Expression language


Flavour provides its own expression language, similar to EL in JSP or Spring Expression Language. Unlike those, Flavour’s expression language is statically typed, which means all type errors are reported during compile time.

Expression Language (henceforth referred to as EL) is basically a subset of Java expressions with some syntactical sugar. EL consists of the following elements:

  • Java identifiers, usually referring to methods, fields and lambda parameters. Examples: foo, Bar, camelCaseName, CONSTANT_NAME, it.
  • String literals surrounded by single quotes. Examples: 'Some string', '', First line\nSecond line.
  • Numeric literals. Examples: 23, 3.14159, 6.626e-34.
  • Standard literals: null, this, true, false.
  • Arithmetic operators: +, -, /, %.
  • Comparison operators: >, <, >=, <=, ==, !=, gt, gt, goe, lt, loe. Note that since < and > may have special meaning in HTML, EL provides aliases for the Java comparison operators.
  • Logical operators: &&, &&, !, and, or, not. Note that since & may have special meaning in HTML, EL provides aliases for the Java logical operators.
  • Conditional operator: <condition> ? <then> : <else>.
  • Invocation operator: functionName(argument1, argument2, ...).
  • Collection subscript operator: collection[index1, index2, ...].
  • Dot operator: ..
  • instanceof operator.
  • Cast operator: (Type) expression.
  • Parentheses: (expression).
  • Lambda expressions.
  • Assignment statement: a = b.

this object

As you already know, a page consists of an HTML template and a view class. To refer to an instance of the view class from an HTML template, you can use this literal. Like in Java, you are not required to do it. Instead of writing, you can write simply foo().

Property access syntax

EL supports JavaBeans convention for properties. Instead of writing foo.getBar() you can write and instead of writing foo.setBar(newValue) your can write = newValue.

Collection access convention

In Java, the subscript operator ([]) is used only to access arrays. EL allows the same syntax for anything that looks like collections. If a value to the left of [] contains a get method, it will be used. For example, since both List<T> and Map<T> have get methods for element access, it’s possible to write something like:

Arrays.asList(2, 4, 8)[1]

instead of

Arrays.asList(2, 4, 8).get(1)

Importing classes

EL does not allow importing classes directly. Instead it’s a feature of templates. Remember, templates are merely HTML pages with special elements. You can use the following processing instruction to make a class or a package available in expressions:

<?import my.pkg1.*?>
<?import my.pkg2.ClassName?>

For example:

<?import java.util.Arrays?>
<std:foreach var="num" in="Arrays.asList(2, 3, 5, 7, 11)">
    <html:text value="num"/>

Lambda expressions

Most expressions you write in components are in fact lambda expressions. So, for example:

<std:foreach var="employee" in="employees">

Is a shortcut for the following full form:

<std:foreach var="employee" in="() -> employees">

Actually, in attribute accepts Supplier<T>, so you should pass zero-parameter lambda there.

Lambdas may be written in the following forms:

(Type1 param1, Type param2, ...) -> expression
Type param -> expression

Where types are optional, in most cases the EL compiler infers them.

For example: -> employee.age > 30).collect(Collectors.toList())

EL supports some shortcuts:

  • When a directive expression accepts a functional type with zero parameter, you can omit the () -> prefix.
  • When a directive expression accepts a functional type with one argument, you can omit the parameter declaration. In this case the parameter is available via it identifier.
  • When a directive expression accepts a functional type with one argument and your lambda is expected to assign value to property, you may omit ` = it suffix. I.e. instead of writing newValue -> = newValue you can write`.

The latter two shortcuts are useful for event handlers, for example:

<input html:value="employee.firstName" html:change="employee.firstName"/>

Which is a shortcut for

<input html:value="() -> employee.firstName" html:change="newValue -> employee.firstName = newValue"/>

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