add isTemplate function

libraries/botbuilder-lg/src/evaluator.ts

@@ -38,6 +38,7 @@ export class Evaluator extends AbstractParseTreeVisitor<any> implements LGFilePa
     public static readonly activityAttachmentFunctionName = 'ActivityAttachment';
     public static readonly fromFileFunctionName = 'fromFile';
     public static readonly templateFunctionName = 'template';
+    public static readonly isTemplateFunctionName = 'isTemplate';
 
     public constructor(templates: LGTemplate[], expressionEngine: ExpressionEngine) {
         super();
@@ -399,6 +400,10 @@ export class Evaluator extends AbstractParseTreeVisitor<any> implements LGFilePa
             return new ExpressionEvaluator(Evaluator.activityAttachmentFunctionName, BuiltInFunctions.apply(this.activityAttachment()), ReturnType.Object, this.validateActivityAttachment);
         }
 
+        if (name === Evaluator.isTemplateFunctionName) {
+            return new ExpressionEvaluator(Evaluator.isTemplateFunctionName, BuiltInFunctions.apply(this.isTemplate()), ReturnType.Boolean, this.validateIsTemplate);
+        }
+
         return baseLookup(name);
     }
 
@@ -419,6 +424,22 @@ export class Evaluator extends AbstractParseTreeVisitor<any> implements LGFilePa
         });
     }
 
+    private readonly isTemplate = (): any => (args: readonly any[]): boolean => {
+        const templateName = args[0].toString();
+        return templateName in this.templateMap;
+    }
+
+    private readonly validateIsTemplate = (expression: Expression): void => {
+        if (expression.children.length !== 1) {
+            throw new Error(`isTemplate should have one parameter`);
+        }
+
+        const children0: Expression = expression.children[0];
+        if (children0.returnType !== ReturnType.Object && children0.returnType !== ReturnType.String) {
+            throw new Error(`${ children0 } can't be used as a template name, must be a string value`);
+        }
+    }
+
     private readonly fromFile = (): any => (args: readonly any[]): any => {
         const filePath: string = path.normalize(ImportResolver.normalizePath(args[0].toString()));
         const resourcePath: string = this.getResourcePath(filePath);

libraries/botbuilder-lg/tests/lg.test.js

@@ -786,4 +786,17 @@ describe('LG', function() {
         var evaled = engine.evaluateTemplate('template', {list:[{}], obj : {a : 'a'}});
         assert.strictEqual(evaled, 'list and obj are both not empty.');        
     });
+
+    it('TestIsTemplateFunction', function() {
+        var engine = new TemplateEngine().addFile(GetExampleFilePath('IsTemplate.lg'));
+
+        var evaled = engine.evaluateTemplate('template2', {templateName:'template1'});
+        assert.strictEqual(evaled, 'template template1 exists');
+
+        var evaled = engine.evaluateTemplate('template2', {templateName:'wPhrase'});
+        assert.strictEqual(evaled, 'template wPhrase exists');
+
+        var evaled = engine.evaluateTemplate('template2', {templateName:'xxx'});
+        assert.strictEqual(evaled, 'template xxx does not exist'); 
+    });
 });

libraries/botbuilder-lg/tests/testData/examples/IsTemplate.lg

@@ -0,0 +1,10 @@
+[import](2.lg)
+
+# template1
+- hi
+
+# template2(templateName)
+- IF: ${isTemplate(templateName)}
+- template ${templateName} exists
+- ELSE:
+- template ${templateName} does not exist