pkl/stdlib/ref.pkl

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/// Type-safe deferred references to values not known at evaluation time.
///
/// References are an advanced API design tool that enables library authors to express
/// domain-specific references to values that may not be known during evaluation.
/// They are particularly suited for configuring execution systems where tasks have well-typed
/// inputs and output.
///
/// WARNING: The API and semantics of references are subject to change in a future release.
/// The Pkl team is soliciting feedback from authors of libraries considering adopting references.
/// For questions and feedback, please reach out via
/// [GitHub Discussions](https://github.com/apple/pkl/discussions) or
/// [create an issue](https://github.com/apple/pkl/issues/new).
@ModuleInfo { minPklVersion = "0.32.0" }
module pkl.ref

/// Creates a deferred [Reference] to a value of type [class] in the given [domain].
///
/// References may only be constructed for single, non-generic class types.
/// To create a reference to other types (generic classes, union, nullable, constrained,
/// typealiases, etc.), it may be necessary to create a wrapper class with a property of the desired
/// type.
/// Example:
/// ```
/// referenceListingString: Reference<Domain, Listing<String>> = Reference(domain, Holder, data).$
///
/// class Holder {
///   $: Listing<String>
/// }
/// ```
external const function Reference<D, T>(
  domain: D(this is Domain),
  `class`: Class<T>,
  data: Any,
): Reference<D, T>

/// A reference to a value that may not exist at evaluation time.
///
/// References are an advanced API design tool that enables library authors to express
/// domain-specific references to values that may not be known during evaluation.
///
/// Instances are created with the [Reference()] constructor method.
/// References consist of four parts:
/// * [Domain]: determines which references are compatible and how references are rendered
///   as strings.
/// * Data: an arbitrary value that may contain domain-specific information about the
///   referenced value.
/// * Path: a [List] of [Access] values indicating how the reference was accessed (by property or
///   subscript).
/// * Referent type: the type of the value that the reference refers to.
///
/// Every time a reference is accessed, either via qualified property access
/// (`<reference>.<property>`) or subscript (`<reference>[<key>]`), a new reference is returned.
/// The new reference shares the same domain and data as the original reference.
/// The new reference's path extends the original reference's with an [Access] instance describing
/// the accessed property name or subscript key.
/// The new reference's referent type is the type of the accessed property or subscript value of the
/// original referent type.
/// Properties with the `external` modifier may not be referenced.
/// Any type constraints within the referent type are erased and type constraints are not allowed
/// in the referent (second) type argument of any Reference type annotations.
///
/// Example:
/// ```
/// import "pkl:ref"
///
/// /// Define a domain class for this configuration system:
/// class Domain extends ref.Domain {
///   /// Define how this domain renders references to strings:
///   function renderReference(reference: ref.Reference<Domain, Any>): String =
///     (
///       List(reference.getData().toString())
///       + reference.getPath().map((it) -> it.property ?? it.key.toString())
///     ).join("/")
/// }
///
/// class Resource {
///   name: String
///   /// Types that provide references will often vend a "root" Reference via a `fixed` property.
///   /// Here, this references [Outputs] and the `data` identifies the enclosing [Resource]. 
///   fixed outputs: ref.Reference<Domain, Outputs> = ref.Reference(new Domain {}, Outputs, name)
/// }
///
/// class Outputs {
///   a: String
///   b: String(length < 5)
///   c: Listing<String>
///   d: Mapping<Foo, Int(isOdd)?>
///   e: Foo | Bar
/// }
///
/// class Foo {
///   x: Int
///
///   function toString(): String = "foo:\(x)"
/// }
///
/// class Bar {
///   x: Float
///
///   function toString(): String = "bar:\(x)"
/// }
///
/// local outputs: ref.Reference<Domain, Outputs> = new Resource { name = "root" }.outputs
///
/// /// Simple property access.
/// /// getPath() == List(new ref.Access { property = "a" })
/// aRef: ref.Reference<Domain, String> = outputs.a
///
/// /// Type constraint erasure.
/// /// getPath() == List(new ref.Access { property = "b" })
/// bRef: ref.Reference<Domain, String> = outputs.b
///
/// /// Simple subscript access.
/// /// getPath() == List(new ref.Access { property = "c" }, new ref.Access { key = 10 })
/// cRef: ref.Reference<Domain, String> = outputs.c[10]
///
/// /// Simple property access:
/// /// getPath() == List(new ref.Access { property = "d" }, new ref.Access { key = new Foo { x = 1 } })
/// /// The type constraint on the Mapping value type argument is also erased.
/// dRef: ref.Reference<Domain, Int?> = outputs.d[new Foo { x = 1 }]
///
/// /// Simple property access:
/// /// getPath() == List(new ref.Access { property = "e" }, new ref.Access { property = "x" })
/// eRef: ref.Reference<Domain, Int | Float> = outputs.e.x
///
/// // Render references as strings:
/// output {
///   renderer {
///     converters {
///       [ref.Reference] = (it) -> it.toString()
///     }
///   }
/// }
/// ```
///
/// Output:
/// ```
/// aRef = "root/a"
/// bRef = "root/b"
/// cRef = "root/c/10"
/// dRef = "root/d/foo:1"
/// eRef = "root/e/x"
/// ```
external class Reference<out D, out T> {
  /// The domain the reference belongs to.
  external function getDomain(): D

  /// Arbitrary data attached to the reference during creation.
  ///
  /// Used for domain-specific purposes such as controlling how the reference is rendered. 
  external function getData(): Any

  /// The path of property and subscript access applied to the reference.
  external function getPath(): List<Access>

  /// Render the reference to a string using its domain's ([getDomain()])
  /// [Domain.referencetoString()] method.
  function toString(): String = getDomain().renderReference(this)
}

/// Represents a configuration domain that a [Reference] may exist within.
///
/// Library authors supporting references should declare a subclass of [Domain] to be shared by all
/// inter-compatible [Reference] instances.
///
/// A domain also determines how [Reference] values are transformed into strings; see
/// [renderReference()].
abstract class Domain {
  /// Must be overridden by domain classes to determine how [Reference] instances are transformed to
  /// strings by [Reference.toString()] and string interpolation.
  abstract function renderReference(reference: Reference<Domain, Any>): String
}

/// Represents a property or subscript access as part of a [Reference]'s path.
class Access {
  /// Indicates if this represents a property access.
  fixed isProperty: Boolean = property != null

  /// Indicates if this represents a subscript access.
  fixed isSubscript: Boolean = property == null

  /// The property that was accessed.
  ///
  /// If non-null, this is a property access. If `null` this is a subscript access.
  property: String(key == null)?

  /// The subscript key that was accessed.
  ///
  /// May be null even when [property] is null, which represents a subscript access with key `null`.
  key: Any
}