From: Michael Heerdegen <michael_heerdegen@web.de>
To: Eli Zaretskii <eliz@gnu.org>
Cc: John Wiegley <jwiegley@gmail.com>,
Emacs Development <emacs-devel@gnu.org>
Subject: Re: emacs-25 1d4887a: Improve documentation of 'pcase'
Date: Sat, 23 Jan 2016 12:38:34 +0100 [thread overview]
Message-ID: <87vb6kzft1.fsf@web.de> (raw)
In-Reply-To: <E1aMvLr-00060z-TI@vcs.savannah.gnu.org> (Eli Zaretskii's message of "Sat, 23 Jan 2016 10:23:28 +0000")
Hi Eli,
thanks for finalizing this stuff.
Some comments:
> -To compare a particular value against various possible cases, the macro
> -@code{pcase} can come handy. It takes the following form:
> +The @code{cond} form lets you choose between alternatives using
> +predicate conditions that compare values of expressions against
> +specific values known and written in advance. However, sometimes it
> +is useful to select alternatives based on more general conditions that
> +distinguish between broad classes of values. The @code{pcase} macro
> +allows to choose between alternatives based on matching the value of
> +an expression against a series of patterns. A pattern can be a
> +literal value (comparison to literal values is what @code{cond}
> does),
That does sound more as a description of `cl-case' -- typo?
> +@defmac pcase expression &rest clauses
> +Evaluate @var{expression} and choose among an arbitrary number of
> +alternatives based on the value of @var{expression}. The possible
> +alternatives are specified by @var{clauses}, each of which must be a
> +list of the form @code{(@var{pattern} @var{body-forms})}.
I think we should write @code{(@var{pattern} . @var{body-forms})}
^
if we mean that BODY-FORMS is a list, or use an ellipsis: "...", as you
do later.
> +The @var{pattern} part of a clause can be of one of two types:
> +@dfn{QPattern}, a pattern quoted with a backquote; or a
> +@dfn{UPattern}, which is not quoted. UPatterns are simpler, so we
> +describe them first.
I had hoped we can get rid of the QPattern/Upattern distinction. Is it
too late to change that? In particular, we wanted to speak of just
patterns instead of Upatterns.
> +@item '@var{val}
> +Matches if the value being matched is @code{equal} to @var{val}.
> +@item @var{atom}
> +Matches any @var{atom}, which can be a keyword, a number, or a string.
> +(These are self-quoting, so this kind of UPattern is actually a
> +shorthand for @code{'@var{atom}}.)
Can we say "matches any (equal) atom"? This makes a difference for
strings.
And actually, this is not true for floats, only for integers (I'm not
sure why).
> +Matches if @var{boolean-expression} evaluates to non-@code{nil}. This
> +allows to include in a UPattern boolean conditions that refer to
> +symbols bound to values (including the value being matched) by
> +previous UPatterns. Typically used inside an @code{and} UPattern, see
> +below. For example, @w{@code{(and x (guard (< x 10)))}} is a pattern
> +which matches any number smaller than 10 and let-binds the variable
> +@code{x} to that number.
Maybe we should use
@w{@code{(and x (pred numberp) (guard (< x 10)))}}
instead in the example, because without the numberp test, the pattern
will raise an error if x is not bound to a number.
> +@table @code
> +@item `(@var{qpattern1} . @var{qpattern2})
> +Matches if the value being matched is a cons cell whose @code{car}
> +matches @var{qpattern1} and whose @code{cdr} matches @var{qpattern2}.
> +@item `[@var{qpattern1} @var{qpattern2} @dots{} @var{qpatternm}]
> +Matches if the value being matched is a vector of length @var{m} whose
> +@code{0}..@code{(@var{m}-1)}th elements match @var{qpattern1},
> +@var{qpattern2} @dots{} @var{qpatternm}, respectively.
> +@item `(,@var{upattern1} ,@var{upattern2} @dots{})
> +Matches if the value being matched is a list whose elements match the
> +corresponding @var{upattern1}, @var{upattern2}, etc.
> +@item @var{atom}
> +Matches if corresponding element of the value being matched is
> +@code{equal} to the specified @var{atom}.
> +@item ,@var{upattern}
> +Matches if the corresponding element of the value being matched
> +matches the specified @var{upattern}.
Please decide if you include the backquote in all examples, or in none.
The thing we name "qpattern" is without backquote, so with the current
wording, I would leave the backquote out.
And these templates are not covering everything possible, e.g. you can
also have
`(,up1 . ,up2)
or
`(,up qp1)
I think I would reorganize that paragraph.
Many, many thanks so far.
Regards,
Michael.
next parent reply other threads:[~2016-01-23 11:38 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20160123102327.23087.15367@vcs.savannah.gnu.org>
[not found] ` <E1aMvLr-00060z-TI@vcs.savannah.gnu.org>
2016-01-23 11:38 ` Michael Heerdegen [this message]
2016-01-23 13:27 ` emacs-25 1d4887a: Improve documentation of 'pcase' Eli Zaretskii
2016-01-25 13:49 ` Michael Heerdegen
2016-01-25 14:36 ` Stefan Monnier
2016-01-25 15:29 ` Michael Heerdegen
2016-01-25 15:56 ` Drew Adams
2016-01-25 16:10 ` Michael Heerdegen
2016-01-25 16:48 ` Drew Adams
2016-01-25 16:15 ` Stefan Monnier
2016-01-25 16:23 ` Eli Zaretskii
2016-01-25 16:43 ` Michael Heerdegen
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