chapters/19.markdown @ 0b93f09f9abf

Variables
=========

Up to this point we've covered single commands.  For the next third of the book
we're going to look at Vimscript as a *programming language*.  This won't be as
instantly gratifying as the rest of what you've learned, but it will lay the
groundwork for the last part of the book, which walks through creating
a full-fledged Vim plugin from scratch.

Let's get started.  The first thing we need to talk about are variables.  Run
the following commands:

    :::vim
    :let foo = "bar"
    :echo foo

Vim will display `bar`.  `foo` is now a variable, and we've assigned it
a string: `"bar"`.  Now run these commands:

    :::vim
    :let foo = 42
    :echo foo

Vim will display `42`, because we've reassigned `foo` to the integer `42`.

From these short examples it may seem like Vimscript is dynamically typed.
That's not the case, but we'll talk more about that later.

Options as Variables
--------------------

You can read and set *options* as variables by using a special syntax.  Run the
following commands:

    :::vim
    :set textwidth=80
    :echo &textwidth

Vim will display `80`.  Using an ampersand in front of a name tells Vim that
you're referring to the option, not a variable that happens to have the same
name.

Let's see how Vim works with boolean options.  Run the following commands:

    :::vim
    :set nowrap
    :echo &wrap

Vim displays `0`.  Now try these commands:

    :::vim
    :set wrap
    :echo &wrap

This time Vim displays `1`.  This is a very strong hint that Vim treats the
integer `0` as "false" and the integer `1` as "true".  It would be reasonable to
assume that Vim treats *any* non-zero integer as "truthy", and this is indeed
the case.

We can also *set* options as variables using the `let` command.  Run the
following commands:

    :::vim
    :let &textwidth = 100
    :set textwidth?

Vim will display `textwidth=100`.

Why would we want to do this when we could just use `set`? Run the following
commands:

    :::vim
    :let &textwidth = &textwidth + 10
    :set textwidth?

This time Vim displays `textwidth=110`.  When you set an option using `set` you
can only set it to a single literal value.  When you use `let` and set it as
a variable you can use the full power of Vimscript to determine the value.

Local Options
-------------

If you want to set the *local* value of an option as a variable, instead of the
*global* value, you need to prefix the variable name.

Open two files in separate splits.  Run the following command:

    :::vim
    :let &l:number = 1

Now switch to the other file and run this command:

    :::vim
    :let &l:number = 0

Notice that the first window has line numbers and the second does not.

Registers as Variables
----------------------

You can also read and set *registers* as variables.  Run the following command:

    :::vim
    :let @a = "hello!"

Now put your cursor somewhere in your text and type `"ap`.  This command tells
Vim to "paste the contents of register `a` here".  We just set the contents of
that register, so Vim pastes `hello!` into your text.

Registers can also be read.  Run the following command:

    :::vim
    :echo @a

Vim will echo `hello!`.

Select a word in your file and yank it with `y`, then run this command:

    :::vim
    :echo @"

Vim will echo the word you just yanked.  The `"` register is the "unnamed"
register, which is where text you yank without specifying a destination will go.

Perform a search in your file with `/someword`, then run the following command:

    :::vim
    :echo @/

Vim will echo the search pattern you just used.  This lets you programmatically
read *and modify* the current search pattern, which can be very useful at times.

Exercises
---------

Go through your `~/.vimrc` file and change some of the `set` and `setlocal`
commands to their `let` forms.  Remember that boolean options still need to be
set to something.

Try setting a boolean option like `wrap` to something other than zero or one.
What happens when you set it to a different number?  What happens if you set it
to a string?

Go back through your `~/.vimrc` file and undo the changes.  You should never use
`let` if `set` will suffice -- it's harder to read.

Read `:help registers` and look over the list of registers you can read and
write.