Throwing subscript setters

[tera]

What is the reason behind prohibiting throwing subscript setters?

struct S {
    subscript(c c: Int) -> Int {
        get { 0 }
        set throws {} // 🛑
    }
    subscript(d d: Int) -> Int {
        get throws { 0 }
        set throws {} // 🛑
    }
}

[xwu (Xiaodi Wu)]

No one has implemented it.

@Chris_Lattner

I consider all of these implementation limitations, not intentional parts of the design of Swift. Patches would be very very welcome to help improve these areas.

@John_McCall

If someone wants to pick this up, I’d be happy to guide them through the implementation. There’s some fairly serious compiler work involved, though, including adding new kinds of apply instruction to SIL.

It is hard, as discussed in SE-0310:

The main purpose of imposing this read-only restriction is to limit the scope of this proposal to a simple, useful, and easy-to-understand feature. Limiting effects specifiers to read-only properties and subscripts in this proposal does not prevent future proposals from offering them for mutable members. For more discussion of why effectful setters are tricky, see the "Extensions considered" section of this proposal.