Why `Double` can pass to `CGFloat?`, but `Double?` cannot?

Using Swift
1 
struct FrameCGFloatAndDouble: View {
    let value = 12.234
    let optional: Double? = 12.234

    var body: some View {
        Text("Hello")
            .frame(minHeight: value)    // okay
            .frame(minHeight: optional) // error: Cannot convert value of type 'Double?' to expected argument type 'CGFloat?'
    }
}

Is this not allowed under SE-0307: Allow interchangeable use of CGFloat and Double types

4 Likes
2

Let's ask the experts: @xedin @hborla

3

Running this in playground I get the error on // okay row:

import Foundation
import PlaygroundSupport
import SwiftUI

struct FrameCGFloatAndDouble: View {
    let value = 12.234
    let optional: Double? = 12.234

    var body: some View {
        Text("Hello")
            .frame(minHeight: value)    // okay
            .frame(minHeight: optional) // error: Cannot convert value of type 'Double?' to expected argument type 'CGFloat?'
    }
}

Screenshot 2021-09-20 at 15.50.57
Screenshot 2021-09-20 at 15.50.571734×560 96 KB

4

I tnink you might be running a version of Xcode which doesn't support Double <-> CGFloat conversions. You can try a snapshot of 5.5 or main and it should work.

1 Like
5

We did it only for value -> optional because optional <-> optional conversions are hard to represent via the mechanism we are using to implement this (calling Double.init or CGFloat.init implicitly) because there could be multiple levels of optionality that have to be matched. I guess one way to implement that might be to use linked flatMap implicitly e.g.

import Foundation

func test(v: Double??) -> CGFloat?? {
  v.flatMap { $0.flatMap { CGFloat($0) } }
}

Might be worth opening a bug on bugs.swift.org as a refinement on the current implementation.

7 Likes
6

https://bugs.swift.org/browse/SR-15218

2 Likes
7

Thank you!

8

I'm not very clear on Optional.map() vs. .flatMap(). I tested your func test() between the two and seems work equally either flatMap() or map().

Is there specific reason to choose flatMap here?

9

I thought that Optional had only flatMap for some reason, but either of them should do :)

1 Like
10

So you added support for Conversion CGFloat? <-> Double?

11

No, this is just a diagnostic. I mentioned on JIRA that adding such a conversion is not a trivial task and outliner a possible approach.

1 Like