terraform, aws_sns_topic_policy use here doc for policy and interpolate variable inside heredoc

I wanted to update my sns target policy using terraform, below is my code. But my terraform plan is failing with below error.

Expected the start of an expression, but found an invalid expression token.
Error: Argument or block definition required

and the second query is how i can interpolate arn name inside the heredoc. Learning terraform so unsure what is going wrong.

resource "aws_sns_topic_policy" "default" {
  arn                   = aws_sns_topic.topic_name.arn
  policy              = <<EOF 
{
  "Version": "2008-10-17",
  "Id": "__default_policy_ID",
  "Statement": [
    {
      "Sid": "__default_statement_ID",
      "Effect": "Allow",
      "Principal": {
        "AWS": "*"
      },
      "Action": [
        "SNS:GetTopicAttributes",
        "SNS:SetTopicAttributes",
        "SNS:AddPermission",
        "SNS:RemovePermission",
        "SNS:DeleteTopic",
        "SNS:Subscribe",
        "SNS:ListSubscriptionsByTopic",
        "SNS:Publish"
      ],
      "Resource": "aws_sns_topic.topic_name.arn",
      "Condition": {
        "StringEquals": {
          "AWS:SourceOwner": "local.account_id"
        }
      }
    },
    {
      "Sid": "AWSEvents_Datasync",
      "Effect": "Allow",
      "Principal": {
        "Service": "events.amazonaws.com"
      },
      "Action": "sns:Publish",
      "Resource": "aws_sns_topic.topic_name.arn"
    }
  ]
}
EOF
}

>Solution :

You haven’t indicated to Terraform that you want to interpolate any of the values inside the heredoc, you have just added them all as plain strings. You need to wrap each interpolated value in ${} to interpolate those values:

resource "aws_sns_topic_policy" "default" {
  arn                   = aws_sns_topic.topic_name.arn
  policy              = <<EOF 
{
  "Version": "2008-10-17",
  "Id": "__default_policy_ID",
  "Statement": [
    {
      "Sid": "__default_statement_ID",
      "Effect": "Allow",
      "Principal": {
        "AWS": "*"
      },
      "Action": [
        "SNS:GetTopicAttributes",
        "SNS:SetTopicAttributes",
        "SNS:AddPermission",
        "SNS:RemovePermission",
        "SNS:DeleteTopic",
        "SNS:Subscribe",
        "SNS:ListSubscriptionsByTopic",
        "SNS:Publish"
      ],
      "Resource": "${aws_sns_topic.topic_name.arn}",
      "Condition": {
        "StringEquals": {
          "AWS:SourceOwner": "${local.account_id}"
        }
      }
    },
    {
      "Sid": "AWSEvents_Datasync",
      "Effect": "Allow",
      "Principal": {
        "Service": "events.amazonaws.com"
      },
      "Action": "sns:Publish",
      "Resource": "${aws_sns_topic.datasync_task_sns[0].arn}"
    }
  ]
}
EOF
}

Note that the AWS Terraform provider has a helper for creating policy documents and generating the policy JSON string. This is often cleaner to use and less error-prone than using heredocs.