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zero-arity-function

Summary: Avoid functions without args

Category: Bugs

Avoid

package policy

first_user() := input.users[0]

Prefer

package policy

first_user := input.users[0]

Rationale​

Note: From Regal v0.39.0, this rule is no longer enabled by default. The reason for this is that OPA's formatter (opa fmt) already removes parentheses from zero-arity function definitions, and as such, this is already covered by the opa-fmt rule. The only time you might still want to enable this rule is if you don't use opa fmt in your workflow (you should!) and still want to enforce this.

Zero-arity functions, or functions without arguments, aren't treated as functions by Rego, but as regular rules. For that reason, they should also be expressed as such. One potential benefit of using functions over rules is that functions don't contribute to the document when a package is evaluated, and as such sometimes used to "hide" information from the result of evaluation. Whether this is a good practice or not, it importantly doesn't work with zero-arity functions, as they are treated as rules and do contribute to the document.

There is an open issue in the OPA project to try and address this in the future, and allow zero-arity functions to be treated as other functions. Until then, the recommendation is to avoid them and just use rules in their place.

Configuration Options​

This linter rule provides the following configuration options:

rules:
  bugs:
    zero-arity-function:
      # one of "error", "warning", "ignore"
      level: error