# Rule based model configuration

Support for rule based configuration is currently incubating. Please be aware that the DSL, APIs and other configuration may change in later Gradle versions.

Rule based model configuration enables configuration logic to itself have dependencies on other elements of configuration, and to make use of the resolved states of those other elements of configuration while performing its own configuration.

## Background

Rule based model configuration facilitates easier domain modelling: communicating intent (i.e. the what) over mechanics (i.e. the how). Domain modelling is a core tenet of Gradle and provides Gradle with several advantages over prior generation build tools such as Apache Ant that focus on the execution model. It allows humans to understand builds at a level that is meaningful to them.

As well as helping humans, a strong domain model also helps the dutiful machines. Plugins can more effectively collaborate around a strong domain model (e.g. plugins can say something about Java applications, such as providing conventions). Very importantly, by having a model of the what instead of the how Gradle can make intelligent choices on just how to do the how.

Gradle’s support for building native software and Play Framework applications already uses this configuration model. Gradle also includes some initial support for building Java libraries using this configuration model.

## Motivations for change

Domain modelling in Gradle isn’t new. The Java plugin’s `SourceSet` concept is an example of domain modelling, as is the modelling of `NativeBinary` in the native plugin suite.

A distinguishing characteristic of Gradle compared to other build tools that also embrace modelling is that Gradle’s model is open and collaborative. Gradle is fundamentally a tool for modelling software construction and then realizing the model, via tasks such as compilation etc. Different domain plugins (e.g. Java, C++, Android) provide models that other plugins can collaborate with and build upon.

While Gradle has long employed sophisticated techniques when it comes to realizing the model (i.e. what we know as building code), the next generation of Gradle builds will employ some of the same techniques to creation of the model itself. By defining build tasks as effectively a graph of dependent functions with explicit inputs and outputs, Gradle is able to order, cache, parallelize and apply other optimizations to the work. Using a “graph of tasks” for the production of software is a long established idea, and necessary given the complexity of software production. The task graph effectively defines the rules of execution that Gradle must follow. The term “Rule based model configuration” refers to applying the same concepts to building the model that builds the task graph.

Another key motivation is performance and scale. Aspects of the current approach that Gradle takes to modelling the build reduce parallelism opportunities and limit scalability. The software model is being designed with the requirements of modern software delivery in mind, where immediate responsiveness is critical for projects large and small.

## Basic Concepts

### The “model space”

The term “model space” is used to refer to the formal model, which can be read and modified by rules.

A counterpart to the model space is the “project space”, which should be familiar to readers. The “project space” is a graph of objects (e.g `project.repositories`, `project.tasks` etc.) having a `Project` as its root. A build script is effectively adding and configuring objects of this graph. For the most part, the “project space” is opaque to Gradle. It is an arbitrary graph of objects that Gradle only partially understands.

Each project also has its own model space, which is distinct from the project space. A key characteristic of the “model space” is that Gradle knows much more about it (which is knowledge that can be put to good use). The objects in the model space are “managed”, to a greater extent than objects in the project space. The origin, structure, state, collaborators and relationships of objects in the model space are first class constructs. This is effectively the characteristic that functionally distinguishes the model space from the project space: the objects of the model space are defined in ways that Gradle can understand them intimately, as opposed to an object that is the result of running relatively opaque code. A “rule” is effectively a building block of this definition.

The model space will eventually replace the project space, becoming the only “space”.

### Rules

The model space is defined by “rules”. A rule is just a function (in the abstract sense) that either produces a model element, or acts upon a model element. Every rule has a single subject and zero or more inputs. Only the subject can be changed by a rule, while the inputs are effectively immutable.

Gradle guarantees that all inputs are fully “realized“ before the rule executes. The process of “realizing” a model element is effectively executing all the rules for which it is the subject, transitioning it to its final state. There is a strong analogy here to Gradle’s task graph and task execution model. Just as tasks depend on each other and Gradle ensures that dependencies are satisfied before executing a task, rules effectively depend on each other (i.e. a rule depends on all rules whose subject is one of the inputs) and Gradle ensures that all dependencies are satisfied before executing the rule.

Model elements are very often defined in terms of other model elements. For example, a compile task’s configuration can be defined in terms of the configuration of the source set that it is compiling. In this scenario, the compile task would be the subject of a rule and the source set an input. Such a rule could configure the task subject based on the source set input without concern for how it was configured, who it was configured by or when the configuration was specified.

There are several ways to declare rules, and in several forms.

## Rule sources

One way to define rules is via a `RuleSource` subclass. If an object extends RuleSource and contains any methods annotated by '@Mutate', then each such method defines a rule. For each such method, the first argument is the subject, and zero or more subsequent arguments may follow and are inputs of the rule.

Example 493. applying a rule source plugin

`build.gradle`

    @Managed
    interface Person {
      void setFirstName(String name)
      String getFirstName()

      void setLastName(String name)
      String getLastName()
    }

    class PersonRules extends RuleSource {
      @Model void person(Person p) {}

      //Create a rule that modifies a Person and takes no other inputs
      @Mutate void setFirstName(Person p) {
        p.firstName = "John"
      }

      //Create a rule that modifies a ModelMap<Task> and takes as input a Person
      @Mutate void createHelloTask(ModelMap<Task> tasks, Person p) {
        tasks.create("hello") {
          doLast {
            println "Hello $p.firstName $p.lastName!"
          }
        }
      }
    }

    apply plugin: PersonRules

Output of `gradle hello`

    > gradle hello
    :hello
    Hello John Smith!

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

Each of the different methods of the rule source are discrete, independent rules. Their order, or the fact that they belong to the same class, do not affect their behavior.

Example 494. a model creation rule

`build.gradle`

    @Model void person(Person p) {}

This rule declares that there is a model element at path `"person"` (defined by the method name), of type `Person`. This is the form of the `Model` type rule for `Managed` types. Here, the person object is the rule subject. The method could potentially have a body, that mutated the person instance. It could also potentially have more parameters, which would be the rule inputs.

Example 495. a model mutation rule

`build.gradle`

    //Create a rule that modifies a Person and takes no other inputs
    @Mutate void setFirstName(Person p) {
      p.firstName = "John"
    }

This `Mutate` rule mutates the person object. The first parameter to the method is the subject. Here, a by-type reference is used as no `Path` annotation is present on the parameter. It could also potentially have more parameters, that would be the rule inputs.

Example 496. creating a task

`build.gradle`

    //Create a rule that modifies a ModelMap<Task> and takes as input a Person
    @Mutate void createHelloTask(ModelMap<Task> tasks, Person p) {
      tasks.create("hello") {
        doLast {
          println "Hello $p.firstName $p.lastName!"
        }
      }
    }

This `Mutate` rule effectively adds a task, by mutating the tasks collection. The subject here is the `"tasks"` node, which is available as a `ModelMap` of `Task`. The only input is our person element. As the person is being used as an input here, it will have been realised before executing this rule. That is, the task container effectively depends on the person element. If there are other configuration rules for the person element, potentially specified in a build script or other plugin, they will also be guaranteed to have been executed.

As `Person` is a `Managed` type in this example, any attempt to modify the person parameter in this method would result in an exception being thrown. Managed objects enforce immutability at the appropriate point in their lifecycle.

Rule source plugins can be packaged and distributed in the same manner as other types of plugins (see Writing Custom Plugins). They also may be applied in the same manner (to project objects) as `Plugin` implementations (i.e. via `Project.apply(java.util.Map)`).

Please see the documentation for `RuleSource` for more information on constraints on how rule sources must be implemented and for more types of rules.

## Advanced Concepts

### Model paths

A model path identifies the location of an element relative to the root of its model space. A common representation is a period-delimited set of names. For example, the model path `"tasks"` is the path to the element that is the task container. Assuming a task whose name is `hello`, the path `"tasks.hello"` is the path to this task.

### Managed model elements

Currently, any kind of Java object can be part of the model space. However, there is a difference between “managed” and “unmanaged” objects.

A “managed” object is transparent and enforces immutability once realized. Being transparent means that its structure is understood by the rule infrastructure and as such each of its properties are also individual elements in the model space.

An “unmanaged” object is opaque to the model space and does not enforce immutability. Over time, more mechanisms will be available for defining managed model elements culminating in all model elements being managed in some way.

Managed models can be defined by attaching the `@Managed` annotation to an interface:

Example 497. a managed type

`build.gradle`

    @Managed
    interface Person {
      void setFirstName(String name)
      String getFirstName()

      void setLastName(String name)
      String getLastName()
    }

By defining a getter/setter pair, you are effectively declaring a managed property. A managed property is a property for which Gradle will enforce semantics such as immutability when a node of the model is not the subject of a rule. Therefore, this example declares properties named firstName and lastName on the managed type Person. These properties will only be writable when the view is mutable, that is to say when the Person is the subject of a `Rule` (see below the explanation for rules).

Managed properties can be of any scalar type. In addition, properties can also be of any type which is itself managed:

Property type | Nullable | Example
--- | --- | ---
`String` | Yes | Example 498. a String property `build.gradle`
    void setFirstName(String name)
    String getFirstName()
`File` | Yes | Example 499. a File property `build.gradle`
    void setHomeDirectory(File homeDir)
    File getHomeDirectory()
`Integer`, `Boolean`, `Byte`, `Short`, `Float`, `Long`, `Double` | Yes | Example 500. a Long property `build.gradle`
    void setId(Long id)
    Long getId()
`int`, `boolean`, `byte`, `short`, `float`, `long`, `double` | No | Example 501. a boolean property `build.gradle`
    void setEmployed(boolean isEmployed)
    boolean isEmployed()
Example 502. an int property `build.gradle`
    void setAge(int age)
    int getAge()
Another managed type. | Only if read/write | Example 503. a managed property `build.gradle`
    void setMother(Person mother)
    Person getMother()
An enumeration type. | Yes | Example 504. an enumeration type property `build.gradle`
    void setMaritalStatus(MaritalStatus status)
    MaritalStatus getMaritalStatus()
A `ManagedSet`. A managed set supports the creation of new named model elements, but not their removal. | Only if read/write | Example 505. a managed set `build.gradle`
    ModelSet<Person> getChildren()
A `Set` or `List` of scalar types. All classic operations on collections are supported: add, remove, clear…​ | Only if read/write | Example 506. a scalar collection `build.gradle`
    void setUserGroups(List<String> groups)
    List<String> getUserGroups()

If the type of a property is itself a managed type, it is possible to declare only a getter, in which case you are declaring a read-only property. A read-only property will be instantiated by Gradle, and cannot be replaced with another object of the same type (for example calling a setter). However, the properties of that property can potentially be changed, if, and only if, the property is the subject of a rule. If it’s not the case, the property is immutable, like any classic read/write managed property, and properties of the property cannot be changed at all.

Managed types can be defined out of interfaces or abstract classes and are usually defined in plugins, which are written either in Java or Groovy. Please see the `Managed` annotation for more information on creating managed model objects.

### Model element types

There are particular types (language types) supported by the model space and can be generalised as follows:

Table 98. Type definitions

Type | Definition
Scalar | A scalar type is one of the following: a primitive type (e.g. `int`) or its boxed type (e.g `Integer`) a `BigInteger` or `BigDecimal` a `String` a `File` an enumeration type
Scalar Collection | A java.util.List or java.util.Set containing one of the scalar types
Managed type | Any class which is a valid managed model (i.e.annotated with @`Managed`)
Managed collection | A `ModelMap` or `ModelSet`

There are various contexts in which these types can be used:

Table 99. Model type support

Context | Supported types
Creating top level model elements | Any managed type `FunctionalSourceSet` (when the `LanguageBasePlugin` plugin has been applied) Subtypes of `LanguageSourceSet` which have been registered via `ComponentType`
Properties of managed model elements | The properties (attributes) of a managed model elements may be one or more of the following: A managed type A type which is annotated with @`Unmanaged` A Scalar Collection A Managed collection containing managed types A Managed collection containing `FunctionalSourceSet`'s (when the `LanguageBasePlugin` plugin has been applied) Subtypes of `LanguageSourceSet` which have been registered via `ComponentType`

### Language source sets

`FunctionalSourceSet`s and subtypes of `LanguageSourceSet` (which have been registered via `ComponentType`) can be added to the model space via rules or via the model DSL.

Example 507. strongly modelling sources sets

`build.gradle`

    apply plugin: 'java-lang'

    //Creating LanguageSourceSets via rules
    class LanguageSourceSetRules extends RuleSource {
        @Model
        void mySourceSet(JavaSourceSet javaSource) {
            javaSource.source.srcDir("src/main/my")
        }
    }
    apply plugin: LanguageSourceSetRules

    //Creating LanguageSourceSets via the model DSL
    model {
        another(JavaSourceSet) {
            source {
                srcDir "src/main/another"
            }
        }
    }

    //Using FunctionalSourceSets
    @Managed
    interface SourceBundle {
        FunctionalSourceSet getFreeSources()
        FunctionalSourceSet getPaidSources()
    }
    model {
        sourceBundle(SourceBundle) {
            freeSources.create("main", JavaSourceSet)
            freeSources.create("resources", JvmResourceSet)
            paidSources.create("main", JavaSourceSet)
            paidSources.create("resources", JvmResourceSet)
        }
    }

Note: The code for this example can be found at `samples/modelRules/language-support` in the ‘-all’ distribution of Gradle.

Output of `gradle help`

    > gradle help
    :help

### References, binding and scopes

As previously mentioned, a rule has a subject and zero or more inputs. The rule’s subject and inputs are declared as “references” and are “bound” to model elements before execution by Gradle. Each rule must effectively forward declare the subject and inputs as references. Precisely how this is done depends on the form of the rule. For example, the rules provided by a `RuleSource` declare references as method parameters.

A reference is either “by-path” or “by-type”.

A “by-type” reference identifies a particular model element by its type. For example, a reference to the `TaskContainer` effectively identifies the `"tasks"` element in the project model space. The model space is not exhaustively searched for candidates for by-type binding; rather, a rule is given a scope (discussed later) that determines the search space for a by-type binding.

A “by-path” reference identifies a particular model element by its path in model space. By-path references are always relative to the rule scope; there is currently no way to path “out” of the scope. All by-path references also have an associated type, but this does not influence what the reference binds to. The element identified by the path must however by type compatible with the reference, or a fatal “binding failure” will occur.

#### Binding scope

Rules are bound within a “scope”, which determines how references bind. Most rules are bound at the project scope (i.e. the root of the model graph for the project). However, rules can be scoped to a node within the graph. The `ModelMap.named(java.lang.String, java.lang.Class)` method is an example of a mechanism for applying scoped rules. Rules declared in the build script using the `model {}` block, or via a `RuleSource` applied as a plugin use the root of the model space as the scope. This can be considered the default scope.

By-path references are always relative to the rule scope. When the scope is the root, this effectively allows binding to any element in the graph. When it is not, then only the children of the scope can be referenced using "by-path" notation.

When binding by-type references, the following elements are considered:

- The scope element itself.

- The immediate children of the scope element.

- The immediate children of the model space (i.e. project space) root.

For the common case, where the rule is effectively scoped to the root, only the immediate children of the root need to be considered.

#### Binding to all elements in a scope matching type

Mutating or validating all elements of a given type in some scope is a common use-case. To accommodate this, rules can be applied via the `@Each` annotation.

In the example below, a `@Defaults` rule is applied to each `FileItem` in the model setting a default file size of "1024". Another rule applies a `RuleSource` to every `DirectoryItem` that makes sure all file sizes are positive and divisible by "16".

Example 508. a DSL example applying a rule to every element in a scope

`build.gradle`

    @Managed interface Item extends Named {}
    @Managed interface FileItem extends Item {
        void setSize(int size)
        int getSize()
    }
    @Managed interface DirectoryItem extends Item {
        ModelMap<Item> getChildren()
    }

    class PluginRules extends RuleSource {
        @Defaults void setDefaultFileSize(@Each FileItem file) {
            file.size = 1024
        }

        @Rules void applyValidateRules(ValidateRules rules, @Each DirectoryItem directory)  {}
    }
    apply plugin: PluginRules

    abstract class ValidateRules extends RuleSource {
        @Validate
        void validateSizeIsPositive(ModelMap<FileItem> files) {
            files.each { file ->
                assert file.size > 0
            }
        }

        @Validate
        void validateSizeDivisibleBySixteen(ModelMap<FileItem> files) {
            files.each { file ->
                assert file.size % 16 == 0
            }
        }
    }

    model {
        root(DirectoryItem) {
            children {
                dir(DirectoryItem) {
                    children {
                        file1(FileItem)
                        file2(FileItem) { size = 2048 }
                    }
                }
                file3(FileItem)
            }
        }
    }

Note: The code for this example can be found at `samples/modelRules/ruleSourcePluginEach` in the ‘-all’ distribution of Gradle.

## The model DSL

In addition to using a RuleSource, it is also possible to declare a model and rules directly in a build script using the “model DSL”.

The model DSL makes heavy use of various Groovy DSL features. Please have a read of the section called “Some Groovy basics” for an introduction to these Groovy features.

The general form of the model DSL is:

    model {
        «rule-definitions»
    }

All rules are nested inside a `model` block. There may be any number of rule definitions inside each `model` block, and there may be any number of `model` blocks in a build script. You can also use a `model` block in build scripts that are applied using `apply from: $uri`.

There are currently 2 kinds of rule that you can define using the model DSL: configuration rules, and creation rules.

### Configuration rules

You can define a rule that configures a particular model element. A configuration rule has the following form:

    model {
        «model-path-to-subject» {
            «configuration code»
        }
    }

Continuing with the example so far of the model element `"person"` of type `Person` being present, the following DSL snippet adds a configuration rule for the person that sets its `lastName` property.

Example 509. DSL configuration rule

`build.gradle`

    model {
        person {
            lastName = "Smith"
        }
    }

A configuration rule specifies a path to the subject that should be configured and a closure containing the code to run when the subject is configured. The closure is executed with the subject passed as the closure delegate. Exactly what code you can provide in the closure depends on the type of the subject. This is discussed below.

You should note that the configuration code is not executed immediately but is instead executed only when the subject is required. This is an important behaviour of model rules and allows Gradle to configure only those elements that are required for the build, which helps reduce build time. For example, let’s run a task that uses the "person" object:

Example 510. Configuration run when required

`build.gradle`

    model {
        person {
            println "configuring person"
            lastName = "Smith"
        }
    }

Output of `gradle showPerson`

    > gradle showPerson
    configuring person
    :showPerson
    Hello John Smith!

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

You can see that before the task is run, the "person" element is configured by running the rule closure. Now let’s run a task that does not require the "person" element:

Example 511. Configuration not run when not required

Output of `gradle somethingElse`

    > gradle somethingElse
    :somethingElse
    Not using person

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

In this instance, you can see that the "person" element is not configured at all.

### Creation rules

It is also possible to create model elements at the root level. The general form of a creation rule is:

    model {
        «element-name»(«element-type») {
            «initialization code»
        }
    }

The following model rule creates the `"person"` element:

Example 512. DSL creation rule

`build.gradle`

    model {
        person(Person) {
            firstName = "John"
        }
    }

A creation rule definition specifies the path of the element to create, plus its public type, represented as a Java interface or class. Only certain types of model elements can be created.

A creation rule may also provide a closure containing the initialization code to run when the element is created. The closure is executed with the element passed as the closure delegate. Exactly what code you can provide in the closure depends on the type of the subject. This is discussed below.

The initialization closure is optional and can be omitted, for example:

Example 513. DSL creation rule without initialization

`build.gradle`

    model {
        barry(Person)
    }

You should note that the initialization code is not executed immediately but is instead executed only when the element is required. The initialization code is executed before any configuration rules are run. For example:

Example 514. Initialization before configuration

`build.gradle`

    model {
        person {
            println "configuring person"
            println "last name is $lastName, should be Smythe"
            lastName = "Smythe"
        }
        person(Person) {
            println "creating person"
            firstName = "John"
            lastName = "Smith"
        }
    }

Output of `gradle showPerson`

    > gradle showPerson
    creating person
    configuring person
    last name is Smith, should be Smythe
    :showPerson
    Hello John Smythe!

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

Notice that the creation rule appears in the build script after the configuration rule, but its code runs before the code of the configuration rule. Gradle collects up all the rules for a particular subject before running any of them, then runs the rules in the appropriate order.

### Model rule closures

Most DSL rules take a closure containing some code to run to configure the subject. The code you can use in this closure depends on the type of the subject of the rule.

You can use the model report to determine the type of a particular model element.

In general, a rule closure may contain arbitrary code, mixed with some type specific DSL syntax.

#### `ModelMap<T>` subject

A `ModelMap` is basically a map of model elements, indexed by some name. When a `ModelMap` is used as the subject of a DSL rule, the rule closure can use any of the methods defined on the `ModelMap` interface.

A rule closure with `ModelMap` as a subject can also include nested creation or configuration rules. These behave in a similar way to the creation and configuration rules that appear directly under the `model` block.

Here is an example of a nested creation rule:

Example 515. Nested DSL creation rule

`build.gradle`

    model {
        people {
            john(Person) {
                firstName = "John"
            }
        }
    }

As before, a nested creation rule defines a name and public type for the element, and optionally, a closure containing code to use to initialize the element. The code is run only when the element is required in the build.

Here is an example of a nested configuration rule:

Example 516. Nested DSL configuration rule

`build.gradle`

    model {
        people {
            john {
                lastName = "Smith"
            }
        }
    }

As before, a nested configuration rule defines the name of the element to configure and a closure containing code to use to configure the element. The code is run only when the element is required in the build.

`ModelMap` introduces several other kinds of rules. For example, you can define a rule that targets each of the elements in the map. The code in the rule closure is executed once for each element in the map, when that element is required. Let’s run a task that requires all of the children of the "people" element:

Example 517. DSL configuration rule for each element in a map

`build.gradle`

    model {
        people {
            john(Person) {
                println "creating $it"
                firstName = "John"
                lastName = "Smith"
            }
            all {
                println "configuring $it"
            }
            barry(Person) {
                println "creating $it"
                firstName = "Barry"
                lastName = "Barry"
            }
        }
    }

Output of `gradle listPeople`

    > gradle listPeople
    creating Person 'people.barry'
    configuring Person 'people.barry'
    creating Person 'people.john'
    configuring Person 'people.john'
    :listPeople
    Hello Barry Barry!
    Hello John Smith!

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

Any method on `ModelMap` that accepts an `Action` as its last parameter can also be used to define a nested rule.

#### `@Managed` type subject

When a managed type is used as the subject of a DSL rule, the rule closure can use any of the methods defined on the managed type interface.

A rule closure can also configure the properties of the element using nested closures. For example:

Example 518. Nested DSL property configuration

`build.gradle`

    model {
        person {
            address {
                city = "Melbourne"
            }
        }
    }

Currently, the nested closures do not define rules and are executed immediately. Please be aware that this behaviour will change in a future Gradle release.

#### All other subjects

For all other types, the rule closure can use any of the methods defined by the type. There is no special DSL defined for these elements.

### Automatic type coercion

Scalar properties in managed types can be assigned `CharSequence` values (e.g. `String`, `GString`, etc.) and they will be converted to the actual property type for you. This works for all scalar types including `File`s, which will be resolved relative to the current project.

Example 519. a DSL example showing type conversions

`build.gradle`

    enum Temperature {
       TOO_HOT,
       TOO_COLD,
       JUST_RIGHT
    }

    @Managed
    interface Item {
       void setName(String n); String getName()

       void setQuantity(int q); int getQuantity()

       void setPrice(float p); float getPrice()

       void setTemperature(Temperature t)
       Temperature getTemperature()

       void setDataFile(File f); File getDataFile()
    }

    class ItemRules extends RuleSource {
       @Model
       void item(Item item) {
          def data = item.dataFile.text.trim()
          def (name, quantity, price, temp) = data.split(',')
          item.name = name
          item.quantity = quantity
          item.price = price
          item.temperature = temp
       }

       @Defaults
       void setDefaults(Item item) {
          item.dataFile = 'data.csv'
       }

       @Mutate
       void createDataTask(ModelMap<Task> tasks, Item item) {
          tasks.create('showData') {
             doLast {
                println """
    Item '$item.name'
       quantity:    $item.quantity
       price:       $item.price
       temperature: $item.temperature"""
             }
          }
       }
    }

    apply plugin: ItemRules

    model {
       item {
          price = "${price * (quantity < 10 ? 2 : 0.5)}"
       }
    }

Note: The code for this example can be found at `samples/modelRules/modelDslCoercion` in the ‘-all’ distribution of Gradle.

In the above example, an `Item` is created and is initialized in `setDefaults()` by providing the path to the data file. In the `item()` method the resolved `File` is parsed to extract and set the data. In the DSL block at the end, the price is adjusted based on the quantity; if there are fewer than 10 remaining the price is doubled, otherwise it is reduced by 50%. The `GString` expression is a valid value since it resolves to a `float` value in string form.

Finally, in `createDataTask()` we add the `showData` task to display all of the configured values.

### Declaring input dependencies

Rules declared in the DSL may depend on other model elements through the use of a special syntax, which is of the form:

    $.«path-to-model-element»

Paths are a period separated list of identifiers. To directly depend on the `firstName` of the person, the following could be used:

    $.person.firstName

Example 520. a DSL rule using inputs

`build.gradle`

    model {
        tasks {
            hello(Task) {
                def p = $.person
                doLast {
                    println "Hello $p.firstName $p.lastName!"
                }
            }
        }
    }

Note: The code for this example can be found at `samples/modelRules/modelDsl` in the ‘-all’ distribution of Gradle.

In the above snippet, the `$.person` construct is an input reference. The construct returns the value of the model element at the specified path, as its default type (i.e. the type advertised by the Model Report). It may appear anywhere in the rule that an expression may normally appear. It is not limited to the right hand side of variable assignments.

The input element is guaranteed to be fully configured before the rule executes. That is, all of the rules that mutate the element are guaranteed to have been previously executed, leaving the target element in its final, immutable, state.

Most model elements enforce immutability when being used as inputs. Any attempt to mutate such an element will result in a runtime error. However, some legacy type objects do not currently implement such checks. Regardless, it is always invalid to attempt to mutate an input to a rule.

#### Using `ModelMap<T>` as an input

When you use a `ModelMap` as input, each item in the map is made available as a property.

## The model report

The built-in `ModelReport` task displays a hierarchical view of the elements in the model space. Each item prefixed with a `+` on the model report is a model element and the visual nesting of these elements correlates to the model path (e.g. `tasks.help`). The model report displays the following details about each model element:

Table 100. Model report - model element details

Detail | Description
Type | This is the underlying type of the model element and is typically a fully qualified class name.
Value | Is conditionally displayed on the report when a model element can be represented as a string.
Creator | Every model element has a creator. A creator signifies the origin of the model element (i.e. what created the model element).
Rules | Is a listing of the rules, excluding the creator rule, which are executed for a given model element. The order in which the rules are displayed reflects the order in which they are executed.

Example 521. model task output

Output of `gradle model`

    > gradle model
    :model

    ------------------------------------------------------------
    Root project
    ------------------------------------------------------------

    + person
          | Type:       Person
          | Creator:     PersonRules#person(Person)
          | Rules:
             ⤷ person { ... } @ build.gradle line 59, column 3
             ⤷ PersonRules#setFirstName(Person)
        + age
              | Type:       int
              | Value:      0
              | Creator:     PersonRules#person(Person)
        + children
              | Type:       org.gradle.model.ModelSet<Person>
              | Creator:     PersonRules#person(Person)
        + employed
              | Type:       boolean
              | Value:      false
              | Creator:     PersonRules#person(Person)
        + father
              | Type:       Person
              | Value:      null
              | Creator:     PersonRules#person(Person)
        + firstName
              | Type:       java.lang.String
              | Value:      John
              | Creator:     PersonRules#person(Person)
        + homeDirectory
              | Type:       java.io.File
              | Value:      null
              | Creator:     PersonRules#person(Person)
        + id
              | Type:       java.lang.Long
              | Value:      null
              | Creator:     PersonRules#person(Person)
        + lastName
              | Type:       java.lang.String
              | Value:      Smith
              | Creator:     PersonRules#person(Person)
        + maritalStatus
              | Type:       MaritalStatus
              | Creator:     PersonRules#person(Person)
        + mother
              | Type:       Person
              | Value:      null
              | Creator:     PersonRules#person(Person)
        + userGroups
              | Type:       java.util.List<java.lang.String>
              | Value:      null
              | Creator:     PersonRules#person(Person)
    + tasks
          | Type:       org.gradle.model.ModelMap<org.gradle.api.Task>
          | Creator:     Project.<init>.tasks()
          | Rules:
             ⤷ PersonRules#createHelloTask(ModelMap<Task>, Person)
        + buildEnvironment
              | Type:       org.gradle.api.tasks.diagnostics.BuildEnvironmentReportTask
              | Value:      task ':buildEnvironment'
              | Creator:     tasks.addPlaceholderAction(buildEnvironment)
              | Rules:
                 ⤷ copyToTaskContainer
        + components
              | Type:       org.gradle.api.reporting.components.ComponentReport
              | Value:      task ':components'
              | Creator:     tasks.addPlaceholderAction(components)
              | Rules:
                 ⤷ copyToTaskContainer
        + dependencies
              | Type:       org.gradle.api.tasks.diagnostics.DependencyReportTask
              | Value:      task ':dependencies'
              | Creator:     tasks.addPlaceholderAction(dependencies)
              | Rules:
                 ⤷ copyToTaskContainer
        + dependencyInsight
              | Type:       org.gradle.api.tasks.diagnostics.DependencyInsightReportTask
              | Value:      task ':dependencyInsight'
              | Creator:     tasks.addPlaceholderAction(dependencyInsight)
              | Rules:
                 ⤷ HelpTasksPlugin.Rules#addDefaultDependenciesReportConfiguration(DependencyInsightReportTask, ServiceRegistry)
                 ⤷ copyToTaskContainer
        + dependentComponents
              | Type:       org.gradle.api.reporting.dependents.DependentComponentsReport
              | Value:      task ':dependentComponents'
              | Creator:     tasks.addPlaceholderAction(dependentComponents)
              | Rules:
                 ⤷ copyToTaskContainer
        + hello
              | Type:       org.gradle.api.Task
              | Value:      task ':hello'
              | Creator:     PersonRules#createHelloTask(ModelMap<Task>, Person) > create(hello)
              | Rules:
                 ⤷ copyToTaskContainer
        + help
              | Type:       org.gradle.configuration.Help
              | Value:      task ':help'
              | Creator:     tasks.addPlaceholderAction(help)
              | Rules:
                 ⤷ copyToTaskContainer
        + init
              | Type:       org.gradle.buildinit.tasks.InitBuild
              | Value:      task ':init'
              | Creator:     tasks.addPlaceholderAction(init)
              | Rules:
                 ⤷ copyToTaskContainer
        + model
              | Type:       org.gradle.api.reporting.model.ModelReport
              | Value:      task ':model'
              | Creator:     tasks.addPlaceholderAction(model)
              | Rules:
                 ⤷ copyToTaskContainer
        + projects
              | Type:       org.gradle.api.tasks.diagnostics.ProjectReportTask
              | Value:      task ':projects'
              | Creator:     tasks.addPlaceholderAction(projects)
              | Rules:
                 ⤷ copyToTaskContainer
        + properties
              | Type:       org.gradle.api.tasks.diagnostics.PropertyReportTask
              | Value:      task ':properties'
              | Creator:     tasks.addPlaceholderAction(properties)
              | Rules:
                 ⤷ copyToTaskContainer
        + tasks
              | Type:       org.gradle.api.tasks.diagnostics.TaskReportTask
              | Value:      task ':tasks'
              | Creator:     tasks.addPlaceholderAction(tasks)
              | Rules:
                 ⤷ copyToTaskContainer
        + wrapper
              | Type:       org.gradle.api.tasks.wrapper.Wrapper
              | Value:      task ':wrapper'
              | Creator:     tasks.addPlaceholderAction(wrapper)
              | Rules:
                 ⤷ copyToTaskContainer

## Limitations and future direction

Rule based model configuration is the future of Gradle. This area is fledgling, but under very active development. Early experiments have demonstrated that this approach is more efficient, able to provide richer diagnostics and authoring assistance and is more extensible. However, there are currently many limitations.

The majority of the development to date has been focused on proving the efficacy of the approach, and building the internal rule execution engine and model graph mechanics. The user facing aspects (e.g the DSL, rule source classes) are yet to be optimized for conciseness and general usability. Likewise, many necessary configuration patterns and constructs are not yet able to be expressed via the API.

In conjunction with the addition of better syntax, a richer toolkit of configuration constructs and generally more expressive power, more tooling will be added that will enable build engineers and users alike to comprehend, modify and extend builds in new ways.

Due to the inherent nature of the rule based approach, it is more efficient at constructing the build model than today’s Gradle. However, in the future Gradle will also leverage the parallelism that this approach enables both at configuration and execution time. Moreover, due to increased transparency of the model Gradle will be able to further reduce build times by caching and pre-computing the build model. Beyond improved general build performance, this will greatly improve the experience when using Gradle from tools such as IDEs.

As this area of Gradle is under active development, it will be changing rapidly. Please be sure to consult the documentation of Gradle corresponding to the version you are using and to watch for changes announced in the release notes for future versions.