Add def (like sum-type)

lib/index.ts

@@ -35,6 +35,9 @@ export type API<N extends string, D extends Definition> = {
     instance: InternalInstance<N, D, keyof D>,
     options: MatchOptions<D, T>
   ) => T;
+  def: <T>(
+		fn: (x: InternalInstance<N, D, keyof D>) => T,
+	) => (x: InternalInstance<N, D, keyof D>) => T
 };
 
 export type Is<R extends string> = `is${R}`;
@@ -89,6 +92,10 @@ function match(instance: any, options: any): any {
   return options[instance.tag](instance.value);
 }
 
+function def(fn: any): any {
+	return (x: any) => fn(x)
+}
+
 function otherwise(tags:string[]) {
     return (fn: any) => 
         Object.fromEntries(
@@ -233,6 +240,7 @@ export function type<N extends string, D extends Definition>(
     fromPathSafe,
     matchOr,
     match,
+    def,
     otherwise: (...args: any[]) => {
       if (args.length === 0) {
         return otherwise(Object.keys(routes))