eintr: Narrowing advantages
The Advantages of Narrowing
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With narrowing, the rest of a buffer is made invisible, as if it weren’t
there. This is an advantage if, for example, you want to replace a word
in one part of a buffer but not in another: you narrow to the part you
want and the replacement is carried out only in that section, not in the
rest of the buffer. Searches will only work within a narrowed region,
not outside of one, so if you are fixing a part of a document, you can
keep yourself from accidentally finding parts you do not need to fix by
narrowing just to the region you want. (The key binding for
‘narrow-to-region’ is ‘C-x n n’.)
However, narrowing does make the rest of the buffer invisible, which
can scare people who inadvertently invoke narrowing and think they have
deleted a part of their file. Moreover, the ‘undo’ command (which is
usually bound to ‘C-x u’) does not turn off narrowing (nor should it),
so people can become quite desperate if they do not know that they can
return the rest of a buffer to visibility with the ‘widen’ command.
(The key binding for ‘widen’ is ‘C-x n w’.)
Narrowing is just as useful to the Lisp interpreter as to a human.
Often, an Emacs Lisp function is designed to work on just part of a
buffer; or conversely, an Emacs Lisp function needs to work on all of a
buffer that has been narrowed. The ‘what-line’ function, for example,
removes the narrowing from a buffer, if it has any narrowing and when it
has finished its job, restores the narrowing to what it was. On the
other hand, the ‘count-lines’ function uses narrowing to restrict itself
to just that portion of the buffer in which it is interested and then
restores the previous situation.
Info Catalog
eintr: Narrowing & Widening
eintr: save-restriction