Re: My possibly wrong use of apply
Hi Christophe, # In order to call xml with the default args and then the particular args, # decided to use apply: (apply xml (quote text text-anchor middle font-size 24 x 120 y 99 Yes, this is not possible. 'apply' can only be used for functions which evaluate their arguments (EXPRs and SUBRs, but not FEXPRs). And 'xml' is an FEXPR: (de xml Lst ... ) The single 'Lst' parameter indicates that the arguments are not evaluated. To understand why 'apply' cannot be used with non-evaluating functions, recall how 'apply' works: It does some kind of internal quoting to the arguments, to _suppress_ the evaluation a function normally does. For example: (setq A 1 B 2 C 3) (apply println '(A B C)) This should print A B C and _not_ 1 2 3 Thus, it is equivalent to (println 'A 'B 'C) and not (println A B C) To achieve that, 'apply' passes the elements in the list (A B C) to some internal structure, which quotes them before calling the function 'println'. 'println' in turn evaluates the arguments, and gets the symbols A, B and C as expected. If - instead of 'println' - a function is called which does not evaluate its arguments, the results will be undefined (implementation-specific). ♪♫ Alex -- UNSUBSCRIBE: mailto:picolisp@software-lab.de?subject=Unsubscribe
Re: My possibly wrong use of apply
On Tue, Jul 8, 2014 at 10:38 AM, Alexander Burger a...@software-lab.de wrote: Hi Christophe, Hi Alex, thanks for this prompt answer. # In order to call xml with the default args and then the particular args, # decided to use apply: (apply xml (quote text text-anchor middle font-size 24 x 120 y 99 Yes, this is not possible. 'apply' can only be used for functions which evaluate their arguments (EXPRs and SUBRs, but not FEXPRs). And 'xml' is an FEXPR: Ouch. For example: (setq A 1 B 2 C 3) (apply println '(A B C)) This should print A B C and _not_ 1 2 3 Thus, it is equivalent to (println 'A 'B 'C) and not (println A B C) Even if I did not face this problem, this is counter-intuitive to me. I guess that there are serious reasons for this behaviour. This may be nice examples for the docs! But I'll understand that you don't want to clutter those pages. If - instead of 'println' - a function is called which does not evaluate its arguments, the results will be undefined (implementation-specific). OK, so what are my options? I'd say: 1) try to write a king of «apply» that would work for me, 2) rewrite xml so that it evaluates its args, then quote the args in my calls to it. Any suggestion? chri -- http://profgra.org/lycee/ (site pro) http://delicious.com/profgraorg (liens, favoris) https://twitter.com/profgraorg -- UNSUBSCRIBE: mailto:picolisp@software-lab.de?subject=Unsubscribe
Re: My possibly wrong use of apply
Hi Christophe, 2) rewrite xml so that it evaluates its args, then quote the args in my calls to it. Exactly! I would do: (de xml (Lst) # See also lib/xm.l and lib/xml.l ... ) and then provide also a quoting function (de xmlq Lst (xml Lst) ) giving a convenient frontend function. another option is calling eval like this: : (eval (cons 'xml X)) where X is the cons tree describing the XML. Of course, you still need to build the X cons tree somehow. Cheers, Tomas -- UNSUBSCRIBE: mailto:picolisp@software-lab.de?subject=Unsubscribe
