Returning `nil` from a `compactMap()` closure fails result type check

Question

In this snippet, I try to map [RegisteredMediaAttachment] to [BasicAttachmentInput], but only for those items that have a parseable uploadUrl.

    private func createSecondaryAttachments(
        _ attachments: [RegisteredMediaAttachment]
    ) -> [BasicAttachmentInput] {
        return attachments.compactMap {
            guard let name = URL(string: $0.registration.uploadUrl) else {
                return nil
            }

            return BasicAttachmentInput(
                name: name,
                contentType: $0.attachment.type.mime(),
                sizeBytes: $0.attachment.data.count,
                mediaId: $0.registration.media.id
            )
        }
    }

However, Swift complains on the line return nil that 'nil' is not compatible with closure result type 'BasicAttachmentInput'

This surprises me. I expect the function to compile and reduce the map result to contain only non-nil values returned from the compactMap closure. What is wrong with the snippet above?

Reconstructionist

to achieve what you want, you could try this: func createSecondaryAttachments(_ attachments: [RegisteredMediaAttachment]) -> [BasicAttachmentInput] { return attachments.compactMap { if let name = URL(string: $0.uploadUrl) { return BasicAttachmentInput(.....) } } }, but to fully explain your error is more difficult. – Assimilation

Is this one of those cases where Swift is not smart enough to infer the type again? Try writing all the types explicitly, attachments.compactMap { (attachment: RegisteredMediaAttachment) -> BasicAttachmentInput? – Catherine

I tried your code with some simple structs ; one with an optional and one without and the function takes an array of the first and returns a compactmap containing the second. I got no errors in either playgrounds nor Xcode 15 – Devisal

I also tried with a string and and a url property as output. Again, no errors. Can you create a minimal reproducible example? – Devisal

Answer 1

Turns out Swift is just failing to infer the types correctly: simply annotating the compactMap() closure with the implied return type solves the issue:

    private func createSecondaryAttachments(
        _ attachments: [RegisteredMediaAttachment]
    ) -> [BasicAttachmentInput] {
        return attachments.compactMap { (attachment: RegisteredMediaAttachment) -> BasicAttachmentInput?
            // Note the explicit return type signature of the closure above ^
            guard let name = URL(string: $0.registration.uploadUrl) else {
                return nil
            }

            return BasicAttachmentInput(
                name: name,
                contentType: $0.attachment.type.mime(),
                sizeBytes: $0.attachment.data.count,
                mediaId: $0.registration.media.id
            )
        }
    }

Reconstructionist

Answer 2

compactMap and error

'nil' is not compatible with closure result type

Also this error is threw when you have a syntax error inside closure(which is ignored)

Berkelium