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If your plan is to fail the program if `err` is non-nil, then you are already using `ApplyT` correctly:
```policyDocument := saml.Arn.ApplyT(yourFunction)```
If you want to handle the error and keep going, you will need to do so inside the `ApplyT`:
```policyDocument := saml.Arn.ApplyT(func(arn string) (*iam.GetPolicyDocumentResult, error) {
  doc, err := iam.GetPolicyDocument(ctx, &iam.GetPolicyDocumentArgs{
    Statements: []iam.GetPolicyDocumentStatement{
      {
        Principals: []iam.GetPolicyDocumentStatementPrincipal{
          {
            Type:        "Federated",
            Identifiers: []string{arn},
          },
        },
      },
    },
    if err != nil {
      return defaultDocument(arn)
     }
     return doc
  })
})```
The first option is more common in Pulumi programs, but both are available.