Re: Lisp's QUOTE and Mathematica's "Hold"

josephoswaldgg@xxxxxxxxxxx wrote: 
Nathan Baum wrote:

It only _looks_ like Lisp is similar to Mathematica because Lisp
expressions can evaluate to Lisp expressions. If Lisp was virtually the
same as Mathematica, programs could only be written to use
self-evaluating types: there'd be no lists or symbols.

  (print '(list + 1 2))

wouldn't print (LIST + 1 2), nor (+ 1 2), but 3. The initial evaluation
of QUOTE would produce (LIST + 1 2), which is not a self-evaluating form
and would therefore be evaluated to produce (+ 1 2), which is not a
self-evaluating form and would therefore be evaluated to produce 3,
which is a self-evaluating form and would therefore not be evaluated
because further evaluation would have no effect.



Careful; remember Common Lisp *has* a variable with the name + (which
evaluates to the previous form form evaluated at the REPL). I'm
surprised no one has been burned by this yet in this discussion.


Hmm. You mean that evaluation would proceed as

  (print '(list + 1 2))
  => (print (list + 1 2))
  => (print (list (print '(list + 1 2)) 1 2))
  => (print (list (print (list + 1 2)) 1 2))
  => (print (list (print (list (print '(list + 1 2)) 1 2)) 1 2))
  => (print (list (print (list (print (list + 1 2)) 1 2)) 1 2))
  => (print (list (print (list (print (list (print '(list + 1 2)) 1 2))
                          1 2)) 1 2))
  => ...

until we got a stack overflow or out of memory error, depending upon how the expansion was implemented.

Presumably, a Lisp which used Mathematica-style evaluation would want to be a Lisp-1, so this kind of problem could not occur.
.

Relevant Pages

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