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rego_type_error: {built-in name} arity mismatch

Arity is a term used to describe the number of arguments a function takes. This error happens when one of Rego's built-in functions is called with an unexpected number of arguments - the wrong arity.


This message is only shown for Rego's built-in functions, not for user-defined functions. For errors containing has arity n, got m argument(s) see this section.

compilationrego_type_error{built-in name} arity mismatch


In the following example, the split function is called with two arguments, but it only takes one:

package policy

import future.keywords.if

allow if "admin" in split("admin,member")

When compiled, this will result in the following error:

1 error occurred: policy.rego:6: rego_type_error: split: arity mismatch
have: (string, ???)
want: (x: string, delimiter: string)

How To Fix It

In order to find the built-in function that is causing this error, you can find the line number in the error message. E.g. in the example above, the error message says:

policy.rego:6: rego_type_error: split: arity mismatch
^ ^ ^
file line number built-in function

Here, the line number is 6 and the built-in function is split.

Once you have found the built-in function, you can look up its expected arguments and arity in the 'Policy Reference' documentation.


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