coding standard for allowed arguments of predicates

coding standard for allowed arguments of predicates

[Roland Winkler]

Is there a coding standard for allowed arguments of predicates?
I have always assumed that predicates should accept any type of
argument;  but I cannot find this documented anywhere.

The context of my question is `iso8601-valid-p'.  It assumes that
its argument must be a string.  Is this a bug?  In my usage example,
the argument of `iso8601-valid-p' could also be nil.

If, indeed, predicates should always accept any type of argument,
I suggest to document this in the elisp manual.

Re: coding standard for allowed arguments of predicates

[Philipp Stephani]

Am Fr., 18. Dez. 2020 um 15:49 Uhr schrieb Roland Winkler <winkler@gnu.org>:
>
> Is there a coding standard for allowed arguments of predicates?
> I have always assumed that predicates should accept any type of
> argument;  but I cannot find this documented anywhere.
>
> The context of my question is `iso8601-valid-p'.  It assumes that
> its argument must be a string.  Is this a bug?  In my usage example,
> the argument of `iso8601-valid-p' could also be nil.
>
> If, indeed, predicates should always accept any type of argument,
> I suggest to document this in the elisp manual.
>

There are lots of predicates that accept only certain types, e.g.
(file-readable-p 123) also signals an error. Typically only the
predicates that check for types (stringp, integerp, listp, etc.)
accept any type.

Re: coding standard for allowed arguments of predicates

[Roland Winkler]

On Fri Dec 18 2020 Philipp Stephani wrote:
> Am Fr., 18. Dez. 2020 um 15:49 Uhr schrieb Roland Winkler <winkler@gnu.org>:
> > The context of my question is `iso8601-valid-p'.  It assumes that
> > its argument must be a string.  Is this a bug?  In my usage example,
> > the argument of `iso8601-valid-p' could also be nil.
> 
> There are lots of predicates that accept only certain types, e.g.
> (file-readable-p 123) also signals an error. Typically only the
> predicates that check for types (stringp, integerp, listp, etc.)
> accept any type.

Thanks; thinking about this once more it makes sense that not all
predicates need to check the type of their arguments.  In the
example of `iso8601-valid-p' one could argue that it is at the
borderline of the class of predicates that check for (in this case:
very particular) types.  But a caller of this predicate can handle
the possibility of non-string arguments, too.