prefer-value-in-head

Summary: Prefer value in rule head

Category: Custom

Avoid

package policy

import rego.v1

pin_as_number := val if {
    is_number(input.pin_code)
    val := to_number(input.pin_code)
}

deny contains message if {
    not input.user
    message := "user attribute missing from input"
}

Prefer

package policy

import rego.v1

pin_as_number := to_number(input.pin_code) if is_number(input.pin_code)

deny contains "user attribute missing from input" if not input.user

Rationale

Rules that return the value assigned in the last expression of the rule body may have the value, or the function returning the value, moved directly to the rule head. This creates more succinct rules, and often allows for rules to be expressed as "one-liners". This is not a general recommendation, but a style preference that a team or organization might want to standardize on. As such, it is placed in the custom category, and must be explicitly enabled in configuration.

The only-scalars configuration option may be used to only suggest moving scalar values (strings, numbers, booleans, null) to the head, and not expressions or functions returning a value. With this option set to true, the following example would be flagged:

deny contains message if {
    not input.user
    # value is a scalar
    message := "user attribute missing from input"
}

But not:

deny contains message if {
    not input.user
    # value returned from a function call, not suggested if `only-scalars` is set to `true`
    message := sprintf("user attribute missing from input: %v", [input])
}

Configuration Options

This linter rule provides the following configuration options:

rules:
  custom:
    prefer-value-in-head:
      # note that all rules in the "custom" category are disabled by default
      # (i.e. level "ignore")
      #
      # one of "error", "warning", "ignore"
      level: error
      # whether to only suggest moving scalar values (strings, numbers, booleans, null)
      # to the head, and not expressions or functions
      only-scalars: false

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!