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

Brian Downing wrote:
> In article <43062cf8$0$22943$ed2619ec@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
> Jon Harrop <usenet@xxxxxxxxxxxxxx> wrote:
>> Right. I think that's exactly the same as "Hold" in Mathematica.
>
> Not really.

Both QUOTE and Hold retain subexpressions in unevaluated form. Perhaps the
similarity ends there...

> (defparameter *test* (quote (+ 2 2))) ==> *TEST* contains (+ 2 2).
>
> test = Hold[2 + 2] ==> test contains Hold[2 + 2]
>
> i.e., Hold "sticks" with the expression, whereas QUOTE is just a signal
> to the evaluator to pass its contents through unevaluated. You can't
> build something as simple as QUOTE in Mathematica because the evaluator
> will keep evaluating until it stops changing - the CL QUOTE would be a
> noop in the Mathematica evaluator.

You can see Hold in Mathematica but not QUOTE in Lisp but isn't that more a
function of the pretty printer rather than the evaluation?

> Note also that CL has a very strong sense of object identity. If I say
> (quote (+ 2 2)), I get the exact same (EQ) cons out that I put in. I
> don't think Mathematica has anything like EQ, so I'm not sure how you'd
> test to see if the Plus[2, 2] in a hold is the same Plus[2, 2] you put
> in earlier.

In[1]:= Hold[2+2] === Hold[2+2]

Out[1]= True

In[2]:= Hold[a+b] === Hold[b+a]

Out[2]= False

So the Mathematica:

Release[Hold[expr]]

is equivalent to the Lisp:

(eval (quote expr))

Beyond that, evaluation is completely different. But just looking at QUOTE
and Hold, there is an uncanny resemblance.

--
Dr Jon D Harrop, Flying Frog Consultancy
http://www.ffconsultancy.com
.

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    ... Jon Harrop wrote: to the evaluator to pass its contents through unevaluated. ... You can't build something as simple as QUOTE in Mathematica because the evaluator will keep evaluating until it stops changing - the CL QUOTE would be a noop in the Mathematica evaluator. ...
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