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defer-assignment

Summary: Assignment can be deferred

Category: Performance

Avoid

package policy

allow if {
    resp := http.send({"method": "GET", "url": "http://example.com"})

    # this check does not depend on the response above
    # and thus the resp := ... assignment can be deferred to
    # after the check
    input.user.name in allowed_users

    resp.status_code == 200

    # more done with response here
}

Prefer

package policy

allow if {
    input.user.name in allowed_users

    # the next expression *does* depend on `resp`
    resp := http.send({"method": "GET", "url": "http://example.com"})

    resp.status_code == 200

    # more done with response here
}

Rationale​

Assignments are normally cheap, but certainly not always. If the right-hand side of an assignment is expensive, deferring the assignment to where it's needed can save a considerable amount of time. Even for less expensive assignments, code tends to be more readable when assignments are placed close to where they're used.

This rule uses a fairly simplistic heuristic to determine if an assignment can be deferred:

  • The next expression is not an assignment
  • The next expression does not depend on the assignment
  • The next expression does not initialize iteration

It is possible that the rule will be improved to cover more cases in the future.

Configuration Options​

This linter rule provides the following configuration options:

rules:
  performance:
    defer-assignment:
      # 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!