Introduce (optional) values for tags

[jotaen]

I think it would be useful to introduce a syntax for tags to have (optional) values assigned to them. For example:

1999-05-31
    4h Worked on #ticket=462

The rules could be this:

• A tag may be followed by an equals sign (=) plus a value.
  • In the above example, the tag itself is still #ticket. In terms of terminology, I’d suggest ticket to be the tag’s name and 462 to be the tag’s value. “Tag” refers to the entire concept.
  • The allowed characters for values are the same as for tags themselves: letters, digits, underscores and dashes. In contrast to the tag name, however, the value is not harmonised to lower-case. So #tag=VALUE ­≠ #tag=value, whereas #TAG=value = #tag=value.
  • The value could optionally be quoted, e.g. #ticket="462" or #ticket='462'. That would allow values to contain more characters, e.g. #ticket="Foo Bar #.:!" → Foo Bar #.:!, whereas without quotes it’d be #ticket=Foo Bar #.:! → Foo, because the space character terminates the tag.
    • The quotes themselves are not part of the values.
    • If the closing quote is missing, the scope of the value ends at the end of the line. (So multiline values are prohibited.)
    • Escaping of quotes wouldn’t be supported, because that would be a bit too technical for klog. So no #ticket='It\'s done'.
• Absent value (#ticket or #ticket= or #ticket="") is treated as empty string. That way, tags without values behave exactly as they do right now.
• The = can only appear once, so #ticket=1=2 is either invalid or treated as #ticket=1.

On the CLI, users could additionally query for tag values. So with the example above:

• klog total --tag ticket would return 4h
• klog total --tag ticket=462 would return 4h
• klog total --tag ticket=1 would return 0h

And klog tags could additionally break it down by value as well:

#code-reviews  7h
#ticket        18h45m
 =465          6h30m
 =689          7h
 =911          5h15m
#conference    24h

Discussion

• An alternative to the = would be a colon, so #ticket:462. I think the meaning is a bit clearer with an equals sign, however, and it would also avoid ambiguity with a colon that appears within the sentence, like Worked on a #ticket: it’s not done yet, though. Therefore, I think the = is the better option.
• The quoting introduces most syntactical complexity. I think it might still be valuable, though, because letters and digits could be limited for some use cases. The default way would be without quotes also.
• If quoting is allowed for values, one could make a point to also allow that for tag names, e.g. #"foo.bar". That looks awkward to me, however, and you could as well use a value then, e.g. #whatever="foo.bar", so I wouldn’t do it.

This is a breaking change, because the = currently terminates the tag. The meaning would change now.

[vladdeSV]

Just wanted to mention that I really like this idea. The decision to use = is a good one.

[jotaen]

Resolved via #181.