set the default for payload to a Lambda to use the state input

packages/@aws-cdk/aws-stepfunctions-tasks/lib/lambda/run-lambda-task.ts

@@ -10,7 +10,7 @@ export interface RunLambdaTaskProps {
   /**
    * The JSON that you want to provide to your Lambda function as input.
    *
-   * @default - No payload
+   * @default - The state input (JSON path '$')
    */
   readonly payload?: sfn.TaskInput;
 
@@ -92,7 +92,7 @@ export class RunLambdaTask implements sfn.IStepFunctionsTask {
       metricDimensions: { LambdaFunctionArn: this.lambdaFunction.functionArn },
       parameters: {
         FunctionName: this.lambdaFunction.functionName,
-        Payload: this.props.payload?.value,
+        Payload: this.props.payload ? this.props.payload.value : sfn.TaskInput.fromDataAt('$').value,
         InvocationType: this.props.invocationType,
         ClientContext: this.props.clientContext,
         Qualifier: this.props.qualifier,

packages/@aws-cdk/aws-stepfunctions-tasks/test/lambda/integ.run-lambda.expected.json

@@ -85,7 +85,7 @@
       "Type": "AWS::Lambda::Function",
       "Properties": {
         "Code": {
-          "ZipFile": "exports.handler = async function(event, context) {\n        return {\n          status: event.statusCode === '200' ? 'SUCCEEDED' : 'FAILED';\n        };\n  };"
+          "ZipFile": "exports.handler = async function(event, context) {\n        return {\n          status: event.statusCode === '200' ? 'SUCCEEDED' : 'FAILED'\n        };\n  };"
         },
         "Handler": "index.handler",
         "Role": {
@@ -175,15 +175,15 @@
               {
                 "Ref": "submitJobLambdaEFB00F3C"
               },
-              "\"},\"OutputPath\":\"$.Payload\",\"Type\":\"Task\",\"Resource\":\"arn:",
+              "\",\"Payload.$\":\"$\"},\"OutputPath\":\"$.Payload\",\"Type\":\"Task\",\"Resource\":\"arn:",
               {
                 "Ref": "AWS::Partition"
               },
               ":states:::lambda:invoke\"},\"Check the job state\":{\"Next\":\"Job Complete?\",\"Parameters\":{\"FunctionName\":\"",
               {
                 "Ref": "checkJobStateLambda4618B7B7"
               },
-              "\",\"Payload.$\":\"$.Payload\"},\"OutputPath\":\"$.Payload\",\"Type\":\"Task\",\"Resource\":\"arn:",
+              "\",\"Payload.$\":\"$\"},\"OutputPath\":\"$.Payload\",\"Type\":\"Task\",\"Resource\":\"arn:",
               {
                 "Ref": "AWS::Partition"
               },

packages/@aws-cdk/aws-stepfunctions-tasks/test/lambda/integ.run-lambda.ts

@@ -5,8 +5,8 @@ import * as tasks from '../../lib';
 
 /*
  * Stack verification steps:
- * The generated State Machine can be executed from the Step Functions console (or CLI)
- * and run with an execution status of `Succeeded`.
+ * The generated State Machine can be executed from the CLI (or Step Functions console)
+ * and runs with an execution status of `Succeeded`.
  *
  * -- aws stepfunctions start-execution --state-machine-arn  <deployed state machine arn> provides execution arn
  * -- aws stepfunctions describe-execution --execution-arn <from previous command> returns a status of `Succeeded`
@@ -33,17 +33,15 @@ const submitJob = new sfn.Task(stack, 'Invoke Handler', {
 const checkJobStateLambda = new Function(stack, 'checkJobStateLambda', {
   code: Code.fromInline(`exports.handler = async function(event, context) {
         return {
-          status: event.statusCode === '200' ? 'SUCCEEDED' : 'FAILED';
+          status: event.statusCode === '200' ? 'SUCCEEDED' : 'FAILED'
         };
   };`),
   runtime: Runtime.NODEJS_10_X,
   handler: 'index.handler',
 });
 
 const checkJobState = new sfn.Task(stack, 'Check the job state', {
-  task: new tasks.RunLambdaTask(checkJobStateLambda, {
-    payload: sfn.TaskInput.fromDataAt('$.Payload'),
-  }),
+  task: new tasks.RunLambdaTask(checkJobStateLambda),
   outputPath: '$.Payload',
 });
 

packages/@aws-cdk/aws-stepfunctions-tasks/test/lambda/run-lambda-task.test.ts

@@ -98,7 +98,39 @@ test('Lambda function can be used in a Task with Task Token', () => {
   });
 });
 
-test('Lambda function can be provided with the task input as the payload', () => {
+test('Lambda function is invoked with the state input as payload by default', () => {
+  const task = new sfn.Task(stack, 'Task', {
+    task: new tasks.RunLambdaTask(fn),
+  });
+  new sfn.StateMachine(stack, 'SM', {
+    definition: task,
+  });
+
+  expect(stack.resolve(task.toStateJson())).toEqual({
+    Type: 'Task',
+    Resource: {
+      'Fn::Join': [
+        '',
+        [
+          'arn:',
+          {
+            Ref: 'AWS::Partition',
+          },
+          ':states:::lambda:invoke',
+        ],
+      ],
+    },
+    End: true,
+    Parameters: {
+      'FunctionName': {
+        Ref: 'Fn9270CBC0',
+      },
+      'Payload.$': '$',
+    },
+  });
+});
+
+test('Lambda function can be provided with the state input as the payload', () => {
   const task = new sfn.Task(stack, 'Task', {
     task: new tasks.RunLambdaTask(fn, {
       payload: sfn.TaskInput.fromDataAt('$'),

packages/@aws-cdk/aws-stepfunctions/README.md

@@ -162,18 +162,14 @@ similar to (for example) `inputPath`.
 [Invoke](https://docs.aws.amazon.com/lambda/latest/dg/API_Invoke.html) a Lambda function.
 
 You can specify the input to your Lambda function through the `payload` attribute.
-By default, no payload is specified so Step Functions invokes Lambda with the empty
-object `{ }` as input.
+By default, Step Functions invokes Lambda function with the state input (JSON path '$')
+as the input.
 
 The following snippet invokes a Lambda Function with the state input as the payload
 by referencing the `$` path.
 
 ```ts
-new sfn.Task(this, 'Invoke with task context', {
-  task: new tasks.RunLambdaTask(myLambda, {
-    payload: sfn.TaskInput.fromDataAt('$'),
-  }),
-});
+new sfn.Task(this, 'Invoke with state input');
 ```
 
 When a function is invoked, the Lambda service sends  [these response
@@ -186,8 +182,10 @@ The following snippet invokes a Lambda Function by referencing the `$.Payload` p
 to reference the output of a Lambda executed before it.
 
 ```ts
-new sfn.Task(this, 'Invoke with empty payload', {
-  task: new tasks.RunLambdaTask(myLambda),
+new sfn.Task(this, 'Invoke with empty object as payload', {
+  task: new tasks.RunLambdaTask(myLambda, {
+    payload: sfn.TaskInput.fromObject({})
+  }),
 });
 
 new sfn.Task(this, 'Invoke with payload field in the state input', {