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function-arg-return

Summary: Return value assigned in function argument

Category: Style

Avoid

package policy

import rego.v1

has_email(user) if {
    indexof(user.email, "@", i)
    i != -1
}

Prefer

package policy

import rego.v1

has_email(user) if {
    i := indexof(user.email, "@")
    i != -1
}

Rationale​

Older Rego policies sometimes contain an unusual way to declare where the return value of a function call should be stored — the last argument of the function. True to its Datalog roots, return values may be stored either using assignment (i.e. :=) or by appending a variable name to the argument list of a function. While both forms are valid, using assignment := consistently is preferred.

Exceptions​

The walk built-in function is a special one, as it's the only one producing a relation. Therefore, it is okay to treat it as one even in style, and:

walk(object, [path, value])

is arguably more idiomatic than:

[path, value] := walk(object)

Configuration Options​

This linter rule provides the following configuration options:

rules:
  style:
    function-arg-return:
      # one of "error", "warning", "ignore"
      level: error
      # list of function names to ignore
      # * by default, walk is excepted from this rule
      # * note that `print` is always ignored as it does not return a value
      except-functions:
        - walk

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!