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if-let*/when-let*/and-let*: Don't recommend (VALUEFORM) form

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* doc/lispref/control.texi (Conditionals):
* lisp/subr.el (if-let*): Document '(_ VALUEFORM)' instead of
'(VALUEFORM)'.
This commit is contained in:
Sean Whitton 2025-10-07 17:31:34 +01:00
parent db7fd704de
commit da47fa2f23
2 changed files with 25 additions and 17 deletions
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21
doc/lispref/control.texi
View file

@ -317,7 +317,7 @@ following way instead:
(do-something result2))
@end example
There's a number of variations on this theme, and they're briefly
There are a number of variations on this theme, and they're briefly
described below.
@defmac if-let* varlist then-form else-forms...
@ -328,10 +328,9 @@ Evaluate each binding in @var{varlist}, stopping if a binding value is
Each element of @code{varlist} has the form @w{@code{(@var{symbol}
@var{value-form})}}: @var{value-form} is evaluated and @var{symbol} is
locally bound to the result. Bindings are sequential, as in @code{let*}
(@pxref{Local Variables}). As a special case, @var{symbol} can be
omitted if only the test result of @var{value-form} is of interest:
@var{value-form} is evaluated and checked for @code{nil}, but its value
is not bound.
(@pxref{Local Variables}). If only the test result of @var{value-form}
is of interest, use @code{_} for @var{symbol}. Then @var{value-form} is
evaluated and checked for @code{nil}, but its value is not bound.
@end defmac
@defmac when-let* varlist then-forms...
@ -343,8 +342,8 @@ form in @var{then-forms}.
@code{varlist} has the form @w{@code{(@var{symbol} @var{value-form})}},
in which @var{value-form} is evaluated and @var{symbol} is locally bound
to the result. Bindings are sequential, as in @code{let*} (@pxref{Local
Variables}). As a special case, @var{symbol} can be omitted if only the
test result of @var{value-form} is of interest: @var{value-form} is
Variables}). If only the test result of @var{value-form}
is of interest, use @code{_} for @var{symbol}. Then @var{value-form} is
evaluated and checked for @code{nil}, but its value is not bound.
@end defmac
@ -358,8 +357,8 @@ the value of the last binding.
@code{varlist} has the form @w{@code{(@var{symbol} @var{value-form})}},
in which @var{value-form} is evaluated and @var{symbol} is locally bound
to the result. Bindings are sequential, as in @code{let*} (@pxref{Local
Variables}). As a special case, @var{symbol} can be omitted if only the
test result of @var{value-form} is of interest: @var{value-form} is
Variables}). If only the test result of @var{value-form} is of
interest, use @code{_} for @var{symbol}. Then @var{value-form} is
evaluated and checked for @code{nil}, but its value is not bound.
@end defmac
@ -382,8 +381,8 @@ established anew.
@code{varlist} has the form @w{@code{(@var{symbol} @var{value-form})}},
in which @var{value-form} is evaluated and @var{symbol} is locally bound
to the result. Bindings are sequential, as in @code{let*} (@pxref{Local
Variables}). As a special case, @var{symbol} can be omitted if only the
test result of @var{value-form} is of interest: @var{value-form} is
Variables}). If only the test result of @var{value-form} is of
interest, use @code{_} for @var{symbol}. Then @var{value-form} is
evaluated and checked for @code{nil}, but its value is not bound.
The return value of @code{while-let} is always @code{nil}.

21
lisp/subr.el
View file

@ -2632,6 +2632,9 @@ Affects only hooks run in the current buffer."
binding))
bindings)))
;; FIXME: Once Emacs 29 is ancient history we can consider a
;; byte-compiler warning. This is because Emacs 29 and older will warn
;; about unused variables with (_ VALUEFORM).
(defmacro if-let* (varlist then &rest else)
"Bind variables according to VARLIST and evaluate THEN or ELSE.
Evaluate each binding in turn, as in `let*', stopping if a
@ -2639,12 +2642,18 @@ binding value is nil. If all are non-nil return the value of
THEN, otherwise the value of the last form in ELSE, or nil if
there are none.
Each element of VARLIST is a list (SYMBOL VALUEFORM) that binds
SYMBOL to the value of VALUEFORM. An element can additionally be
of the form (VALUEFORM), which is evaluated and checked for nil;
i.e. SYMBOL can be omitted if only the test result is of
interest. It can also be of the form SYMBOL, then the binding of
SYMBOL is checked for nil."
Each element of VARLIST is a list (SYMBOL VALUEFORM) that binds SYMBOL
to the value of VALUEFORM. If only the test result is of interest, use
`_' as SYMBOL, i.e. (_ VALUEFORM), in which case VALUEFORM is evaluated
and checked for nil but the result is not bound.
An element of VARLIST can also be of the form SYMBOL, in which case the
binding of SYMBOL is checked for nil, only.
An older form for entries of VARLIST is also supported, where SYMBOL is
omitted, i.e. (VALUEFORM). This means the same as (_ VALUEFORM).
This form is not recommended because many programmers find it
significantly less readable. A future release of Emacs may introduce a
byte-compiler warning for uses of (VALUEFORM) in VARLIST."
(declare (indent 2)
(debug ((&rest [&or symbolp (symbolp form) (form)])
body)))