feat: support for simple extensions dependencies (#265)

Co-authored-by: David Sisson <[email protected]>

site/docs/extensions/index.md

@@ -23,6 +23,27 @@ A Substrait plan can reference one or more YAML files via URI for extension. In
 | Type Variation     | The name as defined on the type variation object.            |
 | Function Signature | A function signature compound name as described below.       |
 
+A YAML file can also reference types and type variations defined in another YAML file. To do this, it must declare the YAML file it depends on using a key-value pair in the `dependencies` key, where the value is the URI to the YAML file, and the key is a valid identifier that can then be used as an identifier-safe alias for the URI. This alias can then be used as a `.`-separated namespace prefix wherever a type class or type variation name is expected.
+
+For example, if the YAML file at `file:///extension_types.yaml` defines a type called `point`, a different YAML file can use the type in a function declaration as follows:
+
+```yaml
+dependencies:
+  ext: file:///extension_types.yaml
+scalar_functions:
+- name: distance
+  description: The distance between two points.
+  impls:
+  - args:
+    - name: a
+      value: ext.point
+    - name: b
+      value: ext.point
+    return: f64
+```
+
+Here, the choice for the name `ext` is arbitrary, as long as it does not conflict with anything else in the YAML file.
+
 ### Function Signature Compound Names
 
 A YAML file may contain one or more functions by the same name. The key used in the function extension declaration to reference a function is a combination of the name of the function along with a list of the required input argument types. The format is as follows:

text/simple_extensions_schema.yaml

@@ -4,6 +4,16 @@ title: Simple Extensions
 additionalProperties: false
 type: object
 properties:
+  dependencies:
+    # For reusing type classes and type variations from other extension files.
+    # The keys are namespace identifiers that you can then use as dot-separated
+    # prefix for type class and type variation names in functions and the base
+    # type class for variations. The values must be extension URIs, following
+    # the same format and conventions as those used in the proto plans.
+    type: object
+    patternProperties:
+      "^[a-zA-Z_\\$][a-zA-Z0-9_\\$]*$":
+        type: string
   types:
     type: array
     minItems: 1