Proposal: Literal identifiers with backtick

[mdmcconnell]

My application has a lot of identifiers that use natural language, things like "Section 1-2a", or "2015 Information Table". Currently, expr can not parse these as single token identifiers for various reasons. Possible workarounds for me include: make ad-hoc subsitutions to my identifiers, such as "Section_1_2a", though that will be messy and error-prone; create a literal('Section 1-2a') method, which would clutter expressions; use single quote 'Section 1-2a' and patch the tree to turn all string nodes into indentifiers, but I would lose the ability to use actual strings in my expressions (porbably ok in my case).

I would like to know if expr would consider allowing backtick to enclose literal identifers, so the above Section 1-2a would be parsed as a single identifier node. As backtick is now an illegal character I can not see backwards compatibility problems - though enabling this feature could be wrapped in an option to ensure safety. Python has deprecated backtick, so a disadvantage could be some divergence from Python-like syntax. Or possibly backtick is reserved for some future use like binary literals. Any thoughts?

[antonmedv]

Can you show an example of what it may look like? Is it for variable access from env?

[mdmcconnell]

Yes, correct my use is for variable access from env (I am using a simple map[string]struct for env - in go, any string could be a key). There would also be the possibility of function name literals and others; maybe that is of use to someone if not to me. With literal identifiers, the following would be a valid expression:
"`Section 1-2a` + `2015 Information Table`"
And would evaluate to the sum of whatever those two variables contain.
I ask about the possibility, because I think it would take me a similar amount of work to add to this to expr as implementing a good workaround.

[antonmedv]

I was thinking about how to add this feature myself. But using ` is better to reserve for raw strings.

For example, in JavaScript you can use global:

globalThis['Section 1-2a'] = 1

Maybe something like env keyword will make sense? env['Section 1-2a']?

Can you research how other languages are doing this?

[mdmcconnell]

Yes, I understand, and it is good to consider carefully how to use one of the last unused characters. env keyword would work well, though possibly break someone's application; my only argument in favour of new syntax is slightly shorter / nicer-looking expressions. I am not familiar with any language that allows totally arbitrary variable names. Some googling indicates Fortran ignored whitespace entirely (not really helpful here). VB apparently allows within square brackets (very similar implementation to backtick, sort of like env[] with the keyword assumed; could be feasible?). Kotlin and Scala it seems allow using backticks, Common LISP using | and PHP may sort of using single quotes.