Moving `GetFlag` inside `State`?

[oxisto]

First of all, I really like your approach and I am currently trying this out in a larger software project that we try to migrate away from viper/cobra. What "bothers" me a little bit is the way flags are accessed. Currently I need to write

cli.GetFlag(state, "my-flag")

This somehow seems redundant, I would either expect to use something like cli.GetFlag("my-flag") to fetch it from a sort of global state - that does not exist (yet?) or state.GetFlag("my-flag"). What was the reasoning behind the current approach?

[mfridman]

Ooof great question.

In an ideal world it would be state.GetFlag[T]("my-flag"). Buuuut, Go doesn't allow parameterized methods -- methods can't introduce their own type paremters independant of the receiver type (golang/go#49085 / design doc). That means we cannot haave this:

// This is illegal
func (s *State) GetFlag[T any](name string) (T, error)

Likewise, can't really make a generic State[T] work since we need to retrieve flags of different types from the same State instance.

If you go down this route, you risk having to define your own flag set.

E.g., https://pkg.go.dev/github.com/peterbourgon/ff/v4#FlagSet

, and this is precisely what I wanted to avoid. I want this to work within the bounds of Go's std lib *flag.FlagSet, and for 95% of cases, that should be sufficient. In the rare case you need arbitrary types, then wire the parsing yourself via Var/Func. Maybe there's an opportunity for a sub-package with some common helpers like StringSlice.

That leaves state.GetFlag("my-flag"). The State changes as you traverse subcommands, so a simple global won't work well (or at least I couldn't find a good way). We could smuggle the State in the context, but you're still left with:

verbose := cli.GetFlag[bool](ctx, "verbose")

And that's how I settled on:

func GetFlag[T any](s *State, name string) T

Provides both type safety and a guarantee that the flag exists and is the expected type, or you get a loud failure that points straight at the programming error during development (only place where I added panics to avoid a , error return for ergonomics).

Sorry, probably a longer answer than you were looking for. But did want to jot this down.

[oxisto]

I totally forgot about the restriction of having type parameters in methods 🙈 Yeah I guess you are sort of stuck on the current API then. Thanks for the (detailed) explanation :)

[mfridman]

I believe this discussion is relevant here golang/go#77273

[oxisto]

I believe this discussion is relevant here golang/go#77273

That would be so awesome