You can call functions from within the template files and embed the call result easily by writing {%=functionName(arg1, arg2, arg3, ...)%} expression in the document template.

This is a short description of the functions implemented in Stencil:

Basic Functions


Accepts any number of arguments, returns the first not-empty value.


Can be used to access the whole template data.



Converts parameter to decimal number. Returns null for missing value.


Decides if a parameter is empty or missing. Useful in conditional statements.


{%if empty(userName) %}Unknown User{%else%}{%=userName%}{%end%}

If the value of userName is missing then Unknown User will be inserted, otherwise the value is used.

The empty() function is useful when we want to either enumerate the contents of an array or hide the whole paragraph when the array is empty.


Creates a list collection from the supplied arguments. Intended to be used with other collections functions.


Converts parameter to integer number. Returns null for missing value.


Selects values under a given key in a sequence of maps. The first parameter is a string which contains what key to select:

Example use cases with data:

{"items": [{"price": 10, "name": "Wood"}, {"price": "20", "name": "Stone"}]}

String functions

These functions deal with textual data.


Joins a list of items with an optional separator.

Example: call join(xs, “,”) to join them with a comma.

Example: call join(xs) to just concatenate the items.


Joins a list of items using two separators. The first separator is used to join the items except for the last item. The second separator is used to join the last item. When two items are supplied, then only the second separator is used.

Example: call joinAnd(xs, ", ", " and ") to get "1, 2, 3 and 4".


Formats a date value according to a given format string.

When called with two arguments:

  1. First argument is a format string.
  2. Second argument is a string containing a date value.

With three arguments:

  1. First argument is an IETF Language Tag string.
  2. Second argument is a format string.
  3. Third argument is a string containing a date value.


  1. This example formats the value of birthDate as a date string: {%=date("yyyy-MM-dd", birthDate) %}
  2. With locale: date("hu", "YYYY MMMM d", "2021-05-20") evaluates to "2021 május 20".

Also, try these formats strings:


It is possible to embed text with basic dynamic formatting using HTML notation. The HTML code will be converted to OOXML and inserted in the document.

Stencil uses a simple parsing algorithm to convert between the formats. At the moment only a limited set of basic formatting is implemented. You can use the following HTML tags:

The rendering throws an exception on invalid HTML input or unexpected HTML tags.


Write the following to embed the content of x as HTML in the document:


You can embed custom xml fragments in the document with the xml() function. The parameter is a string containing the XML nodes to insert.


Calls String.format function.


Like format but first parameter is an IETF Language Tag. For example: formatWithLocale("hu", "%,.2f", number)


This example formats the value of price as a price string: {%=format("$ %(,.2f", price) %}. It may output $ (6,217.58).


The length(x) function returns the length of the value in x:

Returns zero when x is null.


The lowercase(x) function turns its string argument into a lowercase string. For example: lowercase("HELLO") returns "hello".


The str(x) functions convers its non-null arguments into a string. Returns an empty string when all arguments are null.

Numeric functions


Expects one number argument. Rounds the argument to the closest integer. Usage: round(x). For example round(1.1) returns 1.


Expects one number argument. Rounds the argument to the closest integer that is not greater than than the argument. Usage: floor(x)


Expects one number argument. Rounds its argument to the closest integer that is not smaller than the argument. Usage: ceil(x)


Expects two arguments: a value and a list. Checks if list contains the value. Usage: contains("myValue", myList)


Expects one number argument containing a list with numbers. Sums up the numbers and returns result. Usage: sum(myList)

Custom functions

You can register custom implementations of io.github.erdos.stencil.functions.Function or the stencil.functions/call-fn multimethod.

Clojure example:

(defmethod call-fn "first" [_ data]
  (if (seq? data) (first data) data))

Java example:

public class FirstFuncion implements Function {

    public String getName() {
        return "first";

    public Object call(final Object... arguments) throws IllegalArgumentException {
        if (arguments.length != 1) {
            throw new IllegalArgumentException("Unexpected argument count");
        final Object arg = arguments[0];
        if (arg instanceof Iterable<?>) {
            final Iterator<?> iterator = ((Iterable<?>) arg).iterator();
            return iterator.hasNext() ? : null;
        } else {
            return null;

/* .... later you can register it in render stage .... */

API.render(preparedTemplate, fragments, data, Arrays.asList(new FirstFunction()));