Extend valid set of quoted labels

[Gabriella439]

Related to #64

This adds : to the list of supported characters for Dhall labels and
no longer treats the initial character of a quoted label differently

[Profpatsch]

Doesn’t this just move the problem? Now we have an arbitrary list of legal symbols, plus one of them is also used as the separator.

I’m not sure what the more general way what be. Escaping?

[Gabriella439]

The reason I explicitly whitelist allowed characters instead of accepting a broader class of letters is due to the potential for identifier name obfuscation by using visually similar characters.

There actually is a Unicode standard with various alternatives for how to implement broader identifier names:

http://unicode.org/reports/tr31/

... but I decided to skip that for now since it increases the complexity of implementing DHall

[Profpatsch]

Full ack, I’m always for explicit symbol sets. But does it make sense to expand those manually for some use cases? Except if adding : means all yaml symbols are valid now, then it’s probably a good idea.

Update: Just saw #64, which fixes that problem in a general way. So maybe it’s better to not introduce : in symbols? I can imagine that makes syntax highlighting for editors harder to implement (since editors mostly use regex for implementing simple syntax highlighting).

[Gabriella439]

@Profpatsch: Note that this change is only for quoted labels (i.e. surrounded by backticks). That means that a colon is still not allowed in a normal unquoted label:

foo     -- Legal label
fo:o    -- Not a legal label; parsed as "fo : o"
`fo:o`  -- Legal label

This also doesn't prevent editors from using a regular expression for matching symbols. The regular expression is:

[A-Za-z0-9_][A-Za-z0-9_/-]*|`[A-Za-z0-9_/-:]+`

[Profpatsch]

Ah nice, I wasn’t even aware that labels could be quoted with backticks.