Re: Calling a function with undefined symbol

[Jean Abou Samra]

> Le 31 oct. 2022 à 10:45, Heime <heimeborgia@protonmail.com> a écrit :
> 
> 
> ------- Original Message -------
> On Monday, October 31st, 2022 at 9:24 AM, Jean Abou Samra 
> <jean@abou-samra.fr> wrote:
> 
> 
>>>> Le 31 oct. 2022 à 09:59, Heime heimeborgia@protonmail.com a écrit :
>>> 
>>> Yes, I have got really confused about what symbols are exactly. Even
>>> after reading several times. I have never seen anybody make a symbol,
>>> only variables. So, what is a symbol?
>> 
>> 
>> 
>> 
>> Imagine parents who are planning to have a child and decide they will call 
>> it, say Deborah. They will say things like “I like the name Deborah better 
>> than Mary”. Those are sentences about the names themselves, not some 
>> children. The names don’t need children in order to exist as words. Then, 
>> the child is born and sentences like “Deborah weighs 3kg” (referring to the 
>> child named Deborah) start to make sense.
>> 
>> When you do 'symbol in Lisp, this gives you a bare name, or a “symbol”. With 
>> symbol-value, you get what value is associated with this symbol, if any (the 
>> child with that name). But you don’t need a value bound to the symbol (a 
>> child called Deborah) before you start using the symbol itself (the name 
>> “Deborah”).
> 
> The Lisp manual says that a symbol is an object with a name.


This can be interpreted as the correct definition, but also as a wrong 
definition where a symbol is necessarily associated with a defined variable. 
Better said: a symbol is an object that represents a name.


> Then a variable (setq thevar 4) has an associated symbol 'thevar.
> And the function "(defun thefun ()" also has an associated symbol "'thefun".  
> Furthermore one can make just a symbol, let us say 'go,
> which can be passed as an argument to a function "(defun mbcomplt (arg)" 
> using (mbcomplt 'go).


Yes. The symbol is an object like any other (5, "foo", whatever), so what you 
can do with an object, you can do with a symbol, including passing it as an 
argument to a function.


> Then inside the function on can test
> whether the symbol exists with (eq arg 'go).


That is not testing whether the symbol “exists”. It is testing whether arg is 
the symbol 'go.


>  Even though we only have a name without a values.  Is this a good basic 
> understanding 
> of a symbol?


Apart from the last part, yes.