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Coko Rules

Rules in Coko describe how an API should be used. They are functions that are annotated with the @Rule annotation.

In the @Rule annotation you can specify metadata about the rule such as the description of the rule. The metadata will be used for describing the findings in the SARIF output.

If the rule requires some instance of a model, they can be specified as parameters to the rule function.

Each Coko rule must return an implementation of the Evaluator interface, which Codyze can use to evaluate the rule. Coko provides some common evaluators which will be explained in the following sections. The example model will be used for explaining the evaluators.

Example model
class Foo {
    fun constructor() = constructor("Foo") {

    fun first(i: Any?) = op {
        definition("Foo.first") {

    fun second(s: Any?) = op {
        definition("Foo.second") {

class Bar {
    fun second() = op {
        definition("Bar.second") {

Only Evaluator

The only evaluator checks if all calls to an Op are only called with the specified arguments. Therefore, it takes one Op as argument.

Rule example using only
fun `only calls to first with 1 allowed`(foo: Foo) = 

Order Evaluator

The order evaluator checks if functions related to an object are called in the correct order. It takes two arguments, the baseNodes and the order. The baseNodes are the function calls that are the start of the order. Usually, this is either the constructor of a class or some kind of initialization function.

To construct the order, Coko provides a type-safe builder. Within the builder, the order is specified as a regular expression.

The "alphabet" of the order regex is:

If all calls to a modelled function should be considered for the order regardless of the specified signatures, please use the first option. When passing Ops, only functions that match the used signature and argument are considered valid.

The builder provides a set of functions that allow you to add quantifiers to the regex or group them.

Function Regex Description
or | Represents a choice, either the first or the second expression has to be matched
set [] Represents multiple choices, one expression in it has to be matched
maybe * Matches an expression zero or more times
some + Matches an expression one or more times
option ? Matches an expression zero or one time
count(n) {n} Matches an expression exactly n times
atLeast(min) {min,} Matches an expression at least min times
between(min, max) {min, max} Matches an expression at least min and at most max times
Rule example using order
fun `order of Foo`(foo: Foo) = 
    order(foo.constructor()/* (2)! */) { // (1)!
        - foo.first(...) // (3)!
        maybe(foo::second) // (4)!
  1. This starts the type-safe builder for the order.
  2. The Op returned from foo.constructor will be used as query for the function calls that are the starting point for evaluating the order.
  3. This will use the filtered Op returned by foo.first(...) for the order.
  4. This will consider all calls to the function modelled by foo.second() for the order. No filter will be applied.

FollowedBy Evaluator

The followedBy evaluator works similarly like the implication in logic. It takes two Ops and specifies that if the first Op is called then the second Op must be called as well. Compared to the order evaluator, followedBy is more flexible because Ops from different models can be connected.

Rule example using followedBy
fun `if first then second`(foo: Foo, bar: Bar) = 
    foo.first(Wildcard) followedBy bar.second()

Never Evaluator

The never evaluator is used to specify that calls to an Op with the specified arguments are forbidden. It takes one Op as argument.

Rule example using never
fun `never call second with 1`(foo: Foo) =

Last update: 2023-06-06
Created: 2023-06-06