value: add a Maybe[T] type

[creachadair]

A Maybe[T] is a container for an optional single value of arbirary type, which
may be either present or absent.

This implementation does not include more general utility functions using the
type (yet). I will add those as experience demands.

[danderson]

I like ❤️

[creachadair]

I like ❤️

Aha! Now we can bikeshed about the API. I did this to troll @noncombatant but I haven't convinced myself the API is right yet. Ideas welcome!

[noncombatant]

I was doubting the need for Absent but then I figured it out. :)

[creachadair]

I was doubting the need for Absent but then I figured it out. :)

Is there an example that would be helpful for clarifying that to future readers? I could add it to the examples file.

[danderson]

Some bikeshedding for your consideration. To repeat what I say in some of the comments, all these are very weakly held opinions and suggestions, mostly just pointing out where I felt friction reading the API, and throwing out some ideas to see if anything useful sticks. Feel free to ignore all of it, and paint the bikeshed in your preferred hue.

[creachadair]

Some bikeshedding for your consideration. To repeat what I say in some of the comments, all these are very weakly held opinions and suggestions, mostly just pointing out where I felt friction reading the API, and throwing out some ideas to see if anything useful sticks. Feel free to ignore all of it, and paint the bikeshed in your preferred hue.

Thanks! Some very thoughtful suggestions, most of which I took. 🙂

[danderson]

A few more thoughts, but 👍 generally. I'm still not sure how this would feel to wield in actual code, but short of committing it to mds/exp/maybe or something, I don't know how to resolve that short of just seeing what happens and having relaxed views on compatibility 🤷

[creachadair]

Some bikeshedding for your consideration. To repeat what I say in some of the comments, all these are very weakly held opinions and suggestions, mostly just pointing out where I felt friction reading the API, and throwing out some ideas to see if anything useful sticks. Feel free to ignore all of it, and paint the bikeshed in your preferred hue.

I've resisted being too strict about it, though now that there are more dependents outside my own work I might have to revise that a bit. But I chose a module name that is pretty easy to grep for, and I don't mind sending fixes directly, so for now I'm not going to fret too much, especially for a new package.

That said, I would like to play with it some more. I have a few usage patterns in mind:

1. Optional fields in structures. Especially for encoding, for which use it would harmonize well if the omitzero proposal winds up getting implemented 🤞🏻, but could still be useful even if not.

2. Documenting harmless errors, where control flow is annoying but you want better documentation of intent, e.g.,

   id := value.Check(strconv.ParseInt(s, 10, 64)).Or(25).Get()

3. As a nicer way to compose (T, bool) functions, e.g.,

   func MapGet[T comparable, U any](m map[T]U, key T) value.Maybe[U] {
      if v, ok := m[key]; ok {
         return value.Just(v)
      }
      return value.Absent()
   }

But mostly this was an essay in the API, I don't have an in-anger need for it yet.

[creachadair]

I have removed a couple of the top-level helpers that I wasn't convinced are necessary yet, and I'm going to go ahead and merge the basic type plumbing.