Question

I’m analyzing a code and I saw this

[(exp + exp) (+ $1 $3)]

Now I’m wondering what does this do? Does it take the first and third element of that list? Is that set on the left side even considered a list? I’m learning this language but I’ve been having a pretty rough time with it, so you guys have been the best help. Thanks!

Answer 1

Given your previous question, I’m guessing this is in the context of Racket’s parser-tools library. From the docs:

(grammar (non-terminal-id
      ((grammar-id ...) maybe-prec expr)
      ...)
...)

Declares the grammar to be parsed. Each grammar-id can be a token-id
from a group-id named in a tokens declaration, or it can be a
non-terminal-id declared in the grammar declaration. The optional prec
declaration works with the precs declaration. The expr is a “semantic
action,” which is evaluated when the input is found to match its
corresponding production.

Each action is Racket code that has the same scope as its parser’s
definition, except that the variables $1, …, $i are bound, where i
is the number of grammar-ids in the corresponding production. Each $k
is bound to the result of the action for the kth grammar symbol on the
right of the production, if that grammar symbol is a non-terminal, or
the value stored in the token if the grammar symbol is a terminal.