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Update the example and explanation about let and buffer-local bindings.

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Richard M. Stallman 2003-06-04 09:25:00 +00:00
parent 54f7f2a41a
commit beab64dc85
1 changed files with 15 additions and 26 deletions
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41
lispref/variables.texi
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@ -1184,16 +1184,17 @@ the default binding untouched. This means that the default value cannot
be changed with @code{setq} in any buffer; the only way to change it is
with @code{setq-default}.
@strong{Warning:} When a variable has buffer-local values in one or
more buffers, binding the variable with @code{let} and changing to a
different current buffer in which a different binding is in
effect, and then exiting the @code{let}, the variable may not be
restored to the value it had before the @code{let}.
To preserve your sanity, avoid using a variable in that way. If you
use @code{save-excursion} around each piece of code that changes to a
different current buffer, you will not have this problem
(@pxref{Excursions}). Here is an example of what to avoid:
@strong{Warning:} When a variable has buffer-local or frame-local
bindings in one or more buffers, @code{let} rebinds the binding that's
currently in effect. For instance, if the current buffer has a
buffer-local value, @code{let} temporarily rebinds that. If no
buffer-local or frame-local bindings are in effect, @code{let} rebinds
the default value. If inside the @code{let} you then change to a
different current buffer in which a different binding is in effect,
you won't see the @code{let} binding any more. And if you exit the
@code{let} while still in the other buffer, you won't see the
unbinding occur (though it will occur properly). Here is an example
to illustrate:
@example
@group
@ -1208,24 +1209,12 @@ different current buffer, you will not have this problem
;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}}
@var{body}@dots{})
@group
foo @result{} 'a ; @r{we are still in buffer @samp{b}, but exiting the let}
; @r{restored the local value in buffer @samp{a}}
foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},}
; @r{but we don't see that in buffer @samp{b}}
@end group
@group
(set-buffer "a") ; @r{This can be seen here:}
foo @result{} 'a ; @r{we are back to the local value in buffer @samp{a}}
@end group
@end example
@noindent
But @code{save-excursion} as shown here avoids the problem:
@example
@group
(let ((foo 'temp))
(save-excursion
(set-buffer "b")
@var{body}@dots{}))
(set-buffer "a") ; @r{verify the local value was restored}
foo @result{} 'a
@end group
@end example