Created
April 24, 2014 07:17
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Lens inference in shapeless 2.0.0
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import shapeless._ | |
import ops.record.{ Selector, Updater } | |
import record.{ FieldType, field } | |
trait PathLens[T, P] { | |
type Elem | |
def get(t : T) : Elem | |
def set(t : T)(e : Elem) : T | |
} | |
object PathLens { | |
type Aux[T, P, E] = PathLens[T, P] { type Elem = E } | |
implicit def mkPathLens[T, R <: HList, P, E] | |
(implicit | |
gen: LabelledGeneric.Aux[T, R], | |
sel: Selector.Aux[R, P, E], | |
upd: Updater.Aux[R, FieldType[P, E], R] | |
): PathLens.Aux[T, P, E] = | |
new PathLens[T, P] { | |
type Elem = E | |
def get(t : T) : Elem = sel(gen.to(t)) | |
def set(t : T)(e : E) : T = gen.from(upd(gen.to(t), field[P](e))) | |
} | |
} | |
object Demo extends App { | |
case class Person(name : String, age : Int) | |
case class Dog(name : String, age : Int) | |
def updateField[T, F, E](t: T, f: Witness)(e: E)(implicit lens: PathLens.Aux[T, f.T, E]): (T, E) = { | |
val old = lens.get(t) | |
val updated = lens.set(t)(e) | |
(updated, old) | |
} | |
val mary = Person("Mary", 23) | |
val boris = Dog("Boris", 5) | |
// Lens for Person at Int field `age` is inferred | |
val mary1 = updateField(mary, 'age)(24) | |
assert(mary1 == (Person("Mary", 24), 23)) | |
// Lens for Dog at Int field `age` is inferred | |
val boris1 = updateField(boris, 'age)(6) | |
assert(boris1 == (Dog("Boris", 6), 5)) | |
} |
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