How to handle end of file during parsing errors while reading a string?

Question

I was trying to read and eval Lisp forms from a string using a loop, instead of just putting the string into a buffer and using eval-buffer to load everything in the string. That method was mentioned in another thread here.

The problem is that read-from-string signals an error "End of file reached during parsing" and aborts my loop. I was hoping it would return nil, or some indication of the number of characters left, or give me some other way to figure out when to end my loop.

    (while (< pos (length contents))
      (setq pair (read-from-string contents pos))
      (setq obj (car pair))
      (setq pos (cdr pair))
      .. inspect the read obj here
      .. maybe eval it, or toss it, or fix it..
      )

Read-from-string returns starting position for the next read operation in the cdr of the pair returned. So I'm thinking that's so I can use it to start the next read in the right place. But I can't find any examples of how to read a string without getting the EOF error.

Probably it's just that I don't know how handle EOF errors while reading strings. Is there an ideomatic way of handling this kind of situation? Maybe with an unwind-protect or something?

Answer 1

If you want evaluation to return nil when it would normally raise an error, wrap the sexp to be evaluated in ignore-errors:

(ignore-errors (read-from-string contents pos))

See also with-demoted-errors.