Add TextOutputStream to FileDescriptor?

[JetForMe]

Would it make sense to add TextOutputStream conformance to FileDescriptor to support writing something like this:

print("An error occurred", to: FileDescriptor.standardError)

Hmm, I tried doing this:

print("File exists.", to: &FileDescriptor.standardError)       //  Cannot pass immutable value as inout argument: 'standardError' is a get-only property

extension
FileDescriptor : TextOutputStream
{
    public mutating func write(_ inString: String) {
        _ = try? FileDescriptor.standardError.writeAll(inString.utf8)
    }
}

Perhaps those would be better as public static members?

[milseman]

I really like this idea, but I'm concerned that it just doesn't work out in practice. Writes can throw, a file descriptor might not be open for writing, and TextOutputStream is passed inout. Really, I think there needs to be a new/better pattern for streaming textual output than the one we have.

We could also consider throwing overloads of print and dump. Otherwise it might make sense to wait for a file stream concept.

[JetForMe]

For stderr it seems we could just swallow the throw. What about a constrained version, like print(_:toStandard:[.out|.error]) that doesn't throw? Or I'd be okay with throwing ones, too, but I hate to deal with try?in front of all of those.

What wold a stream concept do?

[milseman]

"Stream concept" would be a solution for System that provides the kind of functionality that FILE * does in C. You can write to it, it gets buffered locally, and it will flush on shutdown (or explicitly). The devil's in the details and how exactly we want to model it (e.g. unique ownership vs shared vs actor).

[GeorgeLyon]

In one of my project I faced a similar predicament where I wanted something to act as a TextOutputStream, but possibly throw (and be async). The solution I arrived at was to have a withTextOutputStream method like so:

public func withTextOutputStream(_ body: (inout TextOutputStream) -> Void) async throws {
  …
}

https://github.com/GeorgeLyon/Shwift/blob/f80750153de7ef28accca34d53d30bda95b8329b/Sources/Shell/Builtins.swift#L215-L220

It is a little verbose, but a similar solution might work well for FileDescriptor.