Re: Determine object passed as argument

Heime <[email protected]> writes:

> On Sunday, April 12th, 2026 at 2:38 AM, [email protected] <[email protected]> wrote:
>
>> [email protected] writes:
>> 
>> ...
>> Or,
>> 
>>   (defun lana-du-fpln (datenstruk &optional bname)
>>     (if (cl-typep datenstruk 'symbol)
>>       (symbol-name datenstruk)
>>       ;; Else
>>       "Not a symbol"))
>> 
>>   (lana-du-fpln 'fpln)
>>   => "fpln"
>>   (lana-du-fpln '(a b c))
>>   => "Not a symbol"
>>   (lana-du-fpln "abc")
>>   => "Not a symbol"
>
> No need for boundp as outlines by Jean?
>

1. ‘boundp’ is not needed within the function definition.
The parameter ‘datenstruk’ is definitely bound to the value
that was determined when the function *call* was evaluated:

   a. (lana-du-fpln <some-expression>)
      <some-expression> is evaluated to determine a value.
   b. (lana-du-fpln <its value>)
      The function ‘lana-du-fpln’ is called with the
      argument <its value>.
   c. Within ‘lana-du-fpln’, <its value> is bound to
      ‘datenstruk’.

Where it would be a problem (that is, need to check using
‘boundp’) is if you were using one level of indirection --
if <its value> was itself a symbol from which you wanted
*indirectly* to retrieve its value.

2. In step a, above, if <some-expression> was the name of a
variable which you had not yet bound to some value, then you
might need to use ‘boundp’ there.

   ;; No guard
   (lana-du-fpln needs-a-value)
   => Debugger entered--Lisp error: (void-variable needs-a-value)

   ;; Add a guard
   (when (boundp 'needs-a-value)
     (lana-du-fpln needs-a-value))
   => nil

   ;; No guard needed
   (let (needs-a-value)
     ;; ‘let’ binds ‘needs-a-value’ to nil, by default
     (lana-du-fpln needs-a-value))
   => "nil"
   (symbolp nil)
   => t

For more explanation, see David S. Touretzky’s textbook
(with exercises and answers) "COMMON LISP: A Gentle
Introduction to Symbolic Computation", Chapter 3, where he
introduces his "evaltrace diagram".  Although this is a book
on Common Lisp, almost all of the book applies as well to
Emacs Lisp (although you may need to use Emacs’s ‘cl-lib’
for some expressions).

   https://www.cs.cmu.edu/~dst/LispBook/book.pdf

-- 
The lyf so short, the craft so long to lerne.
- Geoffrey Chaucer, The Parliament of Birds.