Ui.Error, Info, Output should take variadic arguments

[keyneston]

Instead of Ui.Error(string) it should be Ui.Error(string, interface{}...). This should then call fmt.Fprintf(w, string, ...v).

Almost all of my calls to Ui.(Error|Info|Output) end up including a call to fmt.Sprintf.

[mitchellh]

The reason we don't do this is because we use the interface across RPC in a couple of our projects and it didn't deal well with variadics... At this point it would actually break backwards compatibility too (as a lib).

We also follow the pattern of using fmt.Sprintf everywhere. Is it worth breaking compat over? I'm not sure.

[keyneston]

Possibly add Errorf, Outputf, Infof functions?

It would technically change the Ui interface if people have written their own implementations but otherwise would be a noop change for consumers of the library.

[keyneston]

There could also be a UiFormatter interface that implemented Ui and the formatter functions? That way even if people wrote their own implementations of the interface it would be a noop change?

I'm happy to do the work once you give sign off.

[mitchellh]

That's a good idea. I like the idea of introducing a second interface. If you want to do a PR for that, that would be okay to me.

[mitchellh]

Hey @tarrant, given how long its been I think we've made it long enough that this is okay. We don't want to break BC and the actual other reason we don't do variadics is for a weird reason that this works better with our plugin system.