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external-reference

Summary: External reference in function

Category: Style

Avoid

package policy

# Depends on both `input` and `data`
is_preferred_login_method(method) if {
    preferred_login_methods := {login_method |
        some login_method in data.authentication.all_login_methods
        login_method in input.user.login_methods
    }
    method in preferred_login_methods
}

Prefer

package policy

# Depends only on function arguments
is_preferred_login_method(method, user, all_login_methods) if {
    preferred_login_methods := {login_method |
        some login_method in all_login_methods
        login_method in user.login_methods
    }
    method in preferred_login_methods
}

Rationale​

What separates functions from rules is that they accept arguments. While a function also may reference anything from input, data or other rules declared in a policy, these references create dependencies that aren't obvious simply by checking the function signature, and it makes it harder to reuse that function in other contexts. Additionally, functions that only depend on their arguments are easier to test standalone.

Exceptions​

Rego does not provide first-class functions — functions can't be passed as arguments to other functions. Therefore, this rule allows functions to freely reference (i.e. call) other functions, whether built-in functions, or custom functions defined in the same package or elsewhere, and these do not count as "external references" simply because there is not other way to import them into the function body.

package policy

first_name(full_name) := capitalized {
    first_name := split(full_name, " ")[0]

    # while data.utils.capitalize is an external reference, it's not flagged
    # as such, since there is no way to import it via function arguments
    capitalized := data.utils.capitalize(first_name)
}

Changed default behavior since Regal v0.33.0​

While we still consider it a best practice to pass any dependencies of a function in its arguments, the previous (non-configurable) default of not allowing any external references was often considered too distracting. This led to many disabling this rule entirely, or used inline ignore directives where this would be reported. Even in Regal's own policies, there were quite a few locations where the latter option was used.

From v0.33.0 and onwards, this rule's default has been relaxed to allow 2 external references in any given function definition, and the new max-allowed configuration option allows changing this value to whatever feels like a reasonable default. If you previously disabled this rule in your projects, consider enabling it again configured to match your preference.

Configuration Options​

This linter rule provides the following configuration options:

rules:
  style:
    external-reference:
      # one of "error", "warning", "ignore"
      level: error
      # the number of external references to allow for any given function
      #
      # introduced in v0.33.0 and defaults to 2. set to 0 to revert to
      # original behavior to not allow any external references
      max-allowed: 2

Community​

If you think you've found a problem with this rule or its documentation, would like to suggest improvements, new rules, or just talk about Regal in general, please join us in the #regal channel in the Styra Community Slack!