chapters/44.markdown @ d7ca8f55dde3 jlmuir/fix-typo-in-ch-11--1475170122831

Detecting Filetypes
===================

Let's create a Potion file we can use as a sample as we're working on our
plugin.  Create a `factorial.pn` file somewhere and put the following Potion
code inside it:

    :::text
    factorial = (n):
        total = 1
        n to 1 (i):
            total *= i.
        total.

    10 times (i):
        i string print
        '! is: ' print
        factorial (i) string print
        "\n" print.

This code creates a simple factorial function and calls it ten times, printing
the results each time.  Go ahead and run it with `potion factorial.pn`.  The
output should look like this:

    :::text
    0! is: 0
    1! is: 1
    2! is: 2
    3! is: 6
    4! is: 24
    5! is: 120
    6! is: 720
    7! is: 5040
    8! is: 40320
    9! is: 362880

If you don't get this output, or you get an error, stop and figure out what's
gone wrong.  The code should work exactly as-is.

Take some time to understand how the code works.  Refer to the Potion docs
liberally.  It's not critical to understanding Vimscript but it will make you
a better programmer.

Detecting Potion Files
----------------------

Open `factorial.pn` in Vim and run the following command:

    :::vim
    :set filetype?

Vim will display `filetype=` because it doesn't know what a `.pn` file is yet.
Let's fix that.

Create `ftdetect/potion.vim` in your plugin's repo.  Put the following lines
into it:

    :::vim
    au BufNewFile,BufRead *.pn set filetype=potion

This creates a single autocommand: a command to set the filetype of `.pn` files
to `potion`.  Pretty straightforward.

Notice that we *didn't* use an autocommand group like we usually would.  Vim
automatically wraps the contents of `ftdetect/*.vim` files in autocommand groups
for you, so you don't need to worry about it.

Close the `factorial.pn` file and reopen it.  Now run the previous command
again:

    :::vim
    :set filetype?

This time Vim displays `filetype=potion`.  When Vim started up it loaded the
autocommand group inside `~/.vim/bundle/potion/ftdetect/potion.vim`, and when it
opened `factorial.pn` the autocommand fired, setting the `filetype` to `potion`.

Now that we've taught Vim to recognize Potion files we can move on to actually
creating some useful behavior in our plugin.

Exercises
---------

Read `:help ft`.  Don't worry if you don't understand everything there.

Read `:help setfiletype`.

Modify the Potion plugin's `ftdetect/potion.vim` script to use `setfiletype`
instead of `set filetype`.