chapters/50.markdown @ 495eedad2b77

Section Movement Theory
=======================

If you've never used Vim's section movement commands (`[[`, `]]`, `[]` and `][`)
take a second and read the help for them now.  Go ahead and read `:help section`
as well.

Confused yet?  That's okay, so was I the first time I read that stuff.  We're
going to take a quick detour from writing code to learn about how these
movements work, and then in the next chapter we'll make our Potion plugin
support them.

Nroff Files
-----------

The four "section movement" commands are conceptually meant to move around
between "sections" of a file.

All of these commands are designed to work with [nroff files][] by default.
Nroff is a language like LaTeX or Markdown -- it's used to write text that will
be reformatted later (it's actually the format used by UNIX man pages).

Nroff files use a certain set of "macros" to define "section headings".  For
example, here's an excerpt from the `awk` man page:

    :::nroff
    .SH NAME                                                     ***
    awk \- pattern-directed scanning and processing language
    .SH SYNOPSIS                                                 ***
    .B awk
    [
    .BI \-F
    .I fs
    ]
    [
    .BI \-v
    .I var=value
    ]
    [
    .I 'prog'
    |
    .BI \-f
    .I progfile
    ]
    [
    .I file ...
    ]
    .SH DESCRIPTION                                              ***
    .I Awk
    scans each input
    .I file
    for lines that match ...

The lines starting with `.SH` are section headings.  I've marked them with
`***`.  The four section movement commands will move your cursor between these
section heading lines.

Vim considers any line starting with `.` and one of the nroff heading macros to
be a section header, *even when you're not editing an nroff file*!

You can change the macros by changing the `sections` setting, but Vim still
requires a period at the beginning of the line, and the macros must be pairs of
characters, so that setting doesn't add enough flexibility for Potion files.

Braces
------

Section movement commands *also* look for one more thing: an opening or closing
curly brace (`{` or `}`) as the first character on a line.

`[[` and `]]` look for opening braces, while `[]` and `][` look for closing
braces.

This extra "hack" allows you to move between sections of C-like languages
easily.  However, these rules are always the same no matter what type of file
you're in!

Put the following into a buffer:

    :::text
    Test           A B
    Test

    .SH Hello      A B

    Test

    {              A
    Test
    }                B

    Test

    .H World       A B

    Test
    Test           A B

Now run `:set filetype=basic` to tell Vim that this is a BASIC file, and try the
section movement comments.

The `[[` and `]]` commands will move between the lines marked `A`, while `[]`
and `][` move between the lines marked `B`.

This shows us that Vim always uses these same two rules for section movement,
even for languages where neither one makes sense (like BASIC)!

[nroff files]: http://en.wikipedia.org/wiki/Nroff

Exercises
---------

Read `:help section` again, now that you know the story of section movement.

Read `:help sections` just for the fun of it.