Re: Instead of pcase

[Michael Heerdegen via "Emacs development discussions." <emacs-devel@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

> > This is not a valid expression and will be rejected by the reader.
> > Was it a typo or did you think this would be valid?
>
> It was taken from one of the posted examples, perhaps with a typo.
> But that is not relevant.

My question was meant sincerely.  Did you try to understand the syntax?
How?  What was or is still unclear to you?

> Are you saying pcase doesn't have
> constructs like (`(SOMETHING...), or that SOMETHING doesn't use
> periods in places where they are redundant in "normal" ELisp code?

Redundant - no.  May there be different redundant ways to express lists
- yes.

If you use a backquote pattern `SOMETHING in pcase, when SOMETHING
should match a cons (lists are conses), then SOMETHING will typically be
a cons.  The dot reader syntax is the canonical read syntax for conses.
This is not that weird.

> IOW, these details are not the point, the point is something else
> entirely, see below [...]

> My point focuses on the problems this syntax causes to someone who
> doesn't have intimate knowledge and a lot of practice with pcase.

It is repeatedly used a wording like "magical", as if that was an
argument.  Ok, this syntax makes one first wonder how it has to be
interpreted.  Happened to me, too.  But this (the wondering) takes only
seconds.  What happens after that?  Have people then seriously tried to
understand the syntax?  Where and why did they fail?  I heard nothing
about that.  Just a repetition that it looks "strange" or "magical" or
that one would expect a different syntax.

Given that cons cells and lists are fundamental building blocks of the
whole language, I had hoped to hear more about why understanding this
would be hard.  Nobody did that concretely apart from Richard.  And it
might be the case that his rewrite will reuse the backquote syntax.

> > As an alternative one can have a look at the docstring.
>
> It is a disadvantage if one needs to consult doc strings all the time
> while reading code.  And in this case, the doc string is quite long
> and highly non-trivial (and rightfully so, since the DSL is a rather
> complex one).

This is, or should, not be the case.  Sometimes one might need to recall
the argument order of 'let' or 'app', but more when writing code.

I don't ever need to consult the documentation of `pcase' when reading
code, apart from the case when additional pcase macros are involved
(like 'map').  And I don't have a good such a good memory for such
things.

And I use it only in rare circumstances, where possible I prefer `if',
`cond', and `if-let'.

> If one bumps into such constructs very frequently, then eventually one
> will become familiarized with them.  But if the frequency is low
> enough, one could easily forget the details, and then one will need to
> consult the documentation again the next time.

What I want to say is that if you need to consult documentation of
backquote syntax in pcase again and again, you might not have understood
it entirely.  This is my question: do people fail to understand the
concept, or don't they even try?

> Please don't underestimate these difficulties, even if you don't share
> them.

I still try to understand them.


Michael.