HOWTO: Define token length constraints

[drush]

Looking for an approach to be able to define length constraints on tokens. As a canonical example, a math language that only allows variables to be a maximum of 4 characters would work.

GOOD:
var + 6 = var2

BAD:
var + 16 = otherVariable

Error: OtherVariable is too long (variables can be at most 4 characters in length)

[pdubroy]

Hey Darren,

It's not very pretty, but this can be done using a sequence. E.g.:

year = digit digit digit digit
zipCode = digit digit digit digit digit

etc.

I've considered adding first-class support for this, but I haven't yet seen a use case that really warrants it.

(Aside: Thanks for the inaugural post in our Github discussions! 🎉)

[pdubroy]

Just realized that this doesn't quite address the example you gave :-)

To implement a max identifier length, you can make all but the first character optional:

ident = letter alnum? alnum? alnum?

[drush]

This works in the short case, but allowing this to be parametric is important. ie Postgres column name length is max 63 characters.

[pdubroy]

Thanks for the use case.

I think the best way to handle that today is to raise an error in a semantic action. Would that work for your use case? Or is there a reason that it absolutely needs to be in the grammar?