Add variable typing to reference docs

This documents the variable typing introduced in #3167.

Signed-off-by: Roberto Villarreal <[email protected]>

Author: rrjjvv

docs/bake-reference.md

@@ -1081,6 +1081,7 @@ or interpolate them in attribute values in your Bake file.
 
 ```hcl
 variable "TAG" {
+  type = string
   default = "latest"
 }
 
@@ -1102,6 +1103,206 @@ overriding the default `latest` value shown in the previous example.
 $ TAG=dev docker buildx bake webapp-dev
 ```
 
+Variables can also be assigned an explicit type.
+If provided, it will be used to validate the default value (if set), as well as any overrides.
+This is particularly useful when using complex types which are intended to be overridden.
+The previous example could be expanded to apply an arbitrary series of tags.
+```hcl
+variable "TAGS" {
+  default = ["latest"]
+  type = list(string)
+}
+
+target "webapp-dev" {
+  dockerfile = "Dockerfile.webapp"
+  tags = [for tag in TAGS: "docker.io/username/webapp:${tag}"]
+}
+```
+
+This example shows how to generate three tags without changing the file
+or using custom functions/parsing:
+```console
+$ TAGS=dev,latest,2 docker buildx bake webapp-dev
+```
+
+### Variable typing
+
+The following primitive types are available:
+* `string`
+* `number`
+* `bool`
+
+The type is expressed like a keyword; it must be expressed as a literal:
+```hcl
+variable "OK" {
+  type = string
+}
+
+# cannot be an actual string
+variable "BAD" {
+  type = "string"
+}
+
+# cannot be the result of an expression
+variable "ALSO_BAD" {
+  type = lower("string")
+}
+```
+Specifying primitive types can be valuable to show intent (especially when a default is not provided),
+but bake will generally behave as expected without explicit typing.
+
+Complex types are expressed with "type constructors"; they are:
+* `tuple([<type>,...])`
+* `list(<type>)`
+* `set(<type>)`
+* `map(<type>)`
+* `object({<attr>=<type>},...})`
+
+The following are examples of each of those, as well as how the (optional) default value would be expressed:
+```hcl
+# structured way to express "1.2.3-alpha"
+variable "MY_VERSION" {
+  type = tuple([number, number, number, string])
+  default = [1, 2, 3, "alpha"]
+}
+
+# JDK versions used in a matrix build
+variable "JDK_VERSIONS" {
+  type = list(number)
+  default = [11, 17, 21]
+}
+
+# better way to express the previous example; this will also
+# enforce set semantics and allow use of set-based functions
+variable "JDK_VERSIONS" {
+  type = set(number)
+  default = [11, 17, 21]
+}
+
+# with the help of lookup(), translate a 'feature' to a tag
+variable "FEATURE_TO_NAME" {
+  type = map(string)
+  default = {featureA = "slim", featureB = "tiny"}
+}
+
+# map a branch name to a registry location
+variable "PUSH_DESTINATION" {
+  type = object({branch = string, registry = string})
+  default = {branch = "main", registry = "prod-registry.invalid.com"}
+}
+
+# make the previous example more useful with composition
+variable "PUSH_DESTINATIONS" {
+  type = list(object({branch = string, registry = string}))
+  default = [
+    {branch = "develop", registry = "test-registry.invalid.com"},
+    {branch = "main", registry = "prod-registry.invalid.com"},
+  ]
+}
+```
+Note that in each example, the default value would be valid even if typing was not present.
+If typing was omitted, the first three would all be considered `tuple`;
+you would be restricted to functions that operate on `tuple` and, for example, not be able to add elements.
+Similarly, the third and fourth would both be considered `object`, with the limits and semantics of that type.
+In short, in the absence of a type, any value delimited with `[]` is a `tuple`
+and value delimited with `{}` is an `object`.
+Explicit typing for complex types not only opens up the ability to use functions applicable to that specialized type,
+but is also a precondition for providing overrides.
+
+> [!NOTE]
+> See [HCL Type Expressions][typeexpr] page for more details.
+
+### Overriding variables
+
+As mentioned in the [intro to variables](#variable), primitive types (`string`, `number`, and `bool`)
+can be overridden without typing and will generally behave as expected.
+(When explicit typing is not provided, a variable is assumed to be primitive when the default value lacks `{}` or `[]` delimiters;
+a variable with neither typing nor a default value is treated as `string`.)
+Naturally, these same overrides can be used alongside explicit typing too;
+they may help in edge cases where you want `VAR=true` to be a `string`, where without typing,
+it may be a `string` or a `bool` depending on how/where it's used.
+Overriding a variable with a complex type can only be done when the type is provided.
+This is still done via environment variables, but the values can be provided via CSV or JSON.
+
+#### CSV overrides
+
+This is considered the canonical method and is well suited to interactive usage.
+It is assumed that `list` and `set` will be the most common complex type,
+as well as the most common complex type designed to be overridden.
+Thus, there is full CSV support for `list` and `set`
+(and `tuple`; despite being considered a structural type, it is more like a collection type in this regard).
+
+
+There is limited support for `map` and `object` and no support for composite types;
+for these advanced cases, an alternative mechanism [using JSON](#json-overrides) is available.
+
+#### JSON overrides
+
+Overrides can also be provided via JSON.
+This is the only method available for providing some complex types and may be convenient if overrides are already JSON
+(for example, if they come from a JSON API).
+It can also be used when dealing with values are difficult or impossible to specify using CSV (e.g., values containing quotes or commas).
+To use JSON, simply append `_JSON` to the variable name.
+In this contrived example, CSV cannot handle the second value; despite being a supported CSV type, JSON must be used:
+```hcl
+variable "VALS" {
+  type = list(string)
+  default = ["some", "list"]
+}
+```
+```console
+$ cat data.json
+["hello","with,comma","with\"quote"]
+$ VALS_JSON=$(< data.json) docker buildx bake
+
+# CSV equivalent, though the second value cannot be expressed at all 
+$ VALS='hello,"with""quote"' docker buildx bake
+```
+
+This example illustrates some precedence and usage rules:
+```hcl
+variable "FOO" {
+  type = string
+  default = "foo"
+}
+
+variable "FOO_JSON" {
+  type = string
+  default = "foo"
+}
+```
+
+The variable `FOO` can *only* be overridden using CSV because `FOO_JSON`, which would typically used for a JSON override,
+is already a defined variable.
+Since `FOO_JSON` is an actual variable, setting that environment variable would be expected to a CSV value.
+A JSON override *is* possible for this variable, using environment variable `FOO_JSON_JSON`.
+
+```Console
+# These three are all equivalent, setting variable FOO=bar
+$ FOO=bar docker buildx bake <...>
+$ FOO='bar' docker buildx bake <...>
+$ FOO="bar" docker buildx bake <...>
+
+# Sets *only* variable FOO_JSON; FOO is untouched
+$ FOO_JSON=bar docker buildx bake <...>
+
+# This also sets FOO_JSON, but will fail due to not being valid JSON
+$ FOO_JSON_JSON=bar docker buildx bake <...>
+
+# These are all equivalent
+$ cat data.json
+"bar"
+$ FOO_JSON_JSON=$(< data.json) docker buildx bake <...>
+$ FOO_JSON_JSON='"bar"' docker buildx bake <...>
+$ FOO_JSON=bar docker buildx bake <...>
+
+# This results in setting two different variables, both specified as CSV (FOO=bar and FOO_JSON="baz")
+$ FOO=bar FOO_JSON='"baz"' docker buildx bake <...>
+
+# These refer to the same variable with FOO_JSON_JSON having precedence and read as JSON (FOO_JSON=baz)
+$ FOO_JSON=bar FOO_JSON_JSON='"baz"' docker buildx bake <...>
+```
+
 ### Built-in variables
 
 The following variables are built-ins that you can use with Bake without having
@@ -1239,4 +1440,5 @@ target "webapp-dev" {
 [ssh]: https://docs.docker.com/reference/cli/docker/buildx/build/#ssh
 [tag]: https://docs.docker.com/reference/cli/docker/image/build/#tag
 [target]: https://docs.docker.com/reference/cli/docker/image/build/#target
+[typeexpr]: https://github.com/hashicorp/hcl/tree/main/ext/typeexpr
 [userfunc]: https://github.com/hashicorp/hcl/tree/main/ext/userfunc