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Re: A macro and an unwanted containing list in the resulting form


From: Tim X
Subject: Re: A macro and an unwanted containing list in the resulting form
Date: Thu, 24 May 2007 18:47:50 +1000
User-agent: Gnus/5.11 (Gnus v5.11) Emacs/22.1.50 (gnu/linux)

Sebastian Tennant <address@hidden> writes:

> Quoth Pascal Bourguignon <address@hidden>:
>> Sebastian Tennant <address@hidden> writes:
>>>> Is there any reason to make the argument of build-cond an alist? You
>>>>could try
>>>>
>>>> (defmacro build-cond (&rest conds)
>>>>  (append '(cond)
>>>>          (mapcar '(lambda (each)
>>>>                    (cons (list 'equal 'my-var (car each)) (list (cdr 
>>>> each))))
>>>>                  conds)))
>>>>
>>>> and then use
>>>>
>>>>  (build-cond ("hello" . (message "hi"))
>>>>              ("goodbye" . (message "bye"))
>>>
>>> The reason for the alist is the clauses are being passed as one of a
>>> number of arguments to a function call.
>>
>> If you get the a-list as argument to a function, then you don't need a
>> macro to process it!   Just write a loop!
>>
>> (require 'cl)
>>
>> (defun my-function (string clauses)
>>   (loop
>>       for clause in clauses
>>       until (string= string (car clause))
>>       finally (eval (cdr clause))))
>>
>> (my-function "goodbye" '(("hello"   . (message "hi"))
>>                          ("goodbye" . (message "bye"))))
>>
>
> Whoa!  This isn't LISP!  At least it doesn't look like it to me.
> Where are the parentheses?  I suppose I haven't got the first idea how
> the Common Lisp 'loop' function works...???
>
> Sebastian
>
You aren't the first to make those comments. CL's loop is one area that
generally creates considerable debate - som argue its not lispy and is
difficult to master. Others argue its a very powerful construct that shows what
you can do with CL. 

I'm still pretty much a novice with CL and will admit that loop and format (the
CL version) are two aspects of CL that took considerably more mental effort to
understand. I now now reasonably confident with using format, though I
constantly have to check the references. Loop on the other hand is still a
concept I get amazed by and one I have a lot more work to do with before I can
honestly say I'm confident with. However, like most of the stuff I've done with
CL, its been one of the most rewarding languages I've learnt for many many
years. I also find the different development style of working in CL extremely
rewarding after years of C, C++, Java, Tcl and Perl. I now find I've gotten to
the point that its productive for me to 'explore' a problem with CL before I
start implementing the solution in another (more readily accepted) language.
This process often exposes subtle issue that are not evident when working on a
solution from an abstract level and I'm finding it saves me time refactoring or
redesigning things to accomadate the increased understanding that occurs as you
work on a problem. 

Tim


-- 
tcross (at) rapttech dot com dot au


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