Ruby’s Lisp Heritage

There’s an article The Ruby VALUE on O’Reillynet today. It shows a rather neat hack whereby ruby uses a few bits of an integer to specify extra information, like whether the value is nil or not.

It does mean that FixNums only go up to 2³⁰ instead of 2³², but that’s not a serious limitation, because it transparently turns them into BigNums instead.

All of this reminded me of a very similiar situation in another piece of software: Emacs. Like ruby, it uses a few extra bits in the word to signify the type of an object (Integer Type).

Note that Lisp is unlike many other languages in that Lisp objects are self-typing: the primitive type of the object is implicit in the object itself. For example, if an object is a vector, nothing can treat it as a number; Lisp knows it is a vector, not a number.

But it’s not only Emacs, lisps in general have a history of doing this. This leads to the lovely notion that the data has a type as opposed to the variable, which seems to confuse C programmers so much. 🙂