Required outports

[emil14]

Current semantics is this - at least one outports must be used. This has some problems.

Problem

Unhandled Errors

First of all errors could be unhandled. If component has e.g. res T, err error outports then compiler will be fine with you using just res. Yes, this is how Go works but it's not like e.g. Rust works. Go isn't perfect language in that regard IMO.

Yes, there're situations where you sure you be fine. Well, I guess you use Must then? This is especially not a big deal if we implement ? error bypass operator.

Unhandled Defaults

If you match or mod or anything that case something you can actually avoid using :else (or whatever they have for "otherwise".

Might be related to #202.

Proposed Solution: ! Exclamation Mark Operator

Just add ! at the end of the port (not port's data-type because that's info about port, not it's type) and make it required. Compiler will force you to use it. If you only need one port - that will be required one.

component ReadAll(filename string) (res! string, err! error)

In this example all ports of ReadAll are required. It's inport is required because all inports are always required (or because if it doesn't have default value in case we will implement #564). It's outports are both required because they both have !. They both have it because it doesn't make sense not to use result and because the error must be handled.

For components with array-ports (like Match especially - "router" components) the syntax will be what you probably expect it to be:

component Match<T>(data T, [case] T) ([case]! T, else! T)

Again, it only make sense if you use case (at least one slot) and you also have to handle default case.