/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package freemarker.template;

import freemarker.template.utility.ClassUtil;

The common super-interface of the interfaces that stand for the FreeMarker Template Language (FTL) data types. The template language only deals with TemplateModel-s, not directly with plain Java objects. (For example, it doesn't understand Number, but TemplateNumberModel.) This is why the data-model (aka. the "template context" in other languages) is (automatically) mapped to a tree of TemplateModel-s. 
Mapping the plain Java objects to TemplateModel-s (or the other way around sometimes) is the responsibility of the ObjectWrapper (can be set via Configuration.setObjectWrapper(ObjectWrapper)). But not all TemplateModel-s are for wrapping a plain object. For example, a value created within a template is not made to wrap an earlier existing object; it's a value that has always existed in the template language's domain. Users can also write TemplateModel implementations and put them directly into the data-model for full control over how that object is seen from the template. Certain TemplateModel interfaces doesn't even have equivalent in Java. For example the directive type (TemplateDirectiveModel) is like that. 
Because TemplateModel "subclasses" are all interfaces, a value in the template language can have multiple types. However, to prevent ambiguous situations, it's not recommended to make values that implement more than one of these types: string, number, boolean, date. The intended applications are like string+hash, string+method, hash+sequence, etc. 
See Also:
ClassUtil.getFTLTypeDescription(TemplateModel)/**
 * The common super-interface of the interfaces that stand for the FreeMarker Template Language (FTL) data types.
 * The template language only deals with {@link TemplateModel}-s, not directly with plain Java objects. (For example,
 * it doesn't understand {@link java.lang.Number}, but {@link TemplateNumberModel}.) This is why the
 * data-model (aka. the "template context" in other languages) is (automatically) mapped to a tree of
 * {@link TemplateModel}-s.
 * 
 * <p>Mapping the plain Java objects to {@link TemplateModel}-s (or the other way around sometimes) is the
 * responsibility of the {@link ObjectWrapper} (can be set via {@link Configuration#setObjectWrapper(ObjectWrapper)}).
 * But not all {@link TemplateModel}-s are for wrapping a plain object. For example, a value created within a template
 * is not made to wrap an earlier existing object; it's a value that has always existed in the template language's
 * domain. Users can also write {@link TemplateModel} implementations and put them directly into the data-model for
 * full control over how that object is seen from the template. Certain {@link TemplateModel} interfaces doesn't
 * even have equivalent in Java. For example the directive type ({@link TemplateDirectiveModel}) is like that.
 * 
 * <p>Because {@link TemplateModel} "subclasses" are all interfaces, a value in the template language can have multiple
 * types. However, to prevent ambiguous situations, it's not recommended to make values that implement more than one of
 * these types: string, number, boolean, date. The intended applications are like string+hash, string+method,
 * hash+sequence, etc.
 * 
 * @see ClassUtil#getFTLTypeDescription(TemplateModel)
 */
public interface TemplateModel {
    
    
A general-purpose object to represent nothing. It acts as
an empty string, false, empty sequence, empty hash, and
null-returning method model.
/**
     * A general-purpose object to represent nothing. It acts as
     * an empty string, false, empty sequence, empty hash, and
     * null-returning method model.
     */
    TemplateModel NOTHING = GeneralPurposeNothing.getInstance();
}