Ray_Fix
(Ray Fix)
1
Hello,
I have some ID types that are simple ints coming back from a database. I wanted to improve type safety so I don’t accidentally assign product IDs to user IDs, etc. I want to be able to print it and use it as a dictionary key. So it is a trivial wrapper of Int. E.g.
struct CustomerID: Hashable, CustomStringConvertible {
init(_ value: Int) { self.value = value }
let value: Int
var hashValue: Int { return value.hashValue }
var description: String { return String(value) }
}
func ==(lhs: CustomerID, rhs: CustomerID) -> Bool {
return lhs.value == rhs.value
}
While it isn’t too much code, it seems like a lot boilerplate. I tried to reduce the boilerplate with protocol extensions. After a little bit of compiler fighting I came up with:
public protocol IDType: Hashable, CustomStringConvertible {
init(_ value: Int)
var value: Int { get }
}
extension IDType {
public var hashValue: Int { return value.hashValue }
public var description: String { return String(value) }
}
Notes (1) compiler made me declare my protocol public since Hashable and CustomStringConvertible are public. (2) I could not implement init(_ value: Int) in the extension so I have to do that each time (3) Self requirement built into Hashable meant that I could not implement an == for all IDTypes. So the final product looks like the following. Given the additional complexity, doesn’t seem like it is worth it.
public struct ProductID: IDType {
public init(_ value: Int) { self.value = value }
public let value: Int
}
public func ==(lhs: ProductID, rhs: ProductID) -> Bool {
return lhs.value == rhs.value
}
I wonder if the hive mind had any better ideas or different approaches for creating such types with minimal boilerplate. [Is there a possible new language feature coming that will make this more trivial?]
Ray
I have some ID types that are simple ints coming back from a database. I wanted to improve type safety so I don’t accidentally assign product IDs to user IDs, etc. I want to be able to print it and use it as a dictionary key. So it is a trivial wrapper of Int. E.g.
struct CustomerID: Hashable, CustomStringConvertible {
init(_ value: Int) { self.value = value }
let value: Int
var hashValue: Int { return value.hashValue }
var description: String { return String(value) }
}
func ==(lhs: CustomerID, rhs: CustomerID) -> Bool {
return lhs.value == rhs.value
}
Rather than going the protocol route, how about a generic type?
struct ID<IdentifiableType: Identifiable>: Hashable, CustomStringConvertible {
init(_ value: Int) { self.value = value }
let value: Int
var hashValue: Int { return value.hashValue }
var description: String { return String(value) }
}
func ==<T: Identifiable>(lhs: ID<T>, rhs: ID<T>) -> Bool {
return lhs.value == rhs.value
}
protocol Identifiable {
var id: ID<Self>? { get }
}
struct Customer: Identifiable {
var id: ID<Customer>?
}
···
--
Brent Royal-Gordon
Architechies
Ray_Fix
(Ray Fix)
3
Very cool! Thank you.
Notes to self:
ID uses the Identifiable model as sort of a phantom type. Given my current code base, it also convenient for me to do
typealias CustomerID = ID<Customer>
Being able to define new ID types in one line like this is exactly the solution I was looking for.
Ray
···
On May 17, 2016, at 7:24 PM, Brent Royal-Gordon <brent@architechies.com> wrote:
I have some ID types that are simple ints coming back from a database. I wanted to improve type safety so I don’t accidentally assign product IDs to user IDs, etc. I want to be able to print it and use it as a dictionary key. So it is a trivial wrapper of Int. E.g.
struct CustomerID: Hashable, CustomStringConvertible {
init(_ value: Int) { self.value = value }
let value: Int
var hashValue: Int { return value.hashValue }
var description: String { return String(value) }
}
func ==(lhs: CustomerID, rhs: CustomerID) -> Bool {
return lhs.value == rhs.value
}
Rather than going the protocol route, how about a generic type?
struct ID<IdentifiableType: Identifiable>: Hashable, CustomStringConvertible {
init(_ value: Int) { self.value = value }
let value: Int
var hashValue: Int { return value.hashValue }
var description: String { return String(value) }
}
func ==<T: Identifiable>(lhs: ID<T>, rhs: ID<T>) -> Bool {
return lhs.value == rhs.value
}
protocol Identifiable {
var id: ID<Self>? { get }
}
struct Customer: Identifiable {
var id: ID<Customer>?
}
