Providing an explicit type vs type inference - why not both?
30 Jan 2023Type inference is a really nice feature to have but sometimes we have to help the compiler out when what we want to write creates ambiguity.
This post uses a toy helper function that fetches remote JSON to show how we can design its api so that explicitly providing the type isn’t required when the compiler can infer types from context.
Let’s start by defining a pair of functions with a bit of duplication
func loadPost(id: Int) async throws -> Post {
let url = URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Post.self, from: data)
}
func loadPhoto(id: Int) async throws -> Photo {
let url = URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Photo.self, from: data)
}
In the functions above the main changes are the URL to fetch and the type to attempt to JSON decode to.
We could create a helper function to remove the duplication that would look like this:
private func load<Output: Decodable>(url: URL) async throws -> Output {
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Output.self, from: data)
}
This function is generic over an Output that must be decodable and takes care of the networking and decoding tasks.
With this in place our original functions can now become one liners that call through to this helper:
func loadPost(id: Int) async throws -> Post {
try await load(url: URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!)
}
func loadPhoto(id: Int) async throws -> Photo {
try await load(url: URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!)
}
With the above the compiler is happy to infer the type of Output in both cases because it can see that it needs to match the return type of the loadPost or loadPhoto functions.
This is all nice but quickly shows its inflexibility and breaks down if we change our usage slightly.
If I decide that loadPost should really be changed to loadPostTitle instead as callers don’t need the full post object I would try to update my function like this
- func loadPost(id: Int) async throws -> Post {
+ func loadPostTitle(id: Int) async throws -> String {
- try await load(url: URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!)
+ try await load(url: URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!).title
}
With this change the compiler is no long happy and emits this error:
Generic parameter ‘Output’ could not be inferred
We can look at how JSONDecode.decode is defined to see how its api is designed.
Clicking through the header we see
open func decode<T>(_ type: T.Type, from data: Data) throws -> T where T : Decodable
We could replicate this by providing the type explicitly, the updated helper function becomes:
- private func load<Output: Decodable>(url: URL) async throws -> Output {
+ private func load<Output: Decodable>(url: URL, as type: Output.Type) async throws -> Output {
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Output.self, from: data)
}
With this change the compiler now prompts us to update the call sites to explicitly provide the type to decode to
func loadPostTitle(id: Int) async throws -> String {
- try await load(url: URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!).title
+ try await load(url: URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!, as: Post.self).title
}
func loadPhoto(id: Int) async throws -> Photo {
- try await load(url: URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!)
+ try await load(url: URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!, as: Photo.self)
}
With the latest change we have more flexibility but if feels like we’ve lost some brevity in cases where the compiler can infer things. To bring this type inference back we can use a default argument (I think this was first shown to me by my friend Ollie Atkinson many years ago):
- private func load<Output: Decodable>(url: URL, as type: Output.Type) async throws -> Output {
+ private func load<Output: Decodable>(url: URL, as type: Output.Type = Output.self) async throws -> Output {
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Output.self, from: data)
}
With this final change we get a good balance between full flexibility when we need it and type inference when the compiler can figure things out.
func loadPostTitle(id: Int) async throws -> String {
try await load(url: URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!, as: Post.self).title
}
func loadPhoto(id: Int) async throws -> Photo {
- try await load(url: URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!, as: Photo.self)
+ try await load(url: URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!)
}
Conclusion
Making apis intuitive and nice to use can help keep you on track solving problems whilst the code gets out of the way. We’ve probably all used apis that require all our mental energy to remember how to use them, which means we can’t focus on the problem we are trying to solve.
Sample code
Here’s a code listing with scaffolding that you can slap into a playground to explore yourself
import Foundation
struct Post: Decodable {
let id: Int
let title: String
}
struct Photo: Decodable {
let albumId: Int
let id: Int
let title: String
}
enum Original {
static func loadPost(id: Int) async throws -> Post {
let url = URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Post.self, from: data)
}
static func loadPhoto(id: Int) async throws -> Photo {
let url = URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Photo.self, from: data)
}
}
print(try await Original.loadPost(id: 1))
print(try await Original.loadPhoto(id: 1))
enum HelperFunction1 {
static func loadPost(id: Int) async throws -> Post {
try await load(url: URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!)
}
static func loadPhoto(id: Int) async throws -> Photo {
try await load(url: URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!)
}
private static func load<Output: Decodable>(url: URL) async throws -> Output {
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Output.self, from: data)
}
}
print(try await HelperFunction1.loadPost(id: 1))
print(try await HelperFunction1.loadPhoto(id: 1))
enum HelperFunctionWithExplicitType {
static func loadPostTitle(id: Int) async throws -> String {
try await load(url: URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!, as: Post.self).title
}
static func loadPhoto(id: Int) async throws -> Photo {
try await load(url: URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!, as: Photo.self)
}
private static func load<Output: Decodable>(url: URL, as type: Output.Type) async throws -> Output {
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Output.self, from: data)
}
}
print(try await HelperFunctionWithExplicitType.loadPostTitle(id: 1))
print(try await HelperFunctionWithExplicitType.loadPhoto(id: 1))
enum HelperFunctionWithOptionalInference {
static func loadPostTitle(id: Int) async throws -> String {
try await load(url: URL(string: "https://jsonplaceholder.typicode.com/posts/\(id)")!, as: Post.self).title
}
static func loadPhoto(id: Int) async throws -> Photo {
try await load(url: URL(string: "https://jsonplaceholder.typicode.com/photos/\(id)")!)
}
private static func load<Output: Decodable>(url: URL, as type: Output.Type = Output.self) async throws -> Output {
let (data, _) = try await URLSession.shared.data(from: url)
return try JSONDecoder().decode(Output.self, from: data)
}
}
print(try await HelperFunctionWithOptionalInference.loadPostTitle(id: 1))
print(try await HelperFunctionWithOptionalInference.loadPhoto(id: 1))