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Diffstat (limited to 'core/shared/src/main/scala/interface.scala')
| -rw-r--r-- | core/shared/src/main/scala/interface.scala | 157 |
1 files changed, 157 insertions, 0 deletions
diff --git a/core/shared/src/main/scala/interface.scala b/core/shared/src/main/scala/interface.scala new file mode 100644 index 0000000..54f8ce3 --- /dev/null +++ b/core/shared/src/main/scala/interface.scala @@ -0,0 +1,157 @@ +package magnolia + +import language.higherKinds + +/** represents a subtype of a sealed trait + * + * @tparam Typeclass type constructor for the typeclass being derived + * @tparam Type generic type of this parameter */ +trait Subtype[Typeclass[_], Type] { + + /** the type of subtype */ + type SType <: Type + + /** the name of the subtype + * + * This is the fully-qualified name of the type of subclass. */ + def label: String + + /** the typeclass instance associated with this subtype + * + * This is the instance of the type `Typeclass[SType]` which will have been discovered by + * implicit search, or derived by Magnolia. */ + def typeclass: Typeclass[SType] + + /** partial function defined the subset of values of `Type` which have the type of this subtype */ + def cast: PartialFunction[Type, SType] +} + +/** represents a parameter of a case class + * + * @tparam Typeclass type constructor for the typeclass being derived + * @tparam Type generic type of this parameter */ +trait Param[Typeclass[_], Type] { + + /** the type of the parameter being represented + * + * For exmaple, for a case class, + * <pre> + * case class Person(name: String, age: Int) + * </pre> + * the [[Param]] instance corresponding to the `age` parameter would have a [[PType]] equal to + * the type [[scala.Int]]. However, in practice, this type will never be universally quantified. + */ + type PType + + /** the name of the parameter */ + def label: String + + /** the typeclass instance associated with this parameter + * + * This is the instance of the type `Typeclass[PType]` which will have been discovered by + * implicit search, or derived by Magnolia. + * + * Its type is existentially quantified on this [[Param]] instance, and depending on the + * nature of the particular typeclass, it may either accept or produce types which are also + * existentially quantified on this same [[Param]] instance. */ + def typeclass: Typeclass[PType] + + /** provides the default value for this parameter, as defined in the case class constructor */ + def default: Option[PType] + + /** dereferences a value of the case class type, `Type`, to access the value of the parameter + * being represented + * + * When programming generically, against an unknown case class, with unknown parameter names + * and types, it is not possible to directly access the parameter values without reflection, + * which is undesirable. This method, whose implementation is provided by the Magnolia macro, + * will dereference a case class instance to access the parameter corresponding to this + * [[Param]]. + * + * Whilst the type of the resultant parameter value cannot be universally known at the use, its + * type will be existentially quantified on this [[Param]] instance, and the return type of the + * corresponding `typeclass` method will be existentially quantified on the same value. This is + * sufficient for the compiler to determine that the two values are compatible, and the value may + * be applied to the typeclass (in whatever way that particular typeclass provides). + * + * @param param the instance of the case class to be dereferenced + * @return the parameter value */ + def dereference(param: Type): PType +} + +/** represents a case class or case object and the context required to construct a new typeclass + * instance corresponding to it + * + * Instances of [[CaseClass]] provide access to all of the parameters of the case class, the full + * name of the case class type, and a boolean to determine whether the type is a case class or case + * object. + * + * @param typeName the name of the case class + * @param isObject true only if this represents a case object rather than a case class + * @param parametersArray an array of [[Param]] values for this case class + * @tparam Typeclass type constructor for the typeclass being derived + * @tparam Type generic type of this parameter */ +abstract class CaseClass[Typeclass[_], Type] private[magnolia] ( + val typeName: String, + val isObject: Boolean, + val isValueClass: Boolean, + parametersArray: Array[Param[Typeclass, Type]] +) { + + /** constructs a new instance of the case class type + * + * This method will be implemented by the Magnolia macro to make it possible to construct + * instances of case classes generically in user code, that is, without knowing their type + * concretely. + * + * To construct a new case class instance, the method takes a lambda which defines how each + * parameter in the new case class should be constructed. See the [[Param]] class for more + * information on constructing parameter values from a [[Param]] instance. + * + * @param makeParam lambda for converting a generic [[Param]] into the value to be used for + * this parameter in the construction of a new instance of the case class + * @return a new instance of the case class */ + def construct[Return](makeParam: Param[Typeclass, Type] => Return): Type + + /** a sequence of [[Param]] objects representing all of the parameters in the case class + * + * For efficiency, this sequence is implemented by an `Array`, but upcast to a + * [[scala.collection.Seq]] to hide the mutable collection API. */ + def parameters: Seq[Param[Typeclass, Type]] = parametersArray +} + +/** represents a sealed trait and the context required to construct a new typeclass instance + * corresponding to it + * + * Instances of `SealedTrait` provide access to all of the component subtypes of the sealed trait + * which form a coproduct, and to the fully-qualified name of the sealed trait. + * + * @param typeName the name of the sealed trait + * @param subtypesArray an array of [[Subtype]] instances for each subtype in the sealed trait + * @tparam Typeclass type constructor for the typeclass being derived + * @tparam Type generic type of this parameter */ +final class SealedTrait[Typeclass[_], Type](val typeName: String, + subtypesArray: Array[Subtype[Typeclass, Type]]) { + + /** a sequence of all the subtypes of this sealed trait */ + def subtypes: Seq[Subtype[Typeclass, Type]] = subtypesArray + + /** convenience method for delegating typeclass application to the typeclass corresponding to the + * subtype of the sealed trait which matches the type of the `value` + * + * @tparam Return the return type of the lambda, which should be inferred + * @param value the instance of the generic type whose value should be used to match on a + * particular subtype of the sealed trait + * @param handle lambda for applying the value to the typeclass for the particular subtype which + * matches + * @return the result of applying the `handle` lambda to subtype of the sealed trait which + * matches the parameter `value` */ + def dispatch[Return](value: Type)(handle: Subtype[Typeclass, Type] => Return): Return = + subtypes + .map { sub => + sub.cast.andThen { v => + handle(sub) + } + } + .reduce(_ orElse _)(value) +} |