Subject: bug#47427: 26.3; 1. Please define a built-in predicate `plistp', 2. wrong type wrong-type-argument error

From: Lars Ingebrigtsen <[email-masked]>
Date: Sun, 28 Mar 2021 14:50:21 +0200

Drew Adams <[email-masked]> writes:

>  (plist-put (list 'a 'b 'c) "a" 42)
>
> Debugger entered--Lisp error: (wrong-type-argument plistp (a b c))
>   plist-put((a b c) "abc" 42)
>   eval((plist-put (list (quote a) (quote b) (quote c)) "abc" 42))
>
> That's all fine and dandy, except that there is no predicate `plistp'.

The backtrace there doesn't seem to be a result of the example form,
but, yes, the error here isn't very good.  Adding a `plistp' predicate
would perhaps make sense, but it would just be

(and (listp list)
     (zerop (mod (length list) 2)))

and then we have the philosophical issue of "is nil a plist"?  Does
anybody have any opinions?

> Not only that, but the error is _not_, apparently that the first
> arg isn't a proper plist.  For example, this raises no error:
>
>  (plist-put (list 'a 'b 'c) "a" 42)
>
> And it returns the list (a 42 c).  Clearly the error was raised
> not because of an improper plist but because the key to look up
> is a string and the keys in the almost-plist are symbols.

Here you probably meant to say:

(plist-put (list 'a 'b 'c) 'a 42)

And that does indeed not result in any errors, but it's not because of
the stringiness of anything, but because 'a exists in the list, and
plist-put doesn't check whether the list is a plist in that case.  Only
when adding new elements does it check:

(plist-put (list 'a 'b 'c) 'd 42)

This signals an error.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no