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Functions

Like most programming languages, Vimscript has functions. Let's take a look at how to create them, and then talk about some of their quirks.

Run the following command:

:function meow()

You might think this would start defining a function named meow. Unfortunately this is not the case, and we've already run into one of Vimscript's quirks.

Vimscript functions must start with a capital letter if they are unscoped!

Even if you do add a scope to a function (we'll talk about that later) you may as well capitalize the first letter of function names anyway. Most Vimscript coders seem to do it, so don't break the convention.

Okay, let's define a function for real this time. Run the following commands:

:function Meow()
:  echom "Meow!"
:endfunction

This time Vim will happily define the function. Let's try running it:

:call Meow()

Vim will display "Meow!" as expected.

Let's try returning a value. Run the following commands:

:function GetMeow()
:  return "Meow String!"
:endfunction

Now try it out by running this command:

:echom GetMeow()

Vim will call the function and give the result to echom, which will display "Meow String!".

Calling Functions

We can already see that there are two different ways of calling functions in Vimscript.

When you want to call a function directly you use the call command. Run the following commands:

:call Meow()
:call GetMeow()

The first will display "Meow!" but the second doesn't display anything. The return value is thrown away when you use call, so this is only useful when the function has side effects.

The second way to call functions is in expressions. You don't need to use call in this case, you can just name the function. Run the following command:

:echom GetMeow()

As we saw before, this calls GetMeow and passes the return value to echom.

Implicit Returning

Run the following command:

:echom Meow()

This will display two lines: "Meow!" and "0". The first obviously comes from the echom inside of Meow. The second shows us that if a Vimscript function doesn't return a value, it implicitly returns 0. Let's use this to our advantage. Run the following commands:

:function TextwidthIsTooWide()
:  if &l:textwidth ># 80
:    return 1
:  endif
:endfunction

This function uses a lot of important concepts we've seen before:

  • if statements
  • Treating options as variables
  • Localizing those option variables
  • Case-sensitive comparisons

If any of those sound unfamiliar you should go back a few chapters and read about them.

We've now defined a function that will tell us if the textwidth setting is "too wide" in the current buffer (because 80 characters is, of course, the correct width for anything but HTML).

Let's try using it. Run the following commands:

:set textwidth=80
:if TextwidthIsTooWide()
:  echom "WARNING: Wide text!"
:endif

What did we do here?

  • First we set the textwidth globally to 80.
  • The we ran an if statement that checked if TextwidthIsTooWide() was truthy.
  • This wound up not being the case, so the if's body wasn't executed.

Because we never explicitly returned a value, Vim returned 0 from the function, which is falsy. Let's try changing that. Run the following commands:

:setlocal textwidth=100
:if TextwidthIsTooWide()
:  echom "WARNING: Wide text!"
:endif

This time the if statement in the function executes its body, returns 1, and so the if we manually typed in executes its body.

Exercises

Read :help call. Ignore anything about "ranges" for now. How many arguments can you pass to a function? Is this surprising?

Read the first paragraph of :help E124 and find out what characters you're allowed to use in function names. Are underscores okay? Dashes? Accented characters? Unicode characters? If it's not clear from the documentation just try them out and see.

Read :help return. What's the "short form" of that command (which I told you to never use)? Is it what you expected? If not, why not?