HttpInterceptor Errors

Error: Cannot use namespace ‘HttpInterceptor’ as a type

What is wrong with the following?

import { HttpEvent, HttpInterceptor, HttpHandler, HttpRequest } from ‘@angular/common/http’;
import { Observable } from ‘rxjs/Observable’;
import { NgProgress } from ‘./progress.service’;
import ‘rxjs/add/operator/finally’;

// interface HttpInterceptor {
// intercept(req: HttpRequest, next: HttpHandler): Observable>
// }

export declare class NgProgressInterceptor implements HttpInterceptor {
ngProgress: NgProgress;
constructor(ngProgress: NgProgress);
intercept(req: HttpRequest, next: HttpHandler): Observable>;
}