diff --git a/typescript/Maybe.ts b/typescript/Maybe.ts
index 619c2ad..e48a3d8 100644
--- a/typescript/Maybe.ts
+++ b/typescript/Maybe.ts
@@ -40,3 +40,11 @@ export function mustExist<T>(m: T | undefined): T {
     return m;
   }
 }
+
+export function defaultTo<T>(d: T, m: Maybe<T>): T {
+  if (isJust(m)) {
+    return m[SymbolJust];
+  } else {
+    return d;
+  }
+}
diff --git a/typescript/Parse.ts b/typescript/Parse.ts
index 551e2a2..15d80f5 100644
--- a/typescript/Parse.ts
+++ b/typescript/Parse.ts
@@ -1,4 +1,4 @@
-import { isJust, isNothing, Just, just, Maybe, mustExist, Nothing, nothing, SymbolJust } from './Maybe.js';
+import { defaultTo, isJust, isNothing, Just, just, Maybe, mustExist, Nothing, nothing, SymbolJust } from './Maybe.js';
 import { map, primStringToList, split } from './Util.js';
 
 export type Token = {
@@ -155,10 +155,11 @@ export function parseNat(a: Maybe<number>, cs: ReadonlyArray<string>): Result<nu
   }
 
   const [x, ...xs] = cs;
-  const t = parseChar(x);
+  const n = parseChar(x);
 
-  if (t.type === 'digit') {
-    return emitCont(t.val, xs)
+  if (n.type === 'digit') {
+    const na = (defaultTo(0, a) * 10) + n.val;
+    return parseNat(just(na), xs);
   } else {
     return emitBack(cs);
   }