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use-assignment-operator

Summary: Prefer := over = for assignment

Category: Style

Avoid

package policy

import rego.v1

default allow = false

first_name(name) = split(name, " ")[0]

allow if {
    username = input.user.name
    # .. more conditions ..
}

Prefer

package policy

import rego.v1

default allow := false

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

allow {
    username := input.user.name
    # .. more conditions ..
}

Rationale​

Rego has three operators related to assignment and equality:

  • := is the assignment operator, and is only used to assign values to variables
  • == is the equality operator, and is only used to compare values
  • = is the unification operator, and is used both to assign values to variables and compare values

While it often is "harmless" to use the unification operator (=) for assignment, the assignment operator (:=) removes any ambiguities around intent, and prevents some hard to debug issues. Consider:

allow {
    username = input.user.name
    # .. more conditions ..
}

Using the unification operator, username is either assigned (if username isn't defined elsewhere in the policy) or being checked for equality (if username is defined elsewhere in the policy). Using := for assignment, and == for equality comparison removes this ambiguity and make the intent obvious.

In some cases, = and := may be used interchangeably, as the result is the same either way:

first_name(full_name) = split(full_name, " ")[0]
# same as
first_name(full_name) := split(full_name, " ")[0]

Even when that is the case, using := consistently should be considered a best practice.

Configuration Options​

This linter rule provides the following configuration options:

rules:
  style:
    use-assignment-operator:
      # one of "error", "warning", "ignore"
      level: error

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!