There are 4 scalar types:
- Date-like (subtypes: date (no time part), time or date-time)
For each scalar type is a TemplateTypeModel interface, where Type is the name of the type. These interfaces define only one method: type getAsType();. This returns the value of the variable with the Java type (boolean, Number, String and Date respectively).
For historical reasons the interface for string scalars is called TemplateScalarModel, not TemplateStringModel. (It's because in early FreeMarker strings were the only kind of scalars.)
A trivial implementation of these interfaces are available in freemarker.template package with SimpleType class name. However, there is no SimpleBooleanModel; to represent the boolean values you can use the TemplateBooleanModel.TRUE and TemplateBooleanModel.FALSE singletons.
For historical reasons the class for string scalars is called SimpleScalar, not SimpleString.
Scalars are immutable within FTL. When you set the value of a variable in a template, then you replace the TemplateTypeModel instance with another instance, and don't change the value stored in the original instance.
There is a complication around date-like types, because Java API usually does not differentiate java.util.Date-s that store only the date part (April 4, 2003), only the time part (10:19:18 PM), or both (April 4, 2003 10:19:18 PM). To display the value as text correctly (or to do certain other operations), FreeMarker must know what parts of the java.util.Date stores meaningful information, and what parts are unused (usually 0-ed out). Unfortunately, this information is usually only available when the value comes from a database, because most databases have separate date, time and date-time (aka. timestap) types, and java.sql has 3 corresponding java.util.Date subclasses for them.
TemplateDateModel interface has two methods: java.util.Date getAsDate() and int getDateType(). A typical implementation of this interface, stores a java.util.Date object, plus an integer that tells the subtype. The value of this integer must be a constant from the TemplateDateModel interface: DATE, TIME, DATETIME and UNKNOWN.
About UNKNOWN: java.lang and java.util classes are usually converted automatically into TemplateModel implementations be the ObjectWrapper (see object wrapping earlier). If the object wrapper has to wrap a java.util.Date, that is not an instance of a java.sql date class, it can't decide what the subtype is, so it uses UNKNOWN. Later, if the template has to use this variable, and the subtype is needed for the operation, it will stop with error. To prevent this, for the problematic variables the template author must specify the subtype explicitly using the date, time or datetime built-ins (like lastUpdated?datetime). Note that if you use string built-in with format parameter, as foo?string["MM/dd/yyyy"], then FreeMarker doesn't need to know the subtype.