An identifier by itself refers to a scalar in the root hash of the data model:

${foo}

The delimiters mean "print the variable's value here". An expression that has no referent, or whose referent is null, produces no output. Expressions inside template tags don't have these delimiters:

<if foo>
    hello
</if>

This prints "hello" if the boolean value of foo is true. In templates, undefined, null, and empty values evaluate to false; all other values evaluate to true. Undefined, null, and empty values are considered to be equal.

If two identifiers are separated by a dot, the one on the left is a hash; the one on the right is the name of a key in that hash:

<p>First Name: ${book.author.firstName}

When an identifier followed by an expression in brackets, the expression is evaluated at run time as the name of a hash key. The following are all equivalent, if propName equals "bar" and propStart equals "ba":

foo.bar
foo["bar"]
foo[propName]
foo["ba" + "r"]
foo[propStart + "r"]

You can chain together dynamic hash keys. The following are equivalent:

foo.bar.baz.bing
foo["bar"]["baz"]["bing"]

However, the brackets cannot be followed by a dot or an identifier.

In addition to the dot and bracket operators, the following operators are supported in expressions:

Operator	Usage	Meaning
!	!exp	Logical not
+	exp + exp	String concatenation
==	exp == exp	Equality (string comparison)
!=	exp != exp	Inquality (string comparison)
&&	exp && exp	Logical and
||	exp || exp	Logical or

String literals are delimited by double quotes. A backslash in a string literal can be used to escape a double quote. To include a backslash character, use \\. Operators have standard precedence, and you can use nested parentheses.

An identifier followed a pair of parentheses is a method call:

foo()
foo("hello")
deal(king, queen, spade)

A method can return a list, a hash, a scalar, or null. You can chain together method calls if each one returns a hash containing the next method:

foo().bar()

Here, foo() returns a hash in which the key bar's value is a method object that executes bar().

Here are some more examples of syntactically valid expressions:

!(color == "blue" || color == "violet")

"customer" + customer.number

foo && (!(bar || baz) || bing)

eventManager.getEvent(eventID).importance

If you use a boolean expression for its string value (not something you'll often need to do), its string value is "true" if it evaluates to true, otherwise null.

Previous: Introduction

Next: Lists