Why is this for semantics rule of number = digit+ ("." digit+)??

[panlina]

I have grammar:

	number = digit+ ("." digit+)?

and semantic rule:

	number: (integer, dot, decimal) => +(integer.sourceString + dot.sourceString + decimal.sourceString)

I have 2 questions:

• Why the arity should be 3, not 2? It is as if expressions in ? are expanded in place.
• I expect dot and decimal referring to the dot and the decimal part respectively, but as I inspected, both dot.sourceString and decimal.sourceString are ".456" with test case 123.456, while dot.children[0].sourceString and decimal.children[0].sourceString are "." and "456" respectively. This looks strange to me, because there's no such thing whose only child is ./456 while its source is .456. Why is it?

[pdubroy]

I'm really sorry that it took so long to answer this 😬

It is as if expressions in ? are expanded in place.

Yes, that's how Ohm works. The main reason for this is that it's unclear how you would write a separate semantic action for the ? expression separately.

• I expect dot and decimal referring to the dot and the decimal part respectively, but as I inspected, both dot.sourceString and decimal.sourceString are ".456" with test case 123.456, while dot.children[0].sourceString and decimal.children[0].sourceString are "." and "456" respectively. This looks strange to me, because there's no such thing whose only child is ./456 while its source is .456. Why is it?

You're right that it doesn't really make sense. Unfortunately, this is something that we've never been able to find a good solution for. All nodes have a sourceString property, but I think it's not really meaningful for iteration nodes. One thing I considered is just throwing an exception if it's ever called.