diff --git a/scripts/make_exprs.py b/scripts/make_exprs.py
index 97ed36f..df4020f 100755
--- a/scripts/make_exprs.py
+++ b/scripts/make_exprs.py
@@ -4,7 +4,7 @@ from pathlib import Path
 
 EXPR_TYPES = {
     'Binary': ('left: Expr', 'operator: Token', 'right: Expr'),
-    'Grouping': ('expression: Expr'),
+    'Grouping': ('expression: Expr',),
     'Unary': ('operator: Token', 'right: Expr'),
 
     # NOTE: when adding new Literals, add new Literal types, instead of just
@@ -18,7 +18,9 @@ def _gen_expr_decls():
         res.append(f'pub const {expr_type} = struct {{')
 
         for param in expr_params:
-            res.append(f'{param},')
+            res.append(f'    {param},')
+
+        res.append('};\n')
 
     return '\n'.join(res)
 
@@ -28,19 +30,27 @@ def do_base_union(union_name: str):
         'const Token = @import("token.zig").Token;\n'
     ]
 
-    res.extend(_gen_expr_decls())
-    res.append(f'pub const {union_name} = union {{')
+    res.append(_gen_expr_decls())
+
+    res.append(f'pub const {union_name}Type = enum {{')
+    for expr_type in EXPR_TYPES:
+        res.append(f'    {expr_type},')
+    res.append('};\n')
+
+
+    res.append(f'pub const {union_name} = union({union_name}Type) {{')
 
     for expr_type in EXPR_TYPES:
-        res.append(f'{expr_type}: {expr_type},')
+        res.append(f'    {expr_type}: {expr_type},')
 
-    res.append('}\n')
+    res.append('};\n')
 
     return '\n'.join(res)
 
 
 def main():
     expr_file = Path('./src') / 'expr.zig'
+    expr_file.touch()
     expr_file.unlink()
     expr_file.write_text(do_base_union('Expr'))
 
diff --git a/src/expr.zig b/src/expr.zig
new file mode 100644
index 0000000..e00e76f
--- /dev/null
+++ b/src/expr.zig
@@ -0,0 +1,28 @@
+const Token = @import("token.zig").Token;
+
+pub const Binary = struct {
+    left: Expr,
+    operator: Token,
+    right: Expr,
+};
+
+pub const Grouping = struct {
+    expression: Expr,
+};
+
+pub const Unary = struct {
+    operator: Token,
+    right: Expr,
+};
+
+pub const ExprType = enum {
+    Binary,
+    Grouping,
+    Unary,
+};
+
+pub const Expr = union(ExprType) {
+    Binary: Binary,
+    Grouping: Grouping,
+    Unary: Unary,
+};