/
boolinduction.scala
56 lines (45 loc) · 1.83 KB
/
boolinduction.scala
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/*
* Copyright (c) 2013 Miles Sabin
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package shapeless.examples
object BooleanInduction extends App {
import shapeless._
import syntax.singleton._
// Some preliminaries ...
val (wTrue, wFalse) = (true.witness, false.witness)
type True = wTrue.T
type False = wFalse.T
trait If[C <: Boolean, A, B] { type T ; def apply(a: A, b: B): T }
object If {
implicit def ifTrue[A, B] = new If[True, A, B] { type T = A ; def apply(a: A, b: B) = a }
implicit def ifFalse[A, B] = new If[False, A, B] { type T = B ; def apply(a: A, b: B) = b }
}
// Scala translation of:
//
// bool-induction : (P : Bool->Type) (t : P true) (f: P false) (x : Bool) -> P x
// bool-induction P pt pf true = pt
// bool-induction P pt pf false = pf
def boolInduction[P <: { type Case[_ <: Boolean] <: { type T }}, PT, PF]
(p: P)(t: PT)(f: PF)(x: Witness.Lt[Boolean])
(implicit pt: p.Case[True] { type T = PT }, pf: p.Case[False] { type T = PF }, sel: If[x.T, PT, PF]): sel.T = sel(t, f)
// In use ...
object si {
trait Case[B <: Boolean] { type T }
implicit val sit = new Case[True] { type T = String }
implicit val sif = new Case[False] { type T = Int }
}
val bt: String = boolInduction(si)("foo")(23)(true)
val bf: Int = boolInduction(si)("foo")(23)(false)
}