Add ability to encode comments with desc tag

[jbsmith7741]

Resolve issue #75 by adding a new tag 'desc' to describe variables.

example struct

type Person struct {
	Name string `desc:"My Name"`
	Age  int    `desc:"My Age"`
	Dog  Dog    `desc:"My dog"`
}

type Dog struct {
	Name  string `desc:"My Dog"`
	Breed string `desc:"Dog's breed"`
	Pet  Bug    `desc:"Dog's pet"`
}

type Bug struct {
	Weight float64 `desc:"in ounces"`
}

encodes to

# My Name
Name = ""
# My Age
Age = 0
# My dog
[Dog]
  # My Dog
  Name = ""
  # Dog's breed
  Breed = ""
  # Dog's pet
  [Dog.Pet]
    # in ounces
    Weight = 0.0

[zachbadgett]

Maybe we can add a space after the #

[silasdavis]

Would be useful to have this merged. If you want to have a single struct to define your application's config then it is nice to be able to maintain a two-way mapping between your canonical 'init' config with helpful comments and from modified config back to your struct. Without this you end up with the overhead of a manually assembled example/default config with comments and run the risk of them getting out of sync.

[yonderblue]

Would also be useful to decode the comments into something accessible, perhaps that means not using field tags?

[bobheadxi]

I think this looks great, and would be pretty useful for me - any plans to get this merged soon? Paging @BurntSushi

[jbsmith7741]

FYI: I've added this feature in another fork if anyone is interested https://github.com/hydronica/toml

[arp242]

I'm not a huge fan of using struct tags for this. Struct tags are limited in what you can put in there, it's a static string you can never modify (i.e. what if I want to put dynamic data in there?) and it just looks ugly for long strings IMO.

Instead, we could do something like:

type Commenter interface {
    Comment() string
}

And then you can use something like:

type X struct {
    WithComment strCmt
}

type strCmt string

func (strCmt) Comment() string { return "Yo, a comment!" }

Or ... something like that. I'm not sure. It's a bit more boilerplate, but it's a lot more flexible.

How do other encoders solve this problem, if at all? I can't really find an example of another encoder which does something like this.

[arp242]

I'll close this for now as I almost certainly don't want to use struct tags for this. I have some ideas for a better solution, as mentioned, but I need to prototype a few things to see what does and doesn't work. And before I take this on I want to fix some other things, like supporting all of TOML 1.0, fixing a few remaining bugs, and improving the errors a wee bit.

I absolutely want to add this feature on the short term (i.e. weeks, not years), just need to think of the best way to do it. You can track #75 for updates.