On Sunday, May 11, 2014 6:21:08 PM UTC+5:30, Steven D'Aprano wrote: > The point is, it is *logically impossible* for a language to use > precisely the same syntax for value-assignment and variable-assignment. > Consider the variable called "x", which is bound to the value 23. If the > language has a single assignment operator or statement:
Its called set in classic lisp. Here's an emacs lisp session (the oldest lisp I can lay my hands on) The semicolons are comments like python's # *** Welcome to IELM *** Type (describe-mode) for help. ELISP> (set (quote d) "Hello World") "Hello World" ELISP> (set (quote c) (quote d)) d ELISP> ; those quote-s are getting boring ELISP> (set 'b 'c) c ELISP> ; once more short-form a very common case ELISP> (setq a 'b) b ELISP> ; Now unfold the chain ELISP> ; Level 0 ELISP> 'a a ELISP> ; Level 1 ELISP> a b ELISP> ;Level 2 ELISP> (eval a) c ELISP> ;Level 3 ELISP> (eval (eval a)) d ELISP> ;Level 4 ELISP> (eval (eval (eval a))) "Hello World" ELISP> IOW set UNIFORMLY evaluates its 2 arguments. To get usual assignment like behavior of normal programming languages, one (typically) quotes the first argument. This is a sufficiently common case that it gets its own 'special form' -- setq (ie set-quote) However the more general case in which (the name of)* the variable is evaluated at run-time is always available. * "Name of" is strictly not correct because: ELISP> (symbol-name 'a) "a" However if I dont say the "name of..." its hard to speak in an intelligible way. Here is the relevant intro from the elisp manual: A "symbol" in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary Lisp use, ... a symbol's name is unique--no two symbols have the same name. A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. tl;dr: Quote-Eval go up and down the 'meta-language' tower just as Lambda-(function)Apply go up and down the functional tower. Elaborated further: http://blog.languager.org/2013/08/applying-si-on-sicp.html -- https://mail.python.org/mailman/listinfo/python-list
