🚀 go-pugleaf

In article <2698@crcge1.UUCP>, adams@crcge1.UUCP (Drew Adams) writes:
> I  don't  know  what  (pure)  LISP's operational  semantics should be
> called.   If it  weren't already  in use  we might  consider the term
> "denotational" to characterise its *operational* semantics, since the
> built-in recognition of quotation provides the ability to distinguish
> *different levels* of representation of meaning.  

QUOTE is a syntactic construct (as is LAMBDA).  QUOTE is NOT a function.
Under "normal" circumstances "(X Y)" means "look X and Y up in the
current environment, and invoke X with Y as an argument".  However, within
special syntactic constructs (such as QUOTE and LAMBDA), it means something
different.
  * In (LAMBDA (X Y) ...), the "(X Y)", certainly does not have the
    "normal" interpretation.
  * Similarly, in (QUOTE (X Y)), the "(X Y)" is interpreted differently.
This has nothing to do with referential transparency.

> Having such a denotation mechanism available in a language
> facilitates a leveled view of  things.   However, it  would seem that
> most  programmer  use  of  quotation  in LISP  doesn't really involve
> intentional semantic leveling.  The  most frequent  uses of quotation
> in LISP would appear to be:  
> 
> 	* to prevent the attempted evaluation of something which is
> 	  undefined, together with the ensuing error treatment
> 
> 	* to prevent the immediate evaluation of a defined expression 
> 	  because the moment isn't right.
> ...
>
> In other words, QUOTE is  mainly used  by LISP  programmers to offset
> the eagerness  of EVAL.

No, QUOTE construct in LISP is basically a shorthand for making literal
symbols and lists easier to write.  Writing "(QUOTE A B C)" is more
convenient than:
  (LIST (STRING->SYMBOL "A") (STRING->SYMBOL "B") (STRING->SYMBOL "C"))
and has some "EQ" properties would require even more verbosity for the
programmer to simulate.

"Delaying" evaluation is generally better handled LAMBDA and application
rather than QUOTE and EVAL.

The problem is with EVAL, not with QUOTE.  If you remove EVAL from
the language, you have no problem with QUOTE.  If you remove QUOTE from
the language, but keep EVAL, you still have the same problems you would
have had if QUOTE were in the language.

		Steve Vegdahl
		Computer Research Lab
		Tektronix Labs
		Beaverton, Oregon