This Swift project is inspired by previous designs of rule engines in Java. Those engines were driven to a greater or lesser extent by the Dynamic Object Model pattern.
The goal of this project is to design a rule model that relies as much as possible on the Swift (static) type system.
Expressions are the building blocks of rules. Expressions may be composed to create e.g. rule predicates.
Example expressions include:
- basic types like Bool and Int
- arrays consisting of expressions
- key paths
- predicates
- functions.
Expressions adopt the Expression protocol. This protocol requires adopting types to provide an evaluation function:
func eval(in context: C) throws -> Eval
where Eval is an associated type.
Composition of expressions is based on the Eval return type of the eval function rather than on the expression types themselves. Evaluation of a composite expression typically results in a recursive descent evaluation strategy.
The core expression makes few assumptions on the evaluation context, except that key paths used in the expressions must be applicable to the evaluation context.
Most predicates provided by default have a limited number of operands. The classes UnaryOperation and BinaryOperation are just convience classes to re-use the operand properties and initialization.
Use of these classes is not required, however, and neither must predicates be implemented as classes.
Predicates are either:
- Bool values
- composite expressions with boolean Eval type.
Default predicates are defined in the Predicate enum namespace.
Functions allow us to transform expressions into other expressions.
Examples include:
- converting strings into upper- or lower-cased versions
- testing if an optional value is nil.
Default functions are defined in the Function enum namespace.
Expressions are evaluated in a certain context. To allow expressions to access data in the context, key paths can be used because type KeyPath adopts the Expression protocol.
The core expression model does not impose any specific requirement on the context types. Evaluating a key path in a given context checks if the context type and the key path's root type are compatible. If not, an invalidContext error is thrown.
Evaluating key paths is a run-time operation. Constructing key paths involves a compile-time step. This makes it difficult to create e.g. an expression builder allowing users to create expressions involving type-safe access to nested properties in the context. (The expression builder must also guarantee that the key path's value type is compatible with the containing expression.)
The Contextual protocol allows context types to specify building blocks of 'elementary' key paths, out of which more complex key paths can be constructed dynamically in a type-safe way.
The rule model is basically a composite structure consisting of basic types of rules and rule sets consisting of basic rules and nested rule sets.
Basic rules consist of validation rules and data rules to enrich or modify the evaluation context.
Condition-assertion rules allow us to validate input data in the context. Both condition and assertion are expressions.
The semantics is as follows:
- If the condition evaluates to false, the assertion is skipped and the evaluation returns nil. We say that the rule does not match the input data.
- If the condition evaluates to true, the evaluation of the rule is the result of evaluating the assertion. In this case the rule matches the input data.
A Rule set is an ordered collection of basic rules or (nested) rule sets. Like condition-assertion rules, rule sets have a condition (which is also an expression).
The semantics is as follows:
- If the condition evaluates to false, the rules are ignored and the evaluation returns nil. We say that the rule set does not match the input data.
- If the condition evaluates to true, the rule set matches the input data. The evaluation of the rule set is based on the evaluations of the rules in the rule set.
If none of the (nested) rules in the rule set match, the rule set evaluates to nil. This allows us to detect the case when no rule results in either true or false, and to define a proper default behavior.
What happens when one or more rules in a rule set match depends on the matchAll property of the rule set:
- If matchAll is true, the rule set returns true if all matching rules in the rule set evaluate to true, otherwise the result is false (lazy AND semantics).
- If matchAll is false, the rule set returns the result of the first matching rule.
The engine uses a trie structure to link a rule to a path of keys, for instance to the path segments of a URL.
Looking up rules uses two strategies:
- an exact match of the key path
- a longest prefix match.
Suppose that the engine has only one rule R registered with path ["a", "b", "c"].
Looking up a rule for path ["a", "b", "c", "d"] results in rule R when using longest prefix matching, but no rule when using exact match. Looking up a rule for path ["a", "b", "c"] results in rule R, regardless of the strategy.
The code has been tested with Swift 5.1 using XCode 11.2.1, and with Swift 5.2 using XCode 11.6.
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