Re: (*) -> 1

[Jean Louis]

* Tassilo Horn <tsdh@gnu.org> [2023-01-20 16:14]:
> Jean Louis <bugs@gnu.support> writes:
> 
> >> Yes, and I think it's seriously wrong with
> >> 
> >>   : (+)
> >>   -> NIL
> >> 
> >> where its docs say
> >> 
> >>   Returns the sum of all num arguments. When one of the arguments
> >>   evaluates to NIL, it is returned immediately.
> >
> > For some reason PicoLisp is quite different than other Lisp. I have
> > asked author about it.
> >
> > 15:09 <abu[m]> It is a "feature" that NIL propagates through
> >                arithmetics
> 
> Well, but with (*) and (+), there is no single NIL involved!  And in
> Elisp (+ nil), where actually a nil is involved, you get an error.

What we can learn from PicoLisp is that there was no use for (*) ➜ 1
and that programs work, GUI applications and Android/Replicant work,
and there was no use of (*) ➜ 1 so far.

Similarly like author of PicoLisp finding use when (*) ➜ NIL, so I
would also find (very rare) use with errors. Even if NIL, I find use
as I will get error raised in combinations like (+ (*) (* 2 2)),
because (+ nil 4) would raise error.

> > 15:09 <jmarciano> How does it help instead of providing identity
> >                   elements?
> > 15:09 <abu[m]> (*) especially was not contemplated though, it is a
> >                pretty useless call
> > [...]
> > 15:12 <abu[m]> What is a call like (*) useful for?
> >
> > As you see, author also asked naturally why is it useful.
> 
> So go and ask why he thinks (apply '+ ()) -> NIL is more useful than 0
> given that the sum of the empty set of numbers _is_ 0.

I will ask.

But docstring does not speak of empty sets. You introduce "sets" where
there is not direct relation to it.

(+ &rest NUMBERS-OR-MARKERS)

Return sum of any number of arguments, which are numbers or
markers. Of course I get confused.

I ask A, but people say B. 
I ask A, but people say C.
I ask A, but people say D.

No answer about A. But there are many introductions of things not
relevant to function itself. 

I still believe that there is some actual practical use.

`apply' can be used with (apply '+ '(a b)) as why would you need in
apply for addition two arguments?

If list is with one argument, testing with `cadr' will be known,
otherwise, I use `car' instead of `apply'.

> >> So why does it return NIL?  And why do you apparently consider that
> >> useful?  And can something be useful even though it is incorrect?
> >
> > I find it right as with error raising or nil I can find what is
> > wrong.
> 
> It's good to signal an error when the expression is wrong as does Elisp
> with
> 
>   (+ nil)
>   (* 1 2 nil)
>   (apply #'+ (list 1 nil 19))
>   (+ 2 "i am not a number")

That is exactly my point, what you see useful there, I see too. Making
it less error prone with useless default identity elements hides the
real event preceding the operation.

--
Jean

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