YACC nothing

[HarryAnkers]

is there a way for you to have an option of matching a nothing case in yacc. for example: A : B C B C : SOMETHING |

Or would I have to do A : B C B | B B C : SOMETHING

[fyquah]

What you need might be empty rules

I found out about that by googling "Empty sequence" on google, which yields me this stackoverflow post.

[m8pple]

(I responded to this via email on the 13th, but it doesn't seem to have shown up here. Usually GH reflects them back up as issues if you respond.)

---

In bison there is a special token called %epsilon, which can be used to explicitly mark something as empty-by-intent:

https://www.gnu.org/software/bison/manual/html_node/Empty-Rules.html

C : SOMETHING | %empty

There is then some nice stuff which will warn you if you declare a production empty, but it isn't.

Doesn't work in classic yacc though, or older versions of bison. However, you can define a symbol called epsilon:

epsilon : / I am empty /

then use it in your grammar:

C : SOMETHING | epsilon