diff --git a/src/protocol/ast.rs b/src/protocol/ast.rs
index b1711f01c47b37b5fce11546146c2e432357abfb..41a89918e25e80a760da57a086d19596513c87e2 100644
--- a/src/protocol/ast.rs
+++ b/src/protocol/ast.rs
@@ -5,6 +5,7 @@ use std::ops::{Index, IndexMut};
 use super::arena::{Arena, Id};
 use crate::collections::StringRef;
 use crate::protocol::input_source::InputSpan;
+use crate::protocol::TypeId;
 
 /// Helper macro that defines a type alias for a AST element ID. In this case 
 /// only used to alias the `Id<T>` types.
@@ -1015,6 +1016,18 @@ pub enum ProcedureKind {
     Composite,
 }
 
+/// Monomorphed instantiation of a procedure (or the sole instantiation of a
+/// non-polymorphic procedure).
+pub struct ProcedureDefinitionMonomorph {
+    pub argument_types: Vec<TypeId>,
+    pub expr_info: Vec<MonomorphExpressionInfo>
+}
+
+pub struct MonomorphExpressionInfo {
+    pub type_id: TypeId,
+    pub index: i32, // for called procedure's monomorphs, or selected field indices
+}
+
 /// Generic storage for functions, primitive components and composite
 /// components.
 // Note that we will have function definitions for builtin functions as well. In
@@ -1035,7 +1048,7 @@ pub struct ProcedureDefinition {
     pub scope: ScopeId,
     pub body: BlockStatementId,
     // Monomorphization of typed procedures
-
+    pub monomorphs: Vec<ProcedureMonomorph>,
     // Validation/linking
     pub num_expressions_in_body: i32,
 }
@@ -1054,6 +1067,7 @@ impl ProcedureDefinition {
             parameters: Vec::new(),
             scope: ScopeId::new_invalid(),
             body: BlockStatementId::new_invalid(),
+            monomorphs: Vec::new(),
             num_expressions_in_body: -1,
         }
     }
@@ -1568,7 +1582,8 @@ pub struct AssignmentExpression {
     pub right: ExpressionId,
     // Validator/Linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone)]
@@ -1581,7 +1596,8 @@ pub struct BindingExpression {
     pub bound_from: ExpressionId,
     // Validator/Linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone)]
@@ -1595,7 +1611,8 @@ pub struct ConditionalExpression {
     pub false_expression: ExpressionId,
     // Validator/Linking
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -1632,7 +1649,8 @@ pub struct BinaryExpression {
     pub right: ExpressionId,
     // Validator/Linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -1653,7 +1671,8 @@ pub struct UnaryExpression {
     pub expression: ExpressionId,
     // Validator/Linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone)]
@@ -1666,7 +1685,8 @@ pub struct IndexingExpression {
     pub index: ExpressionId,
     // Validator/Linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone)]
@@ -1680,7 +1700,8 @@ pub struct SlicingExpression {
     pub to_index: ExpressionId,
     // Validator/Linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone)]
@@ -1699,7 +1720,8 @@ pub struct SelectExpression {
     pub kind: SelectKind,
     // Validator/Linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone)]
@@ -1712,7 +1734,8 @@ pub struct CastExpression {
     pub subject: ExpressionId,
     // Validator/linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone)]
@@ -1727,7 +1750,8 @@ pub struct CallExpression {
     pub procedure: ProcedureDefinitionId,
     // Validator/Linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -1763,7 +1787,8 @@ pub struct LiteralExpression {
     pub value: Literal,
     // Validator/Linker
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32,
+    // Typing
+    pub expr_index: i32,
 }
 
 #[derive(Debug, Clone)]
@@ -1860,5 +1885,6 @@ pub struct VariableExpression {
     pub declaration: Option<VariableId>,
     pub used_as_binding_target: bool,
     pub parent: ExpressionParent,
-    pub unique_id_in_definition: i32, // used to index into type table after all types are determined
+    // Typing
+    pub expr_index: i32,
 }
\ No newline at end of file