VDL Expression Language Syntax

Elements within a VDL expression language array are accessed using square brackets, inside which the array index of the required element is specified (remember that array indices start at 0) - the following snippet shows how to access the 2nd element of the array.

scenarios[1]

When you dereference a Mosel array entity within a VDL expression, you will need to follow an established rule - because array entities can have multiple indices they cannot be treated as plain arrays, and you need instead to call the array name as a function, passing the indices as parameters.

scenario.entities.MyArray(0).value

For more detail about accessing array and other entities, see Entity Values and Labels.

Inline functions are written within an expression itself:

<vdl-validate pass="=function(entity,value) { return value > 100; }">
</vdl-validate>

Note See Using Validators for information about the <vdl-validate> element.

Inline functions are appropriate if a function is simple and doesn't need to be shared. Another option is to write it in a script block:

<script>
function MyFunc (entity, value) {
    return value < 10;
}
</script>
<vdl-validate pass="=MyFunc"></vdl-validate>

This method provides more space in which to write what might be a more complex function, it can be read and understood more clearly and permits sharing among many VDL elements if needed.

You will often wish to use strings within expressions - perhaps to concatenate them with other strings or returned entity values. Recall that an expression is already enclosed by a pair of double quotes, so it is essential to use single quotes for strings within expressions:

<vdl-column vdl-text="='a'+'bb'+'ccc'"></vdl-column>

The code snippet above would return the value as ' abbcc'.

Several VDL attributes take key-value pairs as their input. For example:

<div vdl-css="hilite: true, showing: false"></div>

The symbol before a colon is called a key, anything after it (up until the end of the attribute string or the next comma) is the value. These are treated as evaluated expressions. If a chosen key contains anything other than alphanumeric characters (a-z, A-Z and 0-9) then the key itself must be placed inside quotes. To avoid clashing with the attribute quotes, use single quotes:

<div vdl-css="'hilite-item': true, 'showing-item':false"></div>

Similarly, when the value is a string, for the same reason, enclose it in single quotes.

A range of standard and boolean operators is available.

Note If in doubt, use the strict comparisons. Use of the roughly equal/not equal operators can lead to unexpected matches.

The VDL expression language implements three logical operators.