1

Hi there,

getting my feet wet with Vapor 4 I want to serve several JSON files as raw Data or String for that matter using Vapor 4.

I have a database which contains besides other stuff UUIDs which also act as filenames for several JSON files.

Currently my code looks like this:

routes.swift:

func routes(_ app: Application) throws {
    let geoJSONController = GeoJSONController()
    
    …

    app.get("MapEditorBackend", ":GeoJSONID", use: geoJSONController.get)
    
    …
}

GeoJSONController.swift:

struct GeoJSONController {
    …

    func get(req: Request) throws -> EventLoopFuture<String> {
        return GeoJSON.find(req.parameters.get("GeoJSONID"), on: req.db)
            .unwrap(or: Abort(.notFound))
            .flatMap { String(decoding: Data.fromFile($0.id!.uuidString), as: UTF8.self) }
            .transform(to: .ok)
    }
    
    func delete(req: Request) throws -> EventLoopFuture<HTTPStatus> {
        return GeoJSON.find(req.parameters.get("GeoJSONID"), on: req.db)
            .unwrap(or: Abort(.notFound))
            .flatMap { $0.delete(on: req.db) }
            .transform(to: .ok)
    }
}

when compiling above code I get those Errors:

SLPN-NB-LSH:MapEditorBackend lars$ ~/toolbox-18.0.0-beta.27/vapor-beta run
/Users/lars/Documents/Projects/MapEditor/MapEditorBackend/MapEditorBackend/Sources/App/Controllers/GeoJSONController.swift:16:14: error: generic parameter 'NewValue' could not be inferred
            .unwrap(or: Abort(.notFound))
             ^
/Users/lars/Documents/Projects/MapEditor/MapEditorBackend/MapEditorBackend/.build/checkouts/swift-nio/Sources/NIO/EventLoopFuture.swift:465:17: note: in call to function 'flatMap(file:line:_:)'
    public func flatMap<NewValue>(file: StaticString = #file, line: UInt = #line, _ callback: @escaping (Value) -> EventLoopFuture<NewValue>) -> EventLoopFuture<NewValue> {
                ^
/Users/lars/Documents/Projects/MapEditor/MapEditorBackend/MapEditorBackend/Sources/App/Controllers/GeoJSONController.swift:17:24: error: cannot convert value of type 'String' to closure result type 'EventLoopFuture<NewValue>'
            .flatMap { String(decoding: Data.fromFile($0.id!.uuidString), as: UTF8.self) }
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/lars/Documents/Projects/MapEditor/MapEditorBackend/MapEditorBackend/Sources/App/Controllers/GeoJSONController.swift:18:29: error: type 'EventLoopFuture<String>' has no member 'ok'
            .transform(to: .ok)
                           ~^~
[2/3] Compiling App routes.swift
Fatal error: result 1: file /Users/lars/toolbox-18.0.0-beta.27/Sources/VaporToolbox/exec.swift, line 50
Illegal instruction: 4

while the delete Function seems to work fine (I used it as a template for my get function) here EventLoopFuture<String> as a return type doesn't seem to work.

What I am doing wrong here? Do I have to wrap the String I create in the flatMap closure somehow?

Thanks in advance,

Lars

2

At a glance I would guess the problem is actually with your string construction. You may want String(data:encoding:) (which produces an optional you will need to unwrap) or you could go straight from the file to the string with a different constructor skipping data altogether. You also should be able to just pass the data to a Response constructor if you’d like to go that route, but you would need to change the signature of your get function in that case.

3

The String initializer is fine. Again just scanning from this forum thread, I think the problem is using flatMap there, flatMap expects you to return an EventLoopFuture but the String initializer just returns a string. Change flatMap to map in the get(req:) handler and that should clean up your build errors.

1 Like
4

Good catch. I change my vote to this.

5

Is that a valid use of .transform(to:)? You should get an HTTP 200 status just for successfully creating a String here so at best I think it is superfluous.

1 Like
6

Another good point! If you want to return the string, you need to remove the transform. The transform(to: .ok) drops the string completely and only returns the status code.

1 Like
7

Thanks to all for your quick help! Changing flatMap to just map and removing the transform almost fixed everything. Now my function looks like this:

    func get(req: Request) throws -> EventLoopFuture<String> {
        return GeoJSON.find(req.parameters.get("GeoJSONID"), on: req.db)
            .unwrap(or: Abort(.notFound))
            .map {
                if let data = try? Data.fromFile($0.id!.uuidString) {
                    return String(decoding: data, as: UTF8.self)
                } else {
                    return ""
                }
        }
    }

and works like a charm!

8

One more suggestion, you may want to be more explicit about the error case here, rather than simply swallowing with try?. You can do something like:

 func get(req: Request) throws -> EventLoopFuture<String> {
        return GeoJSON.find(req.parameters.get("GeoJSONID"), on: req.db)
            .unwrap(or: Abort(.notFound))
            .flatMapThrowing { found in
                try Data.fromFile(found.id!.uuidString)
            }.map { data in 
                String(decoding: data, as: UTF8.self)
            }.flatMapErrorThrowing { error in 
                throw Abort(.internalServerError) // you could do anything in here, or omit `flatMapErrorThrowing ` and propagate the error from the `fromFile` call directly
            }
    }
1 Like