priorities in LALR and Earley

Hey,

Is it correctly understood that priority can be set on rules only for earley and on terminals only for lalr?

I expected to be able to switch seamlessly between the two algorigthms for my unambiguous and non-recursive grammar but I get a lot of parsing errors when switching from earley to larl. I think that most of them are due to priorities on rules not understood by lalr.

Also btw there may be some inconsistency in the index of priority between the two algorithm.

This grammar parses ‘1’ to b with earley:

%import common.INT

start: a | b

a: INT
b.1: INT

while it resolves to A with lalr:

%import common.INT

start: A | B

A: INT
B.1: INT

One has to increment to B.2 to get the grammar to parse ‘1’ to B.

[erezsh]

@night199uk If it overwrote i, that would cause a deterministic error. Anyway, I fixed that line, and the non-determinism persists.

[erezsh]

Yes, there is a difference in how the two algorithms handle priority. It’s not impossible to fix, it’s just a lot of work.

Regarding your example, B.1 is the same as B, because 1 is the default priority. So yeah, you need to use B.2 or higher.

This is the same for Earley. The reason Earley resolves this as a, is because when priorities are even (that is, both 1), it determines according to the order of appearance. In this case, a before b.