempt to capture a summary of the three main design
patterns we've identified for planning-related behaviors, with a few
motivating examples of each one, in the hope that this will be a good
reference and some helpful inspiration for future design work.
It's intentionally not totally comprehensive of all planning behaviors
both because that would duplicate the end-user-oriented documentation and
because it would be burdensome to keep updating this document each time we
add anything new which might fit into these categories. However, we might
add a later feature to this document if it illustrates a new take or
different perspective on one of these patterns.
Extend the documentation on type constraints to include the new default
functionality, including a detailed example of a nested structure with
multiple levels of defaults.
Now that variables parse and retain a set of default values for
object attributes, we must apply the defaults during variable
evaluation. We do so immediately before type conversion, preprocessing
the given value so that conversion will receive the intended defaults as
appropriate.
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.
Rather than maintain a separate graph builder for destroy, use the
normal plan graph with some extra options. Utilize the same pattern as
the validate graph for now, where we take the normal plan graph builder
and inject a new concrete function for the destroy nodes.
All instances in state are being removed for destroy, so we can skip
checking for orphans. Because we want to use the normal plan graph, we
need to be able to still call this during destroy, so flag it off.
Destroy nodes should never participate in references. These edges didn't
come up before, because we weren't building a complete graph including
all temporary values.
- Note that it was added in v1.2, so users know whether they can use it yet.
- Fix nested indentation formatting, so the parent list item stays together.
- Minor copy edits.
- List all of the available lifecycle block arguments before diving into the
docs for each argument, for ease of reading/scanning.
