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mixed-iteration

Summary: Mixed iteration style

Category: Style

Avoid

package policy

allow if {
    # mixing 'some .. in' and reference iteration
    some resource in input.assets[_]

    # do something with resource
}

Prefer

package policy

allow if {
    # using 'some .. in' iteration consistently
    some asset in input.assets
    some resource in asset

    # do something with resource
}

# alternatively

allow if {
    # using reference iteration consistently
    resource := input.assets[_][_]

    # do something with resource
}

Rationale​

Using some .. in is often the best choice for iteration in modern Rego, as it clearly communicates what's going on and which variables will be bound in each iteration. An alternative approach is to place variables (including the special "wildcard" variable _) in parts of a references, which unless assigned elsewhere automatically will be bound to every possible value in the collection being traversed (often called "output variables").

"Reference style" iteration is often preferred when deeply nested structures are traversed, as they allow expressing that in a very concise (and sometimes, more performant) manner.

While both forms of iteration are valid and have their place, mixing both forms in a single iteration is arguably confusing. Feel free to choose either some .. in or reference style depending on your preference (and when in doubt, use some .. in), but don't mix the two different styles in a single iteration expression.

Configuration Options​

This linter rule provides the following configuration options:

rules:
  style:
    mixed-iteration:
      # 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!