Language now supports top level definitions. no more last-expression-is-a-value thing, it's also a def. Host knows to run the special os def

src/parser.ts

@@ -1,5 +1,5 @@
 import type { Token } from './lexer'
-import type { AST, MatchCase, Pattern } from './ast'
+import type { AST, MatchCase, Pattern, Definition } from './ast'
 import { ParseError } from './error'
 
 export class Parser {
@@ -114,8 +114,29 @@ export class Parser {
         }
     }
 
-    parse(): AST {
-        return this.parseExpression();
+    parse(): Definition[] {
+        const definitions: Definition[] = [];
+
+        while (this.current().kind !== 'eof') {
+            definitions.push(this.parseDefinition());
+        }
+
+        return definitions;
+    }
+
+    private parseDefinition(): Definition {
+        const nameToken = this.expect('ident');
+        const name = (nameToken as { value: string }).value;
+
+        this.expect('equals');
+
+        const body = this.parseExpression();
+
+        if (this.current().kind !== 'eof') {
+            this.expect('semicolon');
+        }
+
+        return { kind: 'definition', name, body, ...this.getPos(nameToken) };
     }
 
     private parseExpression(): AST {