Learning Rails

Parentheses Are (Usually) Optional

Example 2-7 works perfectly well, but experienced Rails developers will look at it and wonder why we typed so much. Why? The parentheses around the arguments to h() are (usually) optional. You can produce the same result with the version shown in Example 2-8.

Example 2-8. Escaping instance variables without using the parentheses

<html>
<head><title><%=h @message %> </title></head>
<body>
<h1><%=h @message %></h1>
<p>This is a greeting from app/views/hello/index.html.erb</p>

<p><%=h @bonus %></p>
</body>
</html>

A lot of developers just think of <%=h as the opening to escaped content, and when you’re just dropping an instance variable into the content, this works beautifully.

It doesn’t work, however, when Ruby needs to know where the parentheses are for more complex or ambiguous expressions, such as:

<%=h if @foo.length > 1 then "Sausages" else "Mash" end %>

which will produce the unwanted and mysterious:

>> "syntax error, unexpected kTHEN, expecting $end"

You’ll have to choose for yourself which approach is easiest for you—the idiomatic Ruby approach or the safer but more cluttered approach of making parentheses explicit.

Note

If you want to comment out ERb lines, you can just insert a # symbol after the <%. For example, <%#=h @message %> would do nothing, because of the #.