Parser acting unexpectedly. about mpc HOT 4 CLOSED

That is really weird. I'll have to investigate more and see what is wrong.

from mpc.

I found the problem. The issue is that you are declaring your grammar as predictive: MPCA_LANG_PREDICTIVE.

Predictive means that no backtracking is required and that the first character any of the cases of an or clause always informs how the whole clause is going to be parsed. This is good for performance reasons but can't always be ensured for every grammar.

When you added repeat and rand it meant your grammar was no longer predictive because because both of them start with r. So the parser first tries to parse repeat and gets the first r and fails - at this point it needs to backtrack if it wants to parse rand but it doesn't bother because it is delcared as predictive and carries on blindly trying to parse the next a.

Probably I should remove the MPCA_LANG_PREDICTIVE clause from the maths example to give people a better starting point - and also consider throwing some sort of error when a predictive grammar is asked to backtrack.

Hope that helps,

• Dan

from mpc.

Yes, changing it to MPCA_LANG_DEFAULT makes it work as expected. I assume this is the one I should be using? In any case, thank you very much for your help. :)

from mpc.

Yup, MPCA_LANG_DEFAULT is correct. No problem, and good luck with your project.

from mpc.