add use method

src/handler.ts

@@ -2,6 +2,7 @@ import { ControlFlow, Break, Continue } from "./control_flow"
 
 // Typescript lacks shadowing, so code sometimes can be stupid to avoid it
 
+type Mapper<B, C, Bn, Cn> = (inp: Handler<B, C>) => Handler<Bn, Cn>
 export type HandlerFn<B, C> = (req: C, next: Endpoint<B, C>) => Promise<ControlFlow<B, C>>
 export type InputHandler<B, C> = HandlerFn<B, C> | Handler<B, C>
 
@@ -35,6 +36,10 @@ export class Handler<B, C> {
     this.fn = fn
   }
 
+  use<Bn, Cn>(f: Mapper<B, C, Bn, Cn>): Handler<Bn, Cn> {
+    return f(this)
+  }
+
   then(next: InputHandler<B, C>): Handler<B, C> {
     return new Handler(then(this, next))
   }