Cannot convert value of type 'Double.Type' to expected argument type 'Double' - JSON Decoder

Question

Created Aug ’22

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Hello, I am trying to code an API scraper that will collect Double values in a struct then return them but am getting the title error on the return line of code.

Struct Declaration:

struct ScrapeStruct: Decodable {
  var high: Double
  var low: Double
  var close: Double
  var volume: Double
}

Error Declaration:

enum Error: Error {
  case invalidServerResponse
}

Scrape Function:

class dataFetcher {

func Scraper() async throws -> ScrapeStruct {
   guard let url = URL(string: "someURL")
  else {
     throw Error.invalidServerResponse
    }
  let request = URLRequest(url: url)
  let (data, _) = try await URLSession.shared.data(for: request)
  _ = try JSONDecoder().decode(ScrapeStruct.self, from:data)
  return ScrapeStruct.init(high: Double, low: Double, close: Double, volume: Double) // error is here
  }
}

I am self-taught so there's a chance there are some fundamentel misunderstandings so if that's the case any recommendations on where to learn more in place of a solution are appreciated as well because I can't find anything online that's too helpful.

Answered by Claude31 in 722476022

When you create the struct in

ScrapeStruct.init(high: Double, low: Double, close: Double, volume: Double)

you need to pass values for arguments, not their type (Double)

You should write:

  let value = try JSONDecoder().decode(ScrapeStruct.self, from:data)
  return ScrapeStruct.init(high: value.high, low: value.low, close: value.close, volume: value.volume)

But there is a much simpler way:

  let value = try JSONDecoder().decode(ScrapeStruct.self, from:data)
  return value

There are a lot of JSON tutorials.

This one is good: https://www.raywenderlich.com/3418439-encoding-and-decoding-in-swift

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Answer 1

Claude31 OP

Aug ’22

Accepted Answer

When you create the struct in

ScrapeStruct.init(high: Double, low: Double, close: Double, volume: Double)

you need to pass values for arguments, not their type (Double)

You should write:

  let value = try JSONDecoder().decode(ScrapeStruct.self, from:data)
  return ScrapeStruct.init(high: value.high, low: value.low, close: value.close, volume: value.volume)

But there is a much simpler way:

  let value = try JSONDecoder().decode(ScrapeStruct.self, from:data)
  return value

There are a lot of JSON tutorials.

This one is good: https://www.raywenderlich.com/3418439-encoding-and-decoding-in-swift

1

Answer 2

blackwater00 OP

Aug ’22

Thanks Claude! I've seen your answers on a lot of different questions and you are always very helpful. Now I just have to figure out how to connect the access key and parameters for the API. I think I just need to spend more time reading why everything functions the way it does before jumping in.

For anyone with this issue this is what I changed it too in context for it to work.

func Scraper() async throws -> ScrapeStruct {
   guard let url = URL(string: "someurl")
  else {
     throw Error.invalidServerResponse
    }
  let request = URLRequest(url: url)
  let (data, _) = try await URLSession.shared.data(for: request)
  let scrapeStruct = try JSONDecoder().decode(ScrapeStruct.self, from:data)
  return scrapeStruct
  }

0