diff --git a/src/parser/ast.schema.yaml b/src/parser/ast.schema.yaml
new file mode 100644
index 0000000..fa2f366
--- /dev/null
+++ b/src/parser/ast.schema.yaml
@@ -0,0 +1,123 @@
+$schema: https://json-schema.org/draft/2020-12/schema
+title: Deimos AST generation spec
+type: object
+description: Top level is a map of node names in Pascal case (e.g., CompilationUnit) to node definitions.
+additionalProperties:
+  $ref: "#/$defs/NodeDefinition"
+$defs:
+  NodeDefinition:
+    type: object
+    additionalProperties: false
+    description: A definition of a node type.
+    properties:
+      children:
+        type: array
+        description: Ordered child fields for this node.
+        items:
+          $ref: "#/$defs/StructChildDefinition"
+      rules:
+        type: array
+        description: Alternative parse rules that build this node.
+        items:
+          $ref: "#/$defs/EnumChildDefinition"
+    oneOf:
+      - required:
+          - "children"
+      - required:
+          - "rules"
+  StructChildDefinition:
+    description: A definition of a node's child. Either a bare child name (string) in snake case, or an object.
+    oneOf:
+      - type: string
+        description: Shorthand where child name, var, build, and with are inferred from the given snake-case child name.
+      - $ref: "#/$defs/ChildDefinitionWrapper"
+  ChildDefinitionWrapper:
+    type: object
+    description: Single-key object mapping the child-name to its spec.
+    minProperties: 1
+    maxProperties: 1
+    additionalProperties: false
+    patternProperties:
+      "^[a-z][a-z0-9_]*$":
+        $ref: "#/$defs/ChildDefinition"
+  ChildDefinition:
+    type: object
+    description: One of skip/vec/single child specs.
+    oneOf:
+      - $ref: "#/$defs/SkipChildDefinition"
+      - $ref: "#/$defs/VecChildDefinition"
+      - $ref: "#/$defs/SingleChildDefinition"
+  SkipChildDefinition:
+    type: object
+    additionalProperties: false
+    description: A definition for a child rule that does nothing, i.e., is skipped.
+    properties:
+      rule:
+        type: string
+      skip:
+        type: boolean # note: must be true
+    required:
+      - rule
+      - skip
+  VecChildDefinition:
+    type: object
+    additionalProperties: false
+    description: A definition for a child rule that can be matched multiple times.
+    properties:
+      rule:
+        type: string
+      vec:
+        type: boolean
+    required:
+      - rule
+      - vec
+  SingleChildDefinition:
+    type: object
+    additionalProperties: false
+    description: A definition for a child rule that builds one item.
+    properties:
+      rule:
+        type: string
+      build:
+        oneOf:
+          - type: string
+          - $ref: "#/$defs/SingleChildBuildDefinition"
+    required:
+      - rule
+  SingleChildBuildDefinition:
+    type: object
+    additionalProperties: false
+    description: A definition of what exactly to build for a given child rule.
+    oneOf:
+      - $ref: "#/$defs/BuildBooleanChild"
+  BuildBooleanChild:
+    type: object
+    additionalProperties: false
+    description: A definition for building a boolean child.
+    properties:
+      type:
+        type: string
+        enum:
+          - boolean
+      on:
+        type: string
+        enum:
+          - rule_present
+  EnumChildDefinition:
+    description: A definition of an enum node's child. Either a bare rule (string) in Pascal case, or an object.
+    oneOf:
+      - type: string
+        description: Shorthand where child name, var, build, and with are inferred from the given Pascal-case rule name.
+      - $ref: "#/$defs/LongEnumChildDefinition"
+  LongEnumChildDefinition:
+    type: object
+    additionalProperties: false
+    description: A format for specifying more specific information for an enum child.
+    properties:
+      rule:
+        type: string
+      build:
+        type: string
+    required:
+      - rule
+      - build
diff --git a/src/parser/ast.yaml b/src/parser/ast.yaml
index 1443600..fd982ad 100644
--- a/src/parser/ast.yaml
+++ b/src/parser/ast.yaml
@@ -63,3 +63,22 @@ Class:
     - class_level_declarations:
         rule: ClassLevelDeclaration
         vec: true
+FunctionDeclaration:
+  children:
+    - is_public:
+        rule: Pub
+        build:
+          type: boolean
+          on: rule_present
+    - modifier:
+        rule: Modifier
+        build: FunctionModifier
+    - fn_kw:
+        rule: Fn
+        skip: true
+    - generics:
+        rule: GenericParameters
+    - identifier
+    - parameters
+    - return_type
+    - function_body