chapters/45.markdown @ 436f8ce22479

Basic Syntax Highlighting
=========================

Now that we've gotten the boilerplate out of the way it's time to start writing
some useful code for our Potion plugin.  We'll start with some simple syntax
highlighting.

Create a `syntax/potion.vim` file in your plugin's repo.  Put the following code
into the file:

    :::vim
    if exists("b:current_syntax")
        finish
    endif

    echom "Our syntax highlighting code will go here."

    let b:current_syntax = "potion"

Close Vim, and then open your `factorial.pn` file.  You may or may not see the
message, depending on whether you have any other plugins that perform commands
after this one gets run.  If you run `:messages` you'll definitely see that the
file was indeed loaded.

**Note:**  Whenever I tell you to open the Potion file I want you to do it in
a *new Vim window/instance* instead of in a split/tab.  Opening a new Vim window
causes Vim to reload all your bundled files for that window, whereas using
a split does not.

The lines at the beginning and end of the file are a convention that prevents it
from being loaded if syntax highlighting has already been enabled for this
buffer.

Highlighting Keywords
---------------------

For the rest of this chapter we'll ignore the `if` and `let` boilerplate at the
beginning and end of the file.  Don't remove those lines, just forget about them.

Replace the placeholder `echom` in the file with the following code:

    :::vim
    syntax keyword potionKeyword to times
    highlight link potionKeyword Keyword

Close the `factorial.pn` file and reopen it.  The `to` and `times` words will be
highlighted as keywords in your color scheme!

These two lines show the basic structure of simple syntax highlighting in Vim.
To highlight a piece of syntax:

* You first define a "chunk" of syntax using `syntax keyword` or a related
  command (which we'll talk about later).
* You then link "chunks" to highlighting groups.  A highlighting group is
  something you define in a color scheme, for example "function names should be
  blue".
  
This lets plugin authors define the "chunks" of syntax in ways that make sense
to them, and then link them to common highlighting groups.  It also lets color
scheme creators define colors for a common set of programming constructs so they
don't need to know about individual languages.

Potion has a bunch of other keywords that we haven't used in our toy program, so
let's edit our syntax file to highlight those too:

    :::vim
    syntax keyword potionKeyword loop times to while
    syntax keyword potionKeyword if elsif else
    syntax keyword potionKeyword class return

    highlight link potionKeyword Keyword

First of all: the last line hasn't changed.  We're still telling Vim that
anything in the `potionKeyword` syntax group should be highlighted as
a `Keyword`.

We've now got three lines, each starting with `syntax keyword potionKeyword`.
This shows that running this command multiple times doesn't *reset* the syntax
group -- it adds to it!  This lets you define groups piecemeal.

How you define your groups is up to you:

* You might just toss everything onto one line and be done with it.
* You might prefer to break the lines up so they fit within 80 columns to make
  them easier to read.
* You could have a separate line for each item in a group, to make diffs looks
  nicer.
* You could do what I've done here and group related items together.

Highlighting Functions
----------------------

Another standard Vim highlighting group is `Function`.  Let's add some of the
built-in Potion functions to our highlighting script.  Edit the guts of your
syntax file so it looks like this:

    :::vim
    syntax keyword potionKeyword loop times to while
    syntax keyword potionKeyword if elsif else
    syntax keyword potionKeyword class return

    syntax keyword potionFunction print join string

    highlight link potionKeyword Keyword
    highlight link potionFunction Function

Close and reopen `factorial.pn` and you'll see that the built-in potion
functions are now highlighted.

This works exactly the same way as keyword highlighting.  We've defined a new
syntax group and linked it to a different highlighting group.

Exercises
---------

Think about why the `if exists` and `let` lines at the beginning and end of the
file are useful.  If you can't figure it out, don't worry about it.  I had to
ask Tim Pope to be sure.

Skim over `:help syn-keyword`.  Pay close attention to the part that mentions
`iskeyword`.

Read `:help iskeyword`.

Read `:help group-name` to get an idea of some common highlighting groups that
color scheme authors frequently use.