[swift-users] [swift-evolution] Best way to handle escaping function that might throw

Jacob Bandes-Storch jtbandes at gmail.com
Mon Jan 9 23:28:37 CST 2017

Moving to swift-users list.

No, there's no way to do this today. The point of rethrows is that within
one call site, "f(block)" can be treated as throwing if the block throws,
or not throwing if the block doesn't throw. In your example, once the
FStore object is constructed, the information about the original passed-in
function is lost, so the caller has no way to know whether call() can throw
or not.

If this *were* possible, the information would somehow need to be encoded
in the type system when creating FStore(f: block). That would require
something like dependent typing, or generic-param-based-rethrows, e.g.

struct FStore<T: () throws -> Void> {  // made-up syntax
    let f: T
    func call() rethrows(T) { try f() }  // throws-ness of this function
depends on throws-ness of T

On Mon, Jan 9, 2017 at 9:21 PM, Howard Lovatt via swift-evolution <
swift-evolution at swift.org> wrote:

> Hi,
> If I have an escaping function that I store and then call, I need to
> declare the calling function as throwing, not rethrowing. EG:
>     struct FStore {
>         let f: () throws -> Void
>         init(f: @escaping () throws -> Void) { self.f = f }
>         func call() throws { try f() } // Can't put rethrows here - have
> to use throws
>     }
> Is there a better solution?
> Thanks for any suggestions,
>   -- Howard.
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
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