GNU bug report logs - #70597
Problem in pcase-let?

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Message #58 received at 70597-done <at> debbugs.gnu.org (full text, mbox):

From: Marco Antoniotti <marcoxa <at> gmail.com>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: brubar.cs <at> gmail.com, Eli Zaretskii <eliz <at> gnu.org>,
 70597-done <at> debbugs.gnu.org
Subject: Re: bug#70597: Problem in pcase-let?
Date: Sun, 5 May 2024 10:49:33 +0200

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Hi Stefan,

i usually just to C-h f.  This is what you get from it, in 29.3.

Like `let', but supports destructuring BINDINGS using `pcase' patterns.
BODY should be a list of expressions, and BINDINGS should be a list of
bindings of the form (PATTERN EXP).
All EXPs are evaluated first, and then used to perform destructuring
bindings by matching each EXP against its respective PATTERN.  Then
BODY is evaluated with those bindings in effect.

Each EXP should match its respective PATTERN (i.e. be of structure
compatible to PATTERN); a mismatch may signal an error or may go
undetected, binding variables to arbitrary values, such as nil.

  Probably introduced at or before Emacs version 28.1.

The Info doc you posted is definitively better.

All the best

MA



On Sat, May 4, 2024 at 5:05 PM Stefan Monnier <monnier <at> iro.umontreal.ca>
wrote:

> > sorry, but I do not think that the current doc string and/or the
> > manual explains the situation.
>
> Which "current" are you referring to?  The "current" manual (in
> `master`) has a whole section on "Destructuring with pcase pattern"
> (see below).
>
> Do you still see room for misunderstanding there?
> If so, can you suggest how to change it to make it more clear?
>
>
>         Stefan
>
>
> @node Destructuring with pcase Patterns
> @subsection Destructuring with @code{pcase} Patterns
> @cindex destructuring with pcase patterns
>
> Pcase patterns not only express a condition on the form of the objects
> they can match, but they can also extract sub-fields of those objects.
> For example we can extract 2 elements from a list that is the value of
> the variable @code{my-list} with the following code:
>
> @example
>   (pcase my-list
>     (`(add ,x ,y)  (message "Contains %S and %S" x y)))
> @end example
>
> This will not only extract @code{x} and @code{y} but will additionally
> test that @code{my-list} is a list containing exactly 3 elements and
> whose first element is the symbol @code{add}.  If any of those tests
> fail, @code{pcase} will immediately return @code{nil} without calling
> @code{message}.
>
> Extraction of multiple values stored in an object is known as
> @dfn{destructuring}.  Using @code{pcase} patterns allows you to
> perform @dfn{destructuring binding}, which is similar to a local
> binding (@pxref{Local Variables}), but gives values to multiple
> elements of a variable by extracting those values from an object of
> compatible structure.
>
> The macros described in this section use @code{pcase} patterns to
> perform destructuring binding.  The condition of the object to be of
> compatible structure means that the object must match the pattern,
> because only then the object's subfields can be extracted.  For
> example:
>
> @example
>   (pcase-let ((`(add ,x ,y) my-list))
>     (message "Contains %S and %S" x y))
> @end example
>
> @noindent
> does the same as the previous example, except that it directly tries
> to extract @code{x} and @code{y} from @code{my-list} without first
> verifying if @code{my-list} is a list which has the right number of
> elements and has @code{add} as its first element.  The precise
> behavior when the object does not actually match the pattern is
> undefined, although the body will not be silently skipped: either an
> error is signaled or the body is run with some of the variables
> potentially bound to arbitrary values like @code{nil}.
>
> The pcase patterns that are useful for destructuring bindings are
> generally those described in @ref{Backquote Patterns}, since they
> express a specification of the structure of objects that will match.
>
> For an alternative facility for destructuring binding, see
> @ref{seq-let}.
>
> @defmac pcase-let bindings body <at> dots{}
> Perform destructuring binding of variables according to
> @var{bindings}, and then evaluate @var{body}.
>
> @var{bindings} is a list of bindings of the form @w{@code{(@var{pattern}
> @var{exp})}}, where @var{exp} is an expression to evaluate and
> @var{pattern} is a @code{pcase} pattern.
>
> All @var{exp}s are evaluated first, after which they are matched
> against their respective @var{pattern}, introducing new variable
> bindings that can then be used inside @var{body}.  The variable
> bindings are produced by destructuring binding of elements of
> @var{pattern} to the values of the corresponding elements of the
> evaluated @var{exp}.
>
> Here's a trivial example:
>
> @example
> (pcase-let ((`(,major ,minor)
>              (split-string "image/png" "/")))
>   minor)
>      @result{} "png"
> @end example
> @end defmac
>
> @defmac pcase-let* bindings body <at> dots{}
> Perform destructuring binding of variables according to
> @var{bindings}, and then evaluate @var{body}.
>
> @var{bindings} is a list of bindings of the form @code{(@var{pattern}
> @var{exp})}, where @var{exp} is an expression to evaluate and
> @var{pattern} is a @code{pcase} pattern.  The variable bindings are
> produced by destructuring binding of elements of @var{pattern} to the
> values of the corresponding elements of the evaluated @var{exp}.
>
> Unlike @code{pcase-let}, but similarly to @code{let*}, each @var{exp}
> is matched against its corresponding @var{pattern} before processing
> the next element of @var{bindings}, so the variable bindings
> introduced in each one of the @var{bindings} are available in the
> @var{exp}s of the @var{bindings} that follow it, additionally to
> being available in @var{body}.
> @end defmac
>
> @defmac pcase-dolist (pattern list) body <at> dots{}
> Execute @var{body} once for each element of @var{list}, on each
> iteration performing a destructuring binding of variables in
> @var{pattern} to the values of the corresponding subfields of the
> element of @var{list}.  The bindings are performed as if by
> @code{pcase-let}.  When @var{pattern} is a simple variable, this ends
> up being equivalent to @code{dolist} (@pxref{Iteration}).
> @end defmac
>
> @defmac pcase-setq pattern value <at> dots{}
> Assign values to variables in a @code{setq} form, destructuring each
> @var{value} according to its respective @var{pattern}.
> @end defmac
>
> @defmac pcase-lambda lambda-list &rest body
> This is like @code{lambda}, but allows each argument to be a pattern.
> For instance, here's a simple function that takes a cons cell as the
> argument:
>
> @example
> (setq fun
>       (pcase-lambda (`(,key . ,val))
>         (vector key (* val 10))))
> (funcall fun '(foo . 2))
>     @result{} [foo 20]
> @end example
> @end defmac
>
>
>

-- 
Marco Antoniotti
Somewhere over the Rainbow

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