Add Parse.defaulted

src/index.ts

@@ -102,7 +102,7 @@ export const Parse = {
     throw MismatchError("NaN", val);
   },
 
-  instanceOf<T>(classDef: { new(): T }): (val: unknown) => T {
+  instanceOf<T>(classDef: { new (): T }): (val: unknown) => T {
     return (val: unknown) => {
       if (val instanceof classDef) {
         return val;
@@ -223,6 +223,19 @@ export const Parse = {
     };
   },
 
+  defaulted<T>(
+    parser: (val: unknown) => T,
+    defaultVal: T,
+  ): (val: unknown) => T {
+    return (val: unknown) => {
+      if (val === undefined || val === null) {
+        return defaultVal;
+      }
+
+      return parser(val);
+    };
+  },
+
   oneOf<T extends Parser<unknown>[]>(
     parsers: T,
     shapeName = "Union",