Where is quote (again)?

[Paul Foley]

On Sat, 09 Mar 2002 20:51:02 GMT, logistix  wrote:

> I ain't no lisp expert (so please don't yell at me Paul F. ; ) ), but the
> interpreter is a read-eval-apply loop.

> 1. A statement is READ in
> 2. The arguments are EVALuated (unless short circuited by a quote)
> 3. The evaluated arguements are APPLYed to the function, result is returned

1. A form is READ in
2. The form is evaluated
3. The results are printed   [CL supports multiple values]

The evaluation consists of first evaluating the arguments to
functions, but QUOTE forms are evaluated, too; its value is just its
argument, unevaluated.  QUOTE is one of a small number of hard-wired
things called "special operators" that have special meaning and don't
follow the normal evaluation rules.

E.g., ignoring a few details,

  def EVAL (form):
      # macroexpand form here
      if type(form) is not type([]):
         # if it's a symbol, return its symbol-value
         return form
      elif form[0] is QUOTE:
         if len(form) != 2:
            raise SomeException
         return form[1]
      # other special forms here
      else:
         return apply(form[0], map(EVAL, form[1:]))


[Many implementations don't actually have an interpreter at all, and
implement EVAL as something more like

  (defun eval (form)
    (funcall (compile nil `(lambda () , at form))))

i.e., just compile a function whose body is the form in question, and
immediately execute the newly-compiled function.  `(lambda () , at form)
means essentially "[LAMBDA, []] + form" in Python]

> "N Becker" <nbecker at fred.net> wrote in message
> news:1f5252d4.0203080721.3bfc5c14 at posting.google.com...
>> Back in 1999, I asked "where is quote?".  I was looking for
>> a way to quote a function (delay evaluation).
>> 
>> I wonder if there is anything new.  It looks to me that the
>> best answer is to use lambda.

Quoting code for later evaluation with EVAL is almost guaranteed not
to be what you want, anyway.  LAMBDA is the right thing.


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