Set nested property in Freemarker template - alfresco

Question

I'm working in Alfresco and I'd like to create my own custom component (for a form) extending an exiting one. In my case "number.ftl".
I'd like to provide the ability to set a defaultValue, so that if field.value is empty the default value will be used
default-number.flt
<#if field.control.params.defaultValue?? && !field.value?has_content>
<#assign field.value=field.control.params.defaultValue>
</#if>
<#include "/org/alfresco/components/form/controls/number.ftl" />
It complains about the dot (.) in field.value:
Caused by: freemarker.core.ParseException: Parsing error in template "org/alfresco/components/form/controls/year.ftl" in line 3, column 23:
Encountered ".", but was expecting one of:
"="
"in"
">"
How do I set the variable?
UPDATE
I've tried as suggested by abarisone (but I can't set the variable after importing "number.ftl" or the old value would be used):
<#if field.control.params.curretYearDefaultValue?? && !field.value?has_content>
<#assign value in field>
${.now?string("yy")}
</#assign>
</#if>
<#include "/org/alfresco/components/form/controls/number.ftl" />
but i get:
FreeMarker template error: For "#assign" namespace: Expected a namespace, but this evaluated to an extended_hash+string (org.alfresco.web.scripts.forms.FormUIGet$Field wrapped into f.e.b.StringModel): ==> field [in template "org/alfresco/components/form/controls/year.ftl" at line 3, column 27]
UPDATE2 (solved)
As suggested by ddekany here a working solution
<#if field.control.params.useCurretYearAsDefaultValue?? && field.control.params.useCurretYearAsDefaultValue = "true" && !field.value?has_content>
${field.setValue(.now?string("yyyy"))!}
</#if>
<#include "/org/alfresco/components/form/controls/number.ftl" />

Answer 1

FreeMarker templates don't support data-model modification. They only meant to read the stuff that was already prepared by Java code. You can set top-level variables only because those aren't part of the data-model (they are in a scope in front of the top-level data-model variables). And so FreeMarker doesn't even have a syntax for modifying a subvariable. But, a back door might exists to achieve what you want. Depending on configuration settings and on what field is on Java-level, ${field.setValue(...)} might works. It's quite a hack to do that, mind you. Manipulating form content from a template, that stinks. You should call out to some Java helper method at least.

Answer 2

If you look at the Freemarker assign reference, you will see that when you use assign you usually write <#assign name=value> where name is the name of the variable. It is not expression.
In fact the compiler complains about the fact it was expecting a "=" or "in".
The latter makes sense if you look further to this:
<#assign name in namespacehash>
capture this
</#assign>
explained like that:
If you know what namespaces are: assign directive creates variables in namespaces. Normally it creates the variable in the current namespace (i.e. in the namespace associated with the template where the tag is). However, if you use in namespacehash then you can create/replace a variable of another namespace than the current namespace. For example, here you create/replace variable bgColor of the namespace used for /mylib.ftl:
<#import "/mylib.ftl" as my>
<#assign bgColor="red" in my>
So I can suggest you to import the number.ftl file first and then search for the variable you're wishing to set.