Improve documentation of destructuring-binding macros

* lisp/emacs-lisp/pcase.el (pcase-dolist, pcase-let)
(pcase-let*): Improve the doc strings.

* doc/lispref/sequences.texi (Sequence Functions): Improve
wording and rename arguments of seq-let to be more
descriptive.  Add a cross-reference to "Destructuring with
pcase Patterns".
* doc/lispref/control.texi (Pattern-Matching Conditional):
Improve wording and the menu.
(pcase Macro): Incorporate patch suggested by Paul Eggert
<eggert@cs.ucla.edu>.  Reformat text.
(Destructuring with pcase Patterns): Rename from
"Destructuring patterns", and improve wording and indexing.

doc/lispref/control.texi

@@ -419,65 +419,68 @@ This is not completely equivalent because it can evaluate @var{arg1} or
 @node Pattern-Matching Conditional
 @section Pattern-Matching Conditional
 @cindex pcase
-@cindex pattern matching
+@cindex pattern matching, programming style
 
 Aside from the four basic conditional forms, Emacs Lisp also
 has a pattern-matching conditional form, the @code{pcase} macro,
 a hybrid of @code{cond} and @code{cl-case}
 (@pxref{Conditionals,,,cl,Common Lisp Extensions})
 that overcomes their limitations and introduces
-the @dfn{pattern matching} programming style.
+the @dfn{pattern matching programming style}.
-First, the limitations:
+The limitations that @code{pcase} overcomes are:
 
 @itemize
-@item The @code{cond} form chooses among alternatives
+@item
-by evaluating the predicate @var{condition} of each
+The @code{cond} form chooses among alternatives by evaluating the
-of its clauses (@pxref{Conditionals}).
+predicate @var{condition} of each of its clauses
-The primary limitation is that variables let-bound in @var{condition}
+(@pxref{Conditionals}).  The primary limitation is that variables
-are not available to the clause's @var{body-forms}.
+let-bound in @var{condition} are not available to the clause's
+@var{body-forms}.
 
 Another annoyance (more an inconvenience than a limitation)
 is that when a series of @var{condition} predicates implement
-equality tests, there is a lot of repeated code.
+equality tests, there is a lot of repeated code.  (@code{cl-case}
-For that, why not use @code{cl-case}?
+solves this inconvenience.)
 
 @item
 The @code{cl-case} macro chooses among alternatives by evaluating
 the equality of its first argument against a set of specific
 values.
-The limitations are two-fold:
+
+Its limitations are two-fold:
 
 @enumerate
-@item The equality tests use @code{eql}.
+@item
-@item The values must be known and written in advance.
+The equality tests use @code{eql}.
+@item
+The values must be known and written in advance.
 @end enumerate
 
 @noindent
 These render @code{cl-case} unsuitable for strings or compound
-data structures (e.g., lists or vectors).
+data structures (e.g., lists or vectors).  (@code{cond} doesn't have
-For that, why not use @code{cond}?
+these limitations, but it has others, see above.)
-(And here we end up in a circle.)
 @end itemize
 
 @noindent
 Conceptually, the @code{pcase} macro borrows the first-arg focus
 of @code{cl-case} and the clause-processing flow of @code{cond},
 replacing @var{condition} with a generalization of
-the equality test called @dfn{matching},
+the equality test which is a variant of @dfn{pattern matching},
 and adding facilities so that you can concisely express a
 clause's predicate, and arrange to share let-bindings between
 a clause's predicate and @var{body-forms}.
 
 The concise expression of a predicate is known as a @dfn{pattern}.
-When the predicate, called on the value of the first arg,
+When the predicate, called on the value of the first arg, returns
-returns non-@code{nil}, the pattern matches the value
+non-@code{nil}, we say that ``the pattern matches the value'' (or
-(or sometimes ``the value matches the pattern'').
+sometimes ``the value matches the pattern'').
 
 @menu
-* The @code{pcase} macro: pcase Macro.  Plus examples and caveats.
+* The @code{pcase} macro: pcase Macro.  Includes examples and caveats.
 * Extending @code{pcase}: Extending pcase.  Define new kinds of patterns.
-* Backquote-Style Patterns: Backquote Patterns.  Structural matching.
+* Backquote-Style Patterns: Backquote Patterns.  Structural patterns matching.
-* Destructuring patterns:: Using pcase patterns to extract subfields.
+* Destructuring with pcase Patterns:: Using pcase patterns to extract subfields.
 @end menu
 
 @node pcase Macro
@@ -498,30 +501,30 @@ of the last of @var{body-forms} in the successful clause.
 Otherwise, @code{pcase} evaluates to @code{nil}.
 @end defmac
 
-Each @var{pattern} has to be a @dfn{pcase pattern}, which can either
+@cindex pcase pattern
-use one of the core patterns defined below, or use one of the patterns
+Each @var{pattern} has to be a @dfn{pcase pattern}, which can use
-defined via @code{pcase-defmacro}.
+either one of the core patterns defined below, or one of the patterns
+defined via @code{pcase-defmacro} (@pxref{Extending pcase}).
 
-The rest of this subsection
+The rest of this subsection describes different forms of core
-describes different forms of core patterns,
+patterns, presents some examples, and concludes with important caveats
-presents some examples,
+on using the let-binding facility provided by some pattern forms.  A
-and concludes with important caveats on using the
+core pattern can have the following forms:
-let-binding facility provided by some pattern forms.
-A core pattern can have the following forms:
 
 @table @code
 
 @item _
 Matches any @var{expval}.
-This is known as @dfn{don't care} or @dfn{wildcard}.
+This is also known as @dfn{don't care} or @dfn{wildcard}.
 
 @item '@var{val}
-Matches if @var{expval} is @code{equal} to @var{val}.
+Matches if @var{expval} is equals @var{val}.  The comparison is done
+as if by @code{equal} (@pxref{Equality Predicates}).
 
 @item @var{keyword}
 @itemx @var{integer}
 @itemx @var{string}
-Matches if @var{expval} is @code{equal} to the literal object.
+Matches if @var{expval} equals the literal object.
 This is a special case of @code{'@var{val}}, above,
 possible because literal objects of these types are self-quoting.
 
@@ -533,17 +536,17 @@ Matches any @var{expval}, and additionally let-binds @var{symbol} to
 If @var{symbol} is part of a sequencing pattern @var{seqpat}
 (e.g., by using @code{and}, below), the binding is also available to
 the portion of @var{seqpat} following the appearance of @var{symbol}.
-This usage has some caveats (@pxref{pcase-symbol-caveats,,caveats}).
+This usage has some caveats, see @ref{pcase-symbol-caveats,,caveats}.
 
 Two symbols to avoid are @code{t}, which behaves like @code{_}
-(above) and is deprecated, and @code{nil}, which signals error.
+(above) and is deprecated, and @code{nil}, which signals an error.
 Likewise, it makes no sense to bind keyword symbols
 (@pxref{Constant Variables}).
 
 @item (pred @var{function})
 Matches if the predicate @var{function} returns non-@code{nil}
 when called on @var{expval}.
-@var{function} can have one of the possible forms:
+the predicate @var{function} can have one of the following forms:
 
 @table @asis
 @item function name (a symbol)
@@ -570,20 +573,17 @@ the actual function call becomes: @w{@code{(= 42 @var{expval})}}.
 @item (app @var{function} @var{pattern})
 Matches if @var{function} called on @var{expval} returns a
 value that matches @var{pattern}.
-@var{function} can take one of the
+@var{function} can take one of the forms described for @code{pred},
-forms described for @code{pred}, above.
+above.  Unlike @code{pred}, however, @code{app} tests the result
-Unlike @code{pred}, however,
+against @var{pattern}, rather than against a boolean truth value.
-@code{app} tests the result against @var{pattern},
-rather than against a boolean truth value.
 
 @item (guard @var{boolean-expression})
 Matches if @var{boolean-expression} evaluates to non-@code{nil}.
 
 @item (let @var{pattern} @var{expr})
-Evaluates @var{expr} to get @var{exprval}
+Evaluates @var{expr} to get @var{exprval} and matches if @var{exprval}
-and matches if @var{exprval} matches @var{pattern}.
+matches @var{pattern}.  (It is called @code{let} because @var{pattern}
-(It is called @code{let} because
+can bind symbols to values using @var{symbol}.)
-@var{pattern} can bind symbols to values using @var{symbol}.)
 @end table
 
 @cindex sequencing pattern
@@ -596,18 +596,16 @@ but instead of processing values, they process sub-patterns.
 
 @table @code
 @item (and @var{pattern1}@dots{})
-Attempts to match @var{pattern1}@dots{}, in order,
+Attempts to match @var{pattern1}@dots{}, in order, until one of them
-until one of them fails to match.
+fails to match.  In that case, @code{and} likewise fails to match, and
-In that case, @code{and} likewise fails to match,
+the rest of the sub-patterns are not tested.  If all sub-patterns
-and the rest of the sub-patterns are not tested.
+match, @code{and} matches.
-If all sub-patterns match, @code{and} matches.
 
 @item (or @var{pattern1} @var{pattern2}@dots{})
 Attempts to match @var{pattern1}, @var{pattern2}, @dots{}, in order,
-until one of them succeeds.
+until one of them succeeds.  In that case, @code{or} likewise matches,
-In that case, @code{or} likewise matches,
+and the rest of the sub-patterns are not tested.  (Note that there
-and the rest of the sub-patterns are not tested.
+must be at least two sub-patterns.
-(Note that there must be at least two sub-patterns.
 Simply @w{@code{(or @var{pattern1})}} signals error.)
 @c Issue: Is this correct and intended?
 @c        Are there exceptions, qualifications?
@@ -1042,12 +1040,11 @@ Both use a single backquote construct (@pxref{Backquote}).
 
 This subsection describes @dfn{backquote-style patterns},
 a set of builtin patterns that eases structural matching.
-For background, @xref{Pattern-Matching Conditional}.
+For background, @pxref{Pattern-Matching Conditional}.
 
-@dfn{Backquote-style patterns} are a powerful set of
+Backquote-style patterns are a powerful set of @code{pcase} pattern
-@code{pcase} pattern extensions (created using @code{pcase-defmacro})
+extensions (created using @code{pcase-defmacro}) that make it easy to
-that make it easy to match @var{expval} against
+match @var{expval} against specifications of its @emph{structure}.
-specifications of its @emph{structure}.
 
 For example, to match @var{expval} that must be a list of two
 elements whose first element is a specific string and the second
@@ -1173,87 +1170,102 @@ evaluation results:
 (evaluate '(sub 1 2) nil)                 @result{} error
 @end example
 
-@node Destructuring patterns
+@node Destructuring with pcase Patterns
-@subsection Destructuring Patterns
+@subsection Destructuring with @code{pcase} Patterns
-@cindex destructuring 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.
+they can match, but they can also extract sub-fields of those objects.
-Say we have a list and want to extract 2 elements from it with the
+For example we can extract 2 elements from a list that is the value of
-following code:
+the variable @code{my-list} with the following code:
 
 @example
-  (pcase l
+  (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{l} is a list containing exactly 3 elements and whose
+test that @code{my-list} is a list containing exactly 3 elements and
-first element is the symbol @code{add}.  If any of those tests fail,
+whose first element is the symbol @code{add}.  If any of those tests
-@code{pcase} will directly return @code{nil} without calling
+fail, @code{pcase} will immediately return @code{nil} without calling
 @code{message}.
 
-@dfn{Destructuring} of an object is an operation that extracts
+Extraction of multiple values stored in an object is known as
-multiple values stored in the object, e.g., the 2nd and the 3rd
+@dfn{destructuring}.  Using @code{pcase} patterns allows to perform
-element of a list or a vector.  @dfn{Destructuring binding} is
+@dfn{destructuring binding}, which is similar to a local binding
-similar to a local binding (@pxref{Local Variables}), but it gives
+(@pxref{Local Variables}), but gives values to multiple elements of
-values to multiple elements of a variable by extracting those values
+a variable by extracting those values from an object of compatible
-from an object of compatible structure.
+structure.
 
-The macros described in this section use @dfn{destructuring
+The macros described in this section use @code{pcase} patterns to
-patterns}, which are normal Pcase patterns used in a context where we
+perform destructuring binding.  The condition of the object to be of
-presume that the object does match the pattern, and we only want
+compatible structure means that the object must match the pattern,
-to extract some subfields.  For example:
+because only then the object's subfields can be extracted.  For
+example:
 
 @example
-  (pcase-let ((`(add ,x ,y) l))
+  (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{l} without first verifying
+to extract @code{x} and @code{y} from @code{my-list} without first
-if @code{l} is a list which has the right number of elements and has
+verifying if @code{my-list} is a list which has the right number of
-@code{add} as its first element.
+elements and has @code{add} as its first element.  The precise
-The precise behavior when the object does not actually match the
+behavior when the object does not actually match the pattern is
-pattern is undefined, although the body will not be silently skipped:
+undefined, although the body will not be silently skipped: either an
-either an error is signaled or the body is run with some of the
+error is signaled or the body is run with some of the variables
-variables potentially bound to arbitrary values like @code{nil}.
+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@dots{}
-Bind variables according to @var{bindings} and then eval @var{body}.
+Perform desctructuring 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 destructuring pattern.
+@var{pattern} is a @code{pcase} pattern.
 
-All @var{exp}s are evaluated first after which they are matched
+All @var{exp}s are evaluated first, after which they are matched
 against their respective @var{pattern}, introducing new variable
-bindings which can then be used inside @var{body}.
+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}.
 @end defmac
 
 @defmac pcase-let* bindings body@dots{}
-Bind variables according to @var{bindings} and then eval @var{body}.
+Perform desctructuring 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 destructuring pattern.
+@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 like @code{let*}, each @var{exp} is
+Unlike @code{pcase-let}, but similarly to @code{let*}, each @var{exp}
-matched against its corresponding @var{pattern} before passing to the
+is matched against its corresponding @var{pattern} before processing
-next element of @var{bindings}, so the variables introduced in each
+the next element of @var{bindings}, so the variable bindings
-binding are available in the @var{exp}s that follow it, additionally
+introduced in each one of the @var{bindings} are available in the
-to being available in @var{body}.
+@var{exp}s of the @var{bindings} that follow it, additionally to
+being available in @var{body}.
 @end defmac
 
-@findex dolist
 @defmac pcase-dolist (pattern list) body@dots{}
-This construct executes @var{body} once for each element of
+Execute @var{body} once for each element of @var{list}, on each
-@var{list}, in a context where the variables appearing in the the
+iteration performing a destructuring binding of variables in
-destructuring pattern @var{pattern} are bound to the corresponding
+@var{pattern} to the values of the corresponding subfields of the
-values found in the element.
+element of @var{list}.  The bindings are performed as if by
-When @var{pattern} is a simple variable, this ends up being equivalent
+@code{pcase-let}.  When @var{pattern} is a simple variable, this ends
-to @code{dolist}.
+up being equivalent to @code{dolist} (@pxref{Iteration}).
 @end defmac
 
 

doc/lispref/sequences.texi

@@ -1049,15 +1049,18 @@ that @var{sequence} can be a list, vector or string.  This is
 primarily useful for side-effects.
 @end defmac
 
-@defmac seq-let arguments sequence body@dots{}
+@anchor{seq-let}
+@defmac seq-let var-sequence val-sequence body@dots{}
 @cindex sequence destructuring
-  This macro binds the variables defined in @var{arguments} to the
+  This macro binds the variables defined in @var{var-sequence} to the
-elements of @var{sequence}.  @var{arguments} can themselves include
+values that are the corresponding elements of @var{val-sequence}.
-sequences, allowing for nested destructuring.
+This is known as @dfn{destructuring binding}.  The elements of
+@var{var-sequence} can themselves include sequences, allowing for
+nested destructuring.
 
-The @var{arguments} sequence can also include the @code{&rest} marker
+The @var{var-sequence} sequence can also include the @code{&rest}
-followed by a variable name to be bound to the rest of
+marker followed by a variable name to be bound to the rest of
-@code{sequence}.
+@var{val-sequence}.
 
 @example
 @group
@@ -1081,6 +1084,9 @@ followed by a variable name to be bound to the rest of
 @end group
 @result{} [3 4]
 @end example
+
+The @code{pcase} patterns provide an alternative facility for
+destructuring binding, see @ref{Destructuring with pcase Patterns}.
 @end defmac
 
 @defun seq-random-elt sequence

lisp/emacs-lisp/pcase.el

@@ -264,10 +264,14 @@ variable name being but a special case of it)."
 
 ;;;###autoload
 (defmacro pcase-let* (bindings &rest body)
-  "Like `let*' but where you can use `pcase' patterns for bindings.
+  "Like `let*', but supports destructuring BINDINGS using `pcase' patterns.
-BODY should be an expression, and BINDINGS should be a list of bindings
+As with `pcase-let', BINDINGS are of the form (PATTERN EXP), but the
-of the form (PATTERN EXP).
+EXP in each binding in BINDINGS can use the results of the destructuring
-See `pcase-let' for discussion of how PATTERN is matched."
+bindings that precede it in BINDINGS' order.
+
+Each EXP should match (i.e. be of compatible structure) to its
+respective PATTERN; a mismatch may signal an error or may go
+undetected, binding variables to arbitrary values, such as nil."
   (declare (indent 1)
            (debug ((&rest (pcase-PAT &optional form)) body)))
   (let ((cached (gethash bindings pcase--memoize)))
@@ -280,13 +284,16 @@ See `pcase-let' for discussion of how PATTERN is matched."
 
 ;;;###autoload
 (defmacro pcase-let (bindings &rest body)
-  "Like `let' but where you can use `pcase' patterns for bindings.
+  "Like `let', but supports destructuring BINDINGS using `pcase' patterns.
-BODY should be a list of expressions, and BINDINGS should be a list of bindings
+BODY should be a list of expressions, and BINDINGS should be a list of
-of the form (PATTERN EXP).
+bindings of the form (PATTERN EXP).
-The PATTERNs are only used to extract data, so the code does not test
+All EXPs are evaluated first, and then used to perform destructuring
-whether the data does match the corresponding patterns: a mismatch
+bindings by matching each EXP against its respective PATTERN.  Then
-may signal an error or may go undetected, binding variables to arbitrary
+BODY is evaluated with those bindings in effect.
-values, such as nil."
+
+Each EXP should match (i.e. be of compatible structure) to its
+respective PATTERN; a mismatch may signal an error or may go
+undetected, binding variables to arbitrary values, such as nil."
   (declare (indent 1) (debug pcase-let*))
   (if (null (cdr bindings))
       `(pcase-let* ,bindings ,@body)
@@ -304,11 +311,15 @@ values, such as nil."
 
 ;;;###autoload
 (defmacro pcase-dolist (spec &rest body)
-  "Superset of `dolist' where the VAR binding can be a `pcase' PATTERN.
+  "Eval BODY once for each set of bindings defined by PATTERN and LIST elements.
-More specifically, this is just a shorthand for the following combination
+PATTERN should be a `pcase' pattern describing the structure of
-of `dolist' and `pcase-let':
+LIST elements, and LIST is a list of objects that match PATTERN,
-
+i.e. have a structure that is compatible with PATTERN.
-    (dolist (x LIST) (pcase-let ((PATTERN x)) BODY...))
+For each element of LIST, this macro binds the variables in
+PATTERN to the corresponding subfields of the LIST element, and
+then evaluates BODY with these bindings in effect.  The
+destructuring bindings of variables in PATTERN to the subfields
+of the elements of LIST is performed as if by `pcase-let'.
 \n(fn (PATTERN LIST) BODY...)"
   (declare (indent 1) (debug ((pcase-PAT form) body)))
   (if (pcase--trivial-upat-p (car spec))