Vapor 3 - send a HTTPRequest

Your problem is .map(to: HTTPResponse.self). Map needs to transform its result into a new result regularly, like you would map an array. However, the result of your map-closure returns an EventLoopFuture<HTTPResponse>. This results in your map function returning an EventLoopFuture<EventLoopFuture<HTTPResponse>>.

To avoid this complexity, use flatMap.

var headers: HTTPHeaders = .init()
let body = HTTPBody(string: a)            
let httpReq = HTTPRequest(
    method: .POST,
    url: URL(string: "/post")!,
    headers: headers,
    body: body)

let client = HTTPClient.connect(hostname: "httpbin.org", on: req)

let httpRes = client.flatMap(to: HTTPResponse.self) { client in
    return client.send(httpReq)
}

EDIT: If you want to use the Content APIs you can do so like this:

let data = httpRes.flatMap(to: ExampleData.self) { httpResponse in
    let response = Response(http: httpResponse, using: req)
    return try response.content.decode(ExampleData.self)
}

HTTPClient.connect returns Future<HTTPClient> and it is mapping to a Future<HTTPResponse> not a EventLoopFuture<HTTPResponse>.

If you're expecting a single HTTPResponse use HttpClient.send instead of HTTPClient.connect.

If you're expecting multiple HTTPResponses then .map(to: HTTPResponse.self) must be changed to properly map to a EventLoopFuture<HTTPResponse>