chapters/21.markdown @ 9a3df9479748

Conditionals
============

Every programming language has a way to branch, and in Vimscript that method is
the `if` statement.  The `if` statement is the core of branching in Vim.
There's no `unless` statement like Ruby, so any decision making you do in your
coding will be done with `if`s.

Before we talk about Vim's `if` statement we need to take a short detour to talk
about syntax so we're all on the same page.

Multiple-Line Statements
------------------------

Sometimes can't fit a piece of Vimscript on a single line of code.  We saw this
when we talked about autocommand groups.  Here's a chunk of code we used before:

    :augroup testgroup
    :    autocmd BufWrite * :echom "Baz"
    :augroup END

You can write this as three separate lines in a Vimscript file, which is ideal,
but it gets tedious to write this way when running commands manually.  Instead
you can separate each line with a pipe character (`|`).  Run the following
command:

    :echom "foo" | echom "bar"

Vim will treat that as two separate commands.  Use `:messages` to check the log
if you didn't see both lines appear.

For the rest of this book if you want to manually run a command but don't want
to bother typing in the newlines and colons, feel free to put it all on one line
separated by pipes.

Basic If
--------

Now that we've got that out of the way, run the following commands:

    :if 1
    :    echom "ONE"
    :endif

Vim will display "ONE", because the integer `1` is "truthy".  Now try this
command:

    :if 0
    :    echom "ZERO"
    :endif

Vim will *not* display "ZERO" because the integer `0` is "falsy".  Let's see how
strings behave.  Run this command:

    :if "something"
    :    echom "INDEED"
    :endif

The results may surprise you.  Vim does *not* necessarily treat a non-empty
string as "truthy", so it will not display anything!

Let's dive a bit further down the rabbit hole.  Run this command:

    :if "9024"
    :    echom "WHAT?!"
    :endif

This time Vim *does* display the text!  What's going on here?

To try to wrap our heads around what's going on, run the following two commands:

    :echom "hello" + 10
    :echom "10hello" + 10
    :echom "hello10" + 10

The first command causes Vim to echo "10", the second command echoes "20", and
the third echoes "10" again!

After observing all of these commands we can draw a few informed conclusions
about Vimscript:

* Vim will try to coerce variables (and literals) when necessary.  When `10 + 
  "20foo"` is evaluated Vim will convert `"20foo"` to an integer (which
  results in `20`) and then add it to `10`.
* Strings that start with a number are coerced to that number, otherwise they're
  coerced to `0`.
* Vim will execute the body of an `if` statement when its condition evaluates to
  a non-zero integer, *after* all coersion takes place.

Else and Elseif
---------------

Vim, like Python, supports both "else" and "else if" clauses.  Run the following
commands:

    :if 0
    :    echom "if"
    :elseif "nope!"
    :    echom "elseif"
    :else
    :    echom "finally!"
    :endif

Vim echoes "finally!" because both of the previous conditions evaluate to zero,
which is falsy.

Exercises
---------

Drink a beer to console yourself about Vim's coersion of strings to integers.