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

Jon Harrop wrote: 
Nathan Baum wrote:

This distinction is very significant, because QUOTE only 'protects'
against one round of evaluation.

(EVAL A)   => <the result of evaluating the value of A>
(EVAL 'A)  => <the value of A>
(EVAL ''A) => A

In a hypothetical Mathematica-like dialect of Lisp,

A          => <the value of A>
'A         => <the value of A>
''A        => <the value of A>
(EVAL A)   => <the value of A>
(EVAL 'A)  => <the value of A>
(EVAL ''A) => <the value of A>



No:


You misunderstand me. I mean that the Mathematica equivalent of 'X, which I suppose is just (X), would behave that way.

Mathematica has no _real_ equivalent of Lisp's QUOTE, since Mathematica 'fully evaluates' every expression: i.e. keeps evaluating it until further evaluation will not produce a different value. Hold is possible precisely because Hold[x] evalutes to Hold[x].

Another example of the difference between QUOTE and Hold: = holds its first argument, so assuming b doesn't have a value,

  a = b

assigns b to a, but Evaluate overrides Hold so

  Evaluate[a] = 42

assigns 42 to b, the result of evaluating a. (Or so the Matgematica documentation tells me)

The Lisp function setq quotes its first argument, so

  (setq a 'b)

assigns 'b to a. But eval _doesn't_ override this, and

  (setq (eval a) 42)

is an error.
.

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