Hacker News Reader

Comment by whitten | Hacker News Reader

For those who can explain the difference is this a LISP 1 or a LISP 2 ?

In a Lisp 1 function and variables share the same namespace.

    (defvar foo 5)   ; variable with the name foo
    (defun foo () 5) ; function with the name foo

In a Lisp 1 the defun would overwrite the previous variable declaration. In a Lisp 2 though, both expressions coexist.

Ah, that's really interesting. This one only has a single namespace, but the one I simplified it from actually has two. I guess that makes that one a Lisp 2 :-)

Last week I came across a Usenet post from Kent Pitman (who helped create Common Lisp, and coined the terms Lisp1 and Lisp2 in this paper: https://www.dreamsongs.com/Separation.html) which provides the best explanation I have seen on why Lisp2 is a good idea:

https://groups.google.com/forum/#!msg/comp.lang.lisp/6TkfPLe...

Also as pointed out by KMP later in that thread, block and go tags are separate namespaces, and so are type names. I like to say that it is really Lisp1 vs LispN. Any language with macros and first-class identifiers is going to be capable of having arbitrarily many namespaces.

Reply on news.ycombinator.com