Re: quasiquote limit cases

[Guillermo J. Rozas <gjr@martigny.ai.mit.edu>]

|   Subject: Re: quasiquote limit cases
|   From: Jim Blandy <jimb@red-bean.com>
|   Date: 22 Mar 1998 21:20:57 -0500
|   
|   
|   >    R5RS says:
|   >      ... If no commas appear within the <template>, the result of
|   >      evaluating `<template> is equivalent to the result of evaluating
|   >      '<template>.
|   
|   The intent was clear to me, but I think this is misleading because it
|   says that the *results* are equivalent; the results are values, and
|   thus it's reasonable to wonder about which definition of value
|   equivalence the authors meant.
|   
|   I suggest:
|   
|       If no commas appear within the <template>, the expression
|       `<template> is equivalent to the expression '<template>.
|   
|   It's shorter, and it's clear that one is discussing the equivalence of
|   expressions: equivalence of effect and value.

I actually wonder about this.  I make a distinction between quote and
backquote even in such cases.

When I use quote, I am stating that I will not mutate the object, and that
I may depend on its uniqueness -- i.e. no copying.

When I use backquote, I make no such statement.  In fact, I use it to
create a list initialized with constants that I am willing to mutate.

Thus I think of 

	`(a b c)

as equivalent to 
	
	(list 'a 'b 'c)

while

	'(a b c)

is more like

	;; at top level

	(define <hidden-variable> (list 'a 'b 'c))

	;; at use

	<hidden-variable>

I realize that the reports don't allow me to make that assumption, but
I wonder whether backquote might not be more useful if they did.

This distinction implies deep copying of back-quoted structure.  When
actual insertion is desired, the following clumsy but clear idiom
can be used

	`(foo ,bar ,@'(joe mary frederick))