Transformation Errors

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Transformation errors are returned in the response body to your application. The body is a JSON structure with a format resembling this example:

json

{
  "error": "Received invalid transformation. Does not type check",
  "message": {
    "transformation_error?": true,
    "expression": ["not_a_real_function" 1],
    "type_errors" ["Function "not_a_real_function" does not exist"]
  },
  "request-uuid": "a-unique-identifier-for-the-request"
}

Error	Explanation	Additional `message` fields
Received invalid transformation. Does not type check	One or more type errors were found.	`type_errors`: An array of type errors
Transformations must return one of boolean, nil, number, string	All Forward API functions must return a type that can be serialized.	`return_type`: The type currently returned by the transformation
Invalid expiration year	The year argument passed to the `format_expiration` function was a valid string but not a valid year.	`expiration_year`: The invalid value passed for the year
Invalid expiration month	The month argument passed to the `format_expiration` function was a valid string but not a valid month (01 - 12).	`expiration_month`: The invalid value passed for the month
`encode` received an unsupported charset	The first argument (charset) was not a valid character encoding.	`charset`: the provided charset
`integer` received a non-numeric string	The argument was a valid string but could not be parsed as an integer.	None
slice index range out of bounds for value length	The provided substring start and end index are out of bounds for the given string value	`slice-start`: given start index (inclusive)

slice-stop: given end index (exclusive) value-string: given string value value-length: given string length

Type errors

Before the Forward API performs a transformation, it first type checks the transformation to ensure correctness.

All type errors have Received invalid transformation. Does not type check as the error field. In the message they all include "transformation_error?": true and the triggering transformation as the expression, so those fields aren't listed below.

The type_errors field contains a list of errors detected by the type checker. There are three categories of type error:

Type error category	Example type error	Explanation
Invalid arity	Function "join" expects 2 arguments but received 1 of them.	The function was given an unexpected number of arguments.
Invalid argument type	"join" arg 0: expected string but received number	A function was called with an incorrect argument type. Arguments are 0-indexed.
Invalid function name	Function "not_a_real_function" does not exist	A function that doesn't exist was called.

A transformation may return multiple type errors if multiple arguments have invalid types or if both arity and argument type(s) are invalid.