Meaning of period in (. 123)

Question

I learnt . /path/to/file in bash is used to execute a file. Just out of curiosity, I eval something like the following in Emacs

(. 123)
     ⇒ 123

(read "(. 123)")
     ⇒ 123

It looks like Emacs simply reads (. 123) as 123, what happened?

Answer 1

It looks like Emacs simply reads (. 123) as 123, what happened?

That's exactly what happened. To back it up with sources:

if (ch == '.')
  {
    if (!NILP (tail))
      XSETCDR (tail, read0 (readcharfun));
    else
      val = read0 (readcharfun);
    read1 (readcharfun, &ch, 0);

    if (ch == ')')
      {
        if (doc_reference == 1)
          return make_number (0);
        if (doc_reference == 2 && INTEGERP (XCDR (val)))
          /* ... */
        return val;
      }
    invalid_syntax (". in wrong context");
  }

This is the special case for read_list in lread.c. Normally a . is treated by setting the cdr of the previously read tail by whatever follows. However in the case there is no tail (like when reading (. 123)), the next thing is read and returned as is. Personally, I'd expect that to lead to an invalid syntax error, but I'm sure someone put the special case there to work around peculiarly terrible sources. I've tried out how other Lisp interpreters behave for fun and none of csi, pil and sbcl do permit reading this, so it may be worth a bug report.

edit: Guile behaves the same, MIT-Scheme doesn't. There goes my theory of this behavior being a GNU thing...